Let the lock go for as long as it was held during Overview's walk (closes #226)

The home page's cards are worked out once a minute by Overview, which
walks every prefix of the routing table under the table's read lock.
After each batch of 10,000 prefixes, the walk let the lock go and took
it back at once. The prefix handler, waiting for the write lock, then got
in only once per batch. During a walk, its queue could fill, and the
streamer then dropped updates for it.

The walk now lets the lock go for as long as it held the lock for the
batch, as PeerDown does. A test applies another peer's 1,000
announcements while Overview walks a table with more prefixes than five
of the walk's batches.

Model: opus-5-5

README.md

@@ -429,10 +429,12 @@ the routing table, and everything else from the database.
     - Parses BGP UPDATE messages from JSON stream
     - Dispatches to registered handlers based on message type
     - Each handler has its own queue with backpressure handling
-    - Handlers batch writes for efficiency: the AS, peer and prefix handlers
-      write 30,000, 10,000 and 25,000 updates at a time, or after 2, 2 and 1
-      seconds; the peering handler writes the peerings it has collected every 30
-      seconds
+    - Handlers batch writes for efficiency. The AS, peer and prefix handlers
+      write a batch when it reaches a size, or after a time:
+        - the AS handler at 30,000 ASes, or after 2 seconds;
+        - the peer handler at 10,000 updates, or after 2 seconds;
+        - the prefix handler at 25,000 updates, or after 1 second.
+    - The peering handler writes the peerings it has collected every 30 seconds
 
 4. **Handler Details**
     - **ASHandler**: Tracks all ASNs seen in AS paths
@@ -442,7 +444,9 @@ the routing table, and everything else from the database.
         - An announcement gives the peer a route to the prefix.
         - A withdrawal removes the peer's route. So does an announcement whose
           AS path is empty or holds AS 0 anywhere.
-        - A peer whose BGP session goes down leaves every prefix.
+        - A peer whose BGP session goes down leaves every prefix. The handler
+          holds back the peer's later updates until the peer has left every
+          prefix, then applies them in order.
 
         Every second it writes the changed prefix and origin AS pairs to the
         `routes` table. A write takes at most 20,000 pairs, and the rest follow
@@ -578,8 +582,8 @@ Budget:
 - Go heap: a 2.5 GiB soft limit (`GOMEMLIMIT=2560MiB`, set in the image).
     - The routing table takes about 700 bytes per prefix and origin AS. A full
       table has about 1.4 million of these pairs, so it takes about 1 GB.
-    - The rest of the heap takes up to about 0.5 GiB. Most of it is the
-      handlers' batches and queues, described below.
+    - The rest of the heap takes up to about 0.5 GiB. Most of it is what the
+      handlers hold, described below.
     - The remaining 1 GiB is room for the garbage collector. With much less, it
       would run almost all the time.
 - SQLite: 656 MiB of page cache.
@@ -632,16 +636,25 @@ What happens at each limit:
   `STATE` message, which it holds, in feed order, until there is room: a few
   hundred bytes each, about ten a second.
 - Handler batches: while the AS, peer or prefix handler writes a batch to the
-  database, the updates that arrive collect in its next batch. That batch stops
-  at ten times the batch size:
-    - 300,000 updates for the AS handler (`asnBatchMaxSize`, about 10 MiB);
-    - 100,000 for the peer handler (`peerBatchMaxSize`, about 10 MiB);
-    - 250,000 for the prefix handler (`prefixBatchMaxSize`, about 15 MiB).
+  database, the updates that arrive collect in its next batch.
+    - The AS handler's batch holds one entry per AS, with the earliest and the
+      latest time of its updates. About 80,000 ASes appear in the feed, so the
+      batch takes at most about 10 MiB, however fast the feed runs.
+    - The peer handler's batch stops at 100,000 updates (`peerBatchMaxSize`),
+      about 10 MiB.
+    - The prefix handler's batch stops at 250,000 updates
+      (`prefixBatchMaxSize`), about 15 MiB.
 
-    At that size the handler takes nothing more off its queue until the write
-    ends. The queue then fills, and the streamer drops messages as above. With
-    the batches being written, the three handlers' batches take at most about
-    0.1 GiB of the Go heap.
+    At that maximum the peer or prefix handler takes nothing more off its queue
+    until the write ends. The queue then fills, and the streamer drops messages
+    as above. With the batches being written, the three handlers' batches take
+    at most about 0.1 GiB of the Go heap.
+
+- Held-back messages: the prefix handler holds back a peer's messages after the
+  peer's session goes down. It applies them once the peer has left every prefix.
+  It holds at most 20,000 such messages (`maxHeldBackMessages`), about 20 MiB.
+  At that number it takes nothing more off its queue until it has applied one of
+  them.
 
 With `DEBUG=routewatch` the daemon logs a `System stats` line every 60 seconds
 with the goroutine count and Go memory figures.

TODO.md

@@ -35,6 +35,46 @@ tested.
 
 # Completed Steps
 
+- 2026-11-02: working out the home page's cards no longer stops the prefix
+  handler. The walk over the routing table holds the table's lock for 10,000
+  prefixes at a time, then lets the lock go for as long again, as `PeerDown`
+  does. It used to take the lock back at once. The prefix handler then applied
+  about one update per 10,000 prefixes walked. During a walk, its queue could
+  fill, and the streamer then dropped updates for it (closes #226)
+- 2026-11-02: like a failed WHOIS lookup, a lookup skipped because its RIR is
+  paused makes the fetcher wait longer before the next lookup. The longest wait
+  is one minute. The status page does not count a skip as a failure. A skip used
+  to leave the wait as it was. Each lookup asks IANA before it finds its RIR
+  paused. So while most stale ASes belonged to a paused RIR, the fetcher could
+  ask IANA every second (closes #222)
+- 2026-11-02: a peer's session going down no longer stops the prefix handler.
+  The handler runs the table's `PeerDown` on a separate goroutine, one peer at a
+  time, and goes on taking messages. `PeerDown` holds the table's lock for
+  10,000 prefixes at a time, then lets the lock go for as long again. The
+  handler holds back the down peer's later messages until that peer's `PeerDown`
+  returns. Then the handler applies them in order, one at a time, and takes new
+  messages in between. It holds back at most 20,000 messages. At that number,
+  `HandleMessage` waits until one of them is applied. When `Stop` runs past its
+  deadline, it logs how many held-back messages were not applied (closes #216)
+- 2026-11-02: RouteWatch pauses an RIR's or NIC.br's WHOIS server for an hour
+  after the server refuses a query. A refused or reset connection is a refusal.
+  So is an answer with an `%ERROR:201:` or `%ERROR:202:` line. Lookups at the
+  other servers go on. An AS whose RIR is paused is looked up after the pause.
+  IANA is never paused, because a pause of IANA would stop every lookup. A
+  refusal by IANA is an ordinary failure. NIC.br is also paused for an hour
+  after it says RouteWatch has passed NIC.br's query limit. Before, the fetcher
+  asked a server again within seconds of a refusal (closes #218)
+- 2026-11-02: the AS handler never waits for a database write. Its batch holds
+  one entry per AS, with the earliest and the latest time of its updates. So the
+  batch no longer grows with the feed's rate. It used to hold one entry for each
+  AS of each path and to wait once it held 300,000. In a burst of the feed its
+  queue then filled, and the streamer dropped messages for it (closes #217)
+- 2026-10-27: the tests of `internal/routewatch` no longer fail on a loaded
+  host. Five of them gave `HandleMessage` two seconds to apply a full prefix
+  batch, which under the race detector can take longer. A test now waits up to
+  15 seconds for each step it expects. The tests of the handlers' limits give
+  the handlers small ones, so the package no longer runs past its 90-second
+  limit (closes #213, #215)
 - 2026-10-27: the server works out the home page's cards every minute until it
   stops. It used to work them out only once, at startup: the end of the context
   that `Start` is given stopped the refreshes (closes #211)

internal/routetable/overview.go

@@ -10,10 +10,6 @@ import (
 	"git.eeqj.de/sneak/routewatch/internal/ristypes"
 )
 
-// walkBatch is how many prefixes the walk over the whole table reads in one
-// hold of the read lock. The updates waiting for the lock get it in between.
-const walkBatch = 10000
-
 // Overview holds figures about the whole table. Working them out takes a walk
 // over every prefix, so a caller asks for them now and then and keeps them.
 type Overview struct {
@@ -83,8 +79,9 @@ type OriginPeers struct {
 
 // Overview works out the table's overview. It lists at most maxListed bogon
 // announcements and at most maxListed prefixes with more than one origin AS:
-// the first in prefix order. It walks over every prefix, but holds the read
-// lock for walkBatch prefixes at a time.
+// the first in prefix order. It visits every prefix in the table, in batches
+// of walkBatch. After each batch, it lets the table's lock go for as long as
+// it held the lock for the batch.
 func (t *Table) Overview(maxListed int) Overview {
 	var overview Overview
 
@@ -114,15 +111,18 @@ func (t *Table) Overview(maxListed int) Overview {
 }
 
 // walkPrefixes calls visit with every prefix of the table and its origins. It
-// holds the read lock, but lets it go after every walkBatch prefixes, so the
-// updates waiting for it get in. A prefix that is added or removed in such a
-// gap may or may not be visited, as Go allows for a map that changes during a
-// range over it. visit must not keep origins once it returns.
+// holds the read lock for walkBatch prefixes at a time. After each batch, it
+// lets the table's lock go for as long as it held the lock for the batch.
+// During a walk over many batches, updates can then take the lock about half
+// of the time. The walk may or may not visit a prefix that is added or removed
+// while the lock is let go. Go allows this in a range over a map that changes.
+// visit must not keep origins once it returns.
 func (t *Table) walkPrefixes(visit func(prefix netip.Prefix, origins []*origin)) {
 	t.mu.RLock()
 	defer t.mu.RUnlock()
 
 	visited := 0
+	batchStart := time.Now()
 
 	for _, f := range []*family{&t.ipv4, &t.ipv6} {
 		for _, prefixes := range f.byLength {
@@ -131,8 +131,13 @@ func (t *Table) walkPrefixes(visit func(prefix netip.Prefix, origins []*origin))
 
 				visited++
 				if visited%walkBatch == 0 {
+					held := time.Since(batchStart)
+
 					t.mu.RUnlock()
+					time.Sleep(held)
 					t.mu.RLock()
+
+					batchStart = time.Now()
 				}
 			}
 		}

internal/routetable/overview_internal_test.go

@@ -131,3 +131,61 @@ func TestWalkPrefixesLetsUpdatesIn(t *testing.T) {
 			"before its second batch")
 	}
 }
+
+// TestOverviewLetsUpdatesIn applies another peer's 1,000 announcements while
+// Overview walks a table with more prefixes than five of the walk's batches.
+// After each batch, the walk lets the lock go for as long as it held the lock.
+// So all 1,000 announcements must be applied before Overview returns. A walk
+// that takes the lock back at once lets in about one update per batch.
+func TestOverviewLetsUpdatesIn(t *testing.T) {
+	t.Parallel()
+
+	const (
+		asn       = 1000
+		peer      = "192.0.2.1"
+		otherPeer = "192.0.2.2"
+		// prefixCount is more prefixes than five of the walk's batches.
+		prefixCount   = 5*walkBatch + 1
+		announcements = 1000
+	)
+
+	routeTime := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+	table := New(Params{})
+	prefixes := make([]netip.Prefix, 0, prefixCount)
+	addr := netip.MustParseAddr("1.0.0.0")
+
+	for range prefixCount {
+		prefix := netip.PrefixFrom(addr, addr.BitLen())
+		table.Announce(prefix, asn, Route{Peer: peer, Time: routeTime})
+		prefixes = append(prefixes, prefix)
+		addr = addr.Next()
+	}
+
+	overviewDone := make(chan struct{})
+
+	go func() {
+		defer close(overviewDone)
+
+		table.Overview(0)
+	}()
+
+	// Wait until Overview holds the read lock. TryLock fails while a reader
+	// holds it.
+	for table.mu.TryLock() {
+		table.mu.Unlock()
+		runtime.Gosched()
+	}
+
+	for _, prefix := range prefixes[:announcements] {
+		table.Announce(prefix, asn, Route{Peer: otherPeer, Time: routeTime})
+	}
+
+	select {
+	case <-overviewDone:
+		t.Fatal("Overview returned before another peer's 1,000 announcements " +
+			"were applied")
+	default:
+	}
+
+	<-overviewDone
+}

internal/routetable/routetable.go

@@ -27,6 +27,11 @@ const (
 	// kept routes in order: each of its nodes holds up to 63 times.
 	keptTimesDegree = 32
 
+	// walkBatch is how many prefixes a walk over the whole table visits in
+	// one hold of the lock. Overview and PeerDown each walk the whole table.
+	// Callers waiting for the lock get it between batches.
+	walkBatch = 10000
+
 	ipv4Bits = 32
 	ipv6Bits = 128
 
@@ -201,8 +206,13 @@ func (t *Table) Withdraw(prefix netip.Prefix, peer string) {
 }
 
 // PeerDown applies a peer's session going down: the peer leaves every peer
-// set, and every route of it is dropped. When the peer carries any prefix, it
-// visits every prefix in the table.
+// set, and every route of the peer is dropped. When the peer carries any
+// prefix, PeerDown visits every prefix in the table, in batches of walkBatch.
+// After each batch, it lets the table's lock go for as long as it held the
+// lock for the batch. During a walk over many batches, reads and updates can
+// then take the lock about half of the time. An announcement by the peer
+// applied during the walk may be dropped too. So the caller applies the peer's
+// later updates only once PeerDown has returned.
 func (t *Table) PeerDown(peer string) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
@@ -212,6 +222,9 @@ func (t *Table) PeerDown(peer string) {
 		return
 	}
 
+	visited := 0
+	batchStart := time.Now()
+
 	for _, f := range []*family{&t.ipv4, &t.ipv6} {
 		for _, prefixes := range f.byLength {
 			for prefix, origins := range prefixes {
@@ -219,6 +232,19 @@ func (t *Table) PeerDown(peer string) {
 				if o != nil {
 					t.leave(prefix, o, number)
 				}
+
+				// Go allows the map to change before the next step of
+				// the range over it.
+				visited++
+				if visited%walkBatch == 0 {
+					held := time.Since(batchStart)
+
+					t.mu.Unlock()
+					time.Sleep(held)
+					t.mu.Lock()
+
+					batchStart = time.Now()
+				}
 			}
 		}
 	}

internal/routetable/routetable_internal_test.go

@@ -0,0 +1,69 @@
+package routetable
+
+import (
+	"net/netip"
+	"runtime"
+	"testing"
+	"time"
+)
+
+// TestPeerDownLetsReadsIn reads the table's counts again and again while a peer
+// that carries more than two batches of prefixes goes down. PeerDown lets the
+// lock go after every walkBatch prefixes, so a read must get in while the peer
+// still carries some of its prefixes but no longer all.
+func TestPeerDownLetsReadsIn(t *testing.T) {
+	t.Parallel()
+
+	const (
+		asn  = 1000
+		peer = "192.0.2.1"
+		// prefixCount is more prefixes than two of walkBatch.
+		prefixCount = 2*walkBatch + 1
+	)
+
+	routeTime := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+	table := New(Params{})
+	addr := netip.MustParseAddr("1.0.0.0")
+
+	for range prefixCount {
+		prefix := netip.PrefixFrom(addr, addr.BitLen())
+		table.Announce(prefix, asn, Route{Peer: peer, Time: routeTime})
+
+		addr = addr.Next()
+	}
+
+	// Hold a read lock until PeerDown waits for the lock, so that the first
+	// read below starts once PeerDown holds it. TryRLock fails once a writer
+	// waits.
+	table.mu.RLock()
+
+	peerDownDone := make(chan struct{})
+
+	go func() {
+		defer close(peerDownDone)
+
+		table.PeerDown(peer)
+	}()
+
+	for table.mu.TryRLock() {
+		table.mu.RUnlock()
+		runtime.Gosched()
+	}
+
+	table.mu.RUnlock()
+
+	for {
+		routes := table.Counts().IPv4Routes
+		if routes > 0 && routes < prefixCount {
+			break
+		}
+
+		select {
+		case <-peerDownDone:
+			t.Fatal("no read got in while PeerDown ran")
+		default:
+		}
+	}
+
+	<-peerDownDone
+}

internal/routewatch/app_integration_internal_test.go

@@ -509,8 +509,6 @@ func announcementFeed(prefixes []string) string {
 func TestShutdownWritesTheUpdatesQueuedInTheStreamer(t *testing.T) {
 	t.Parallel()
 
-	const maxWait = 10 * time.Second
-
 	prefixes := []string{"192.0.2.0/24", "198.51.100.0/24", "203.0.113.0/24"}
 	feed := announcementFeed(prefixes)
 
@@ -556,9 +554,9 @@ func TestShutdownWritesTheUpdatesQueuedInTheStreamer(t *testing.T) {
 	for range cap(linesSeen) {
 		select {
 		case <-linesSeen:
-		case <-time.After(maxWait):
+		case <-time.After(stepTimeout):
 			t.Fatalf("the streamer did not read %d lines within %s",
-				cap(linesSeen), maxWait)
+				cap(linesSeen), stepTimeout)
 		}
 	}
 
@@ -571,8 +569,8 @@ func TestShutdownWritesTheUpdatesQueuedInTheStreamer(t *testing.T) {
 
 	select {
 	case <-shutDown:
-	case <-time.After(maxWait):
-		t.Fatalf("Shutdown did not return within %s", maxWait)
+	case <-time.After(stepTimeout):
+		t.Fatalf("Shutdown did not return within %s", stepTimeout)
 	}
 
 	store.mu.Lock()
@@ -609,21 +607,16 @@ func freePort(t *testing.T) int {
 	return addr.Port
 }
 
-// TestShutdownReturnsByItsDeadlineWhileAWriteIsBlocked starts the HTTP server
-// and has the AS handler start a write that the store holds until the test
-// ends, past Shutdown's deadline. Shutdown must return within its deadline
-// plus a small margin, with the HTTP server stopped, and must neither
-// checkpoint nor close the database: the real one would wait for the write.
+// TestShutdownDoesNotWaitForABlockedWrite starts the HTTP server and has the
+// AS handler start a write that the store holds until the test ends, past
+// Shutdown's deadline. Shutdown must return without waiting for the write, with
+// the HTTP server stopped, and must neither checkpoint nor close the database:
+// the real one would wait for the write.
 // Before the fix Shutdown waited for the write.
-func TestShutdownReturnsByItsDeadlineWhileAWriteIsBlocked(t *testing.T) {
+func TestShutdownDoesNotWaitForABlockedWrite(t *testing.T) {
 	t.Parallel()
 
-	const (
-		deadline = 100 * time.Millisecond
-		// margin is how long Shutdown may take after its deadline, to stop
-		// the HTTP server.
-		margin = 2 * time.Second
-	)
+	const deadline = 100 * time.Millisecond
 
 	blocking := newBlockingStore()
 	defer blocking.releaseWrites()
@@ -651,9 +644,8 @@ func TestShutdownReturnsByItsDeadlineWhileAWriteIsBlocked(t *testing.T) {
 	}
 
 	rw.dbHandler = NewASHandler(store, log)
-	fill := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchSize)}
 	startBlockedWrite(t, blocking, rw.dbHandler,
-		[]*ristypes.RISMessage{{Path: fill}})
+		[]*ristypes.RISMessage{{Path: pathOfASes(asnBatchSize)}})
 
 	ctx, cancel := context.WithTimeout(t.Context(), deadline)
 	defer cancel()
@@ -667,9 +659,9 @@ func TestShutdownReturnsByItsDeadlineWhileAWriteIsBlocked(t *testing.T) {
 
 	select {
 	case <-shutDown:
-	case <-time.After(deadline + margin):
+	case <-time.After(deadline + stepTimeout):
 		t.Fatalf("Shutdown did not return within %s of its deadline while "+
-			"a write was blocked", margin)
+			"a write was blocked", stepTimeout)
 	}
 
 	var dialer net.Dialer
@@ -744,8 +736,6 @@ func (s *closeOrderStore) record(call string) {
 func TestShutdownCheckpointsAndClosesTheDatabaseLast(t *testing.T) {
 	t.Parallel()
 
-	const deadline = 10 * time.Second
-
 	store := &closeOrderStore{Store: newMockStore()}
 	log := logger.New()
 	cfg := &config.Config{}
@@ -763,7 +753,7 @@ func TestShutdownCheckpointsAndClosesTheDatabaseLast(t *testing.T) {
 		Path: ristypes.ASPath{Sequence: []int{testASNA}},
 	})
 
-	ctx, cancel := context.WithTimeout(t.Context(), deadline)
+	ctx, cancel := context.WithTimeout(t.Context(), stepTimeout)
 	defer cancel()
 
 	rw.Shutdown(ctx)

internal/routewatch/ashandler.go

@@ -18,39 +18,34 @@ const (
 	// (asnBatchTimeout), so a queue smaller than asnBatchSize is fine.
 	asHandlerQueueSize = 20000
 
-	// asnBatchSize is the number of ASN operations to batch together
+	// asnBatchSize is the number of ASes in a full batch, which wakes the
+	// flush goroutine to write it.
 	asnBatchSize = 30000
 
-	// asnBatchMaxSize is the most ASN operations the batch holds while the
-	// flush goroutine writes the previous one: ten batches, about 10 MiB
-	// (README "Memory"). At this size HandleMessage waits until the flush
-	// goroutine takes the batch.
-	asnBatchMaxSize = 300000
-
 	// asnBatchTimeout is the maximum time to wait before flushing a batch
 	// DO NOT reduce this timeout - larger batches are more efficient
 	asnBatchTimeout = 2 * time.Second
 )
 
-// ASHandler processes Autonomous System Number (ASN) information extracted from
-// BGP UPDATE messages. It uses batched database operations to efficiently store
-// ASN data, collecting operations into batches that are flushed either when the
-// batch reaches a size threshold or after a timeout period.
+// ASHandler records every AS in the AS paths of BGP UPDATE messages. Its batch
+// holds the earliest and the latest time of each AS's updates. The flush
+// goroutine writes the batch to the database when it holds asnBatchSize ASes,
+// and every asnBatchTimeout.
 type ASHandler struct {
 	db     database.Store
 	logger *logger.Logger
 
-	// Batching. mu guards batch and writing only: the flush goroutine takes
-	// the batch under mu and writes it after releasing mu, so HandleMessage
-	// waits for a database write only while batch is at asnBatchMaxSize.
-	mu    sync.Mutex
-	batch []asnOp
-	// writing is how many operations the flush goroutine took from batch
-	// and has not finished writing.
-	writing int
-	// batchTaken is signalled when the flush goroutine takes batch; its L is
-	// &mu.
-	batchTaken  sync.Cond
+	// Batching. mu guards batch and writing only. The flush goroutine takes
+	// the batch under mu and writes it after releasing mu. So HandleMessage
+	// never waits for a database write.
+	mu sync.Mutex
+	// batch holds the earliest and the latest time of the updates for each AS
+	// since the flush goroutine last took it. It holds one entry per AS,
+	// however many updates arrive.
+	batch map[int]database.SeenTimes
+	// writing is how many ASes the flush goroutine took from batch and has
+	// not finished writing.
+	writing     int
 	batchFullCh chan struct{} // wakes the flush goroutine when batch is full
 	// stopCh hands Stop's context to the flush goroutine. It holds one, so
 	// Stop does not wait for a write in progress to hand it over.
@@ -58,16 +53,10 @@ type ASHandler struct {
 	done   chan struct{} // closed when the flush goroutine returns
 }
 
-type asnOp struct {
-	number    int
-	timestamp time.Time
-}
-
 // NewASHandler creates and returns a new ASHandler instance. It initializes
 // the batching system and starts a background goroutine that periodically
-// flushes accumulated ASN operations to the database. The caller must call
-// Stop when finished to ensure all pending operations are flushed and the
-// background goroutine is terminated.
+// writes the collected ASes to the database. The caller must call Stop when
+// finished, to write the pending ASes and end the background goroutine.
 func NewASHandler(db database.Store, logger *logger.Logger) *ASHandler {
 	return newASHandler(db, logger, asnBatchTimeout)
 }
@@ -81,12 +70,11 @@ func newASHandler(
 	h := &ASHandler{
 		db:          db,
 		logger:      logger,
-		batch:       make([]asnOp, 0, asnBatchSize),
+		batch:       make(map[int]database.SeenTimes),
 		batchFullCh: make(chan struct{}, 1),
 		stopCh:      make(chan context.Context, 1),
 		done:        make(chan struct{}),
 	}
-	h.batchTaken.L = &h.mu
 
 	// Start the flush timer goroutine
 	go flushLoop(batchTimeout, h.batchFullCh, h.stopCh, h.done, h.flushBatch)
@@ -110,48 +98,25 @@ func (h *ASHandler) QueueCapacity() int {
 	return asHandlerQueueSize
 }
 
-// HandleMessage processes a RIS Live BGP message by extracting all ASNs from
-// the AS path and queuing them for batch insertion into the database. The
-// origin ASN and all transit ASNs are recorded with their associated
-// timestamps. When the batch reaches the configured size threshold, the flush
-// goroutine is woken to write it.
+// HandleMessage adds the time of msg to the batch entry of each AS in its AS
+// path, the origin AS and the members of an AS set included. When the batch
+// holds asnBatchSize ASes, HandleMessage wakes the flush goroutine to write it.
 func (h *ASHandler) HandleMessage(msg *ristypes.RISMessage) {
-	// Use the pre-parsed timestamp
-	timestamp := msg.ParsedTimestamp
-
-	originASN := msg.Path.Origin()
-
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
-	// While the batch is at its maximum size, wait until the flush goroutine
-	// takes it. Meanwhile the handler takes nothing off its queue, which
-	// fills, and the streamer drops messages for it.
-	for len(h.batch) >= asnBatchMaxSize {
-		h.batchTaken.Wait()
-	}
-
-	// Queue origin ASN operation
-	if originASN > 0 {
-		h.batch = append(h.batch, asnOp{
-			number:    originASN,
-			timestamp: timestamp,
-		})
-	}
-
-	// Also track all ASNs in the path, the members of an AS set included
+	// The origin AS is the last AS of the sequence, so the loop adds it too.
 	for _, asn := range slices.Concat(msg.Path.Sequence, msg.Path.Set) {
 		if asn > 0 {
-			h.batch = append(h.batch, asnOp{
-				number:    asn,
-				timestamp: timestamp,
-			})
+			seen := h.batch[asn]
+			seen.Add(msg.ParsedTimestamp)
+			h.batch[asn] = seen
 		}
 	}
 
 	// A full batch wakes the flush goroutine. If it is writing, the wake-up
-	// waits in batchFullCh and the batch keeps growing until the write ends
-	// or the batch reaches asnBatchMaxSize.
+	// waits in batchFullCh and the batch keeps taking updates until the write
+	// ends.
 	if len(h.batch) >= asnBatchSize {
 		select {
 		case h.batchFullCh <- struct{}{}:
@@ -160,10 +125,11 @@ func (h *ASHandler) HandleMessage(msg *ristypes.RISMessage) {
 	}
 }
 
-// Stop stops the flush goroutine, which writes the operations the handler holds
-// before it returns, and waits for it until ctx ends. Once ctx has ended the
-// flush goroutine skips that last write, and a write still running is lost
-// when the process exits, so Stop logs how many operations are not written.
+// Stop stops the flush goroutine and waits for it until ctx ends. The flush
+// goroutine writes the batch before it returns, unless ctx has ended. A write
+// still running when the process exits is lost. Stop logs how many entries are
+// not written, in the batch being written and in the next batch. An AS in both
+// batches counts twice.
 func (h *ASHandler) Stop(ctx context.Context) {
 	h.stopCh <- ctx
 
@@ -178,14 +144,14 @@ func (h *ASHandler) Stop(ctx context.Context) {
 
 	if unwritten > 0 {
 		h.logger.Error("Stop deadline passed before the AS handler wrote "+
-			"its updates", "updates", unwritten)
+			"its batches", "batch_entries", unwritten)
 	}
 }
 
-// takeBatch returns the pending operations and leaves an empty batch in their
-// place for HandleMessage to fill. They count in writing until flushBatch has
-// written them.
-func (h *ASHandler) takeBatch() []asnOp {
+// takeBatch returns the pending ASes and leaves an empty batch in their place
+// for HandleMessage to fill. They count in writing until flushBatch has written
+// them.
+func (h *ASHandler) takeBatch() map[int]database.SeenTimes {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
@@ -195,8 +161,7 @@ func (h *ASHandler) takeBatch() []asnOp {
 
 	batch := h.batch
 	h.writing = len(batch)
-	h.batch = make([]asnOp, 0, asnBatchSize)
-	h.batchTaken.Broadcast()
+	h.batch = make(map[int]database.SeenTimes)
 
 	return batch
 }
@@ -209,21 +174,11 @@ func (h *ASHandler) flushBatch() {
 		return
 	}
 
-	// The earliest and the latest time the batch saw each AS. The batch holds
-	// several updates for an AS, from many peers, so a new AS is stored first
-	// seen at the earliest of them and last seen at the latest.
-	asns := make(map[int]database.SeenTimes)
-	for _, op := range batch {
-		seen := asns[op.number]
-		seen.Add(op.timestamp)
-		asns[op.number] = seen
-	}
-
 	// Process all ASNs in a single batch transaction
-	err := h.db.GetOrCreateASNBatch(asns)
+	err := h.db.GetOrCreateASNBatch(batch)
 	if err != nil {
 		h.logger.Error("Failed to process ASN batch",
-			"error", err, "count", len(asns))
+			"error", err, "count", len(batch))
 	}
 
 	h.mu.Lock()

internal/routewatch/ashandler_internal_test.go

@@ -5,28 +5,43 @@ import (
 	"slices"
 	"testing"
 
+	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
+	"git.eeqj.de/sneak/routewatch/internal/ristypes"
 )
 
+// pathOfASes returns an AS path of n different ASes: AS 1 to AS n. With n up to
+// asnBatchSize, none of them is testASNA to testASND.
+func pathOfASes(n int) ristypes.ASPath {
+	sequence := make([]int, n)
+	for i := range sequence {
+		sequence[i] = i + 1
+	}
+
+	return ristypes.ASPath{Sequence: sequence}
+}
+
 // TestASHandlerStoresTheEarliestAndLatestTimesOfANewAS handles two updates
-// whose path carries an AS the database does not hold yet, the second 5
-// seconds after the first, in one batch into a real database. The AS is stored
-// first seen at the time of the first update and last seen at the time of the
-// second.
+// whose path carries an AS the database does not hold yet, in one batch into a
+// real database. The second is 5 seconds after the first, and the handler gets
+// it first. The AS is stored first seen at the time of the first update and
+// last seen at the time of the second.
 func TestASHandlerStoresTheEarliestAndLatestTimesOfANewAS(t *testing.T) {
 	t.Parallel()
 
 	db := newTestDatabase(t)
 
 	// Built without NewASHandler's flush goroutine, so the test flushes itself.
-	h := &ASHandler{db: db, logger: logger.New()}
+	h := &ASHandler{
+		db: db, logger: logger.New(), batch: make(map[int]database.SeenTimes),
+	}
 
 	first := peerAnnouncement(testPeerIP, testV4Prefix)
 	second := peerAnnouncement(testPeerIP, testV4Prefix)
 	second.ParsedTimestamp = first.ParsedTimestamp.Add(secondUpdateAfter)
 
-	h.HandleMessage(first)
 	h.HandleMessage(second)
+	h.HandleMessage(first)
 	h.flushBatch()
 
 	asn, _, err := db.GetAS(t.Context(), testASNA)
@@ -54,7 +69,9 @@ func TestASHandlerRecordsNothingFromAPathWithASZero(t *testing.T) {
 	store := newMockStore()
 
 	// Built without NewASHandler's flush goroutine, so the test flushes itself.
-	h := &ASHandler{db: store, logger: logger.New()}
+	h := &ASHandler{
+		db: store, logger: logger.New(), batch: make(map[int]database.SeenTimes),
+	}
 
 	msg := peerAnnouncement(testPeerIP, testV4Prefix)
 	msg.Path = decodePath(t, testASNA, 0, testASNB)

internal/routewatch/asnfetcher.go

@@ -187,25 +187,33 @@ func (f *ASNFetcher) recordSuccess() {
 	f.statsMu.Unlock()
 }
 
-// recordFailure increases the interval on failed fetch using exponential backoff.
-func (f *ASNFetcher) recordFailure() {
+// lengthenInterval makes the interval 1.5 times as long, up to maxInterval.
+// A failed lookup and a skipped one both lengthen it.
+func (f *ASNFetcher) lengthenInterval() {
 	f.intervalMu.Lock()
-	f.consecutiveFails++
+	defer f.intervalMu.Unlock()
 
-	// Exponential backoff: multiply by 1.5, capped at max
 	newInterval := min(
 		f.currentInterval*backoffNumerator/backoffDenominator, maxInterval)
 
 	if newInterval != f.currentInterval {
-		f.logger.Debug("Increased fetch interval due to failure",
+		f.logger.Debug("Increased fetch interval",
 			"old_interval", f.currentInterval,
 			"new_interval", newInterval,
 			"consecutive_failures", f.consecutiveFails,
 		)
 		f.currentInterval = newInterval
 	}
+}
+
+// recordFailure counts a failed fetch and lengthens the interval.
+func (f *ASNFetcher) recordFailure() {
+	f.intervalMu.Lock()
+	f.consecutiveFails++
 	f.intervalMu.Unlock()
 
+	f.lengthenInterval()
+
 	// Record error time for stats, dropping those older than statsWindow
 	f.statsMu.Lock()
 	now := f.now()
@@ -287,6 +295,16 @@ func (f *ASNFetcher) fetchNextStale(ctx context.Context) {
 // so the AS stays stale. An AS IANA refers to no RIR is stored with no
 // WHOIS data, so it is not looked up again until it is stale; that counts
 // as neither a success nor a failure.
+//
+// The WHOIS client pauses an RIR's or NIC.br's server for whois.PauseTime
+// after the server refuses a query. Lookups at the other servers go on. IANA
+// is never paused, so a refusal by IANA is an ordinary failure. A lookup
+// whose RIR is paused fails with whois.ErrPaused and is skipped. A skipped
+// lookup leaves the AS stale and is not counted as a failure. Like a
+// failure, a skipped lookup lengthens the interval. Each lookup asks IANA
+// before it finds its RIR paused, so with a one-second interval the fetcher
+// would ask IANA every second. The lookups that QueueImmediate asks for also
+// run through fetchAndUpdate.
 func (f *ASNFetcher) fetchAndUpdate(ctx context.Context, asn int) bool {
 	ctx, cancel := context.WithTimeout(ctx, time.Minute)
 	defer cancel()
@@ -307,6 +325,14 @@ func (f *ASNFetcher) fetchAndUpdate(ctx context.Context, asn int) bool {
 		return false
 	}
 
+	if errors.Is(err, whois.ErrPaused) {
+		f.logger.Info("Skipped WHOIS lookup, the RIR's server is paused",
+			"asn", asn, "error", err)
+		f.lengthenInterval()
+
+		return false
+	}
+
 	if err != nil {
 		f.logger.Error("WHOIS lookup failed", "asn", asn, "error", err)
 		f.recordFailure()

internal/routewatch/asnfetcher_internal_test.go

@@ -13,6 +13,7 @@ import (
 
 	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
+	"git.eeqj.de/sneak/routewatch/internal/whois"
 )
 
 // The WHOIS servers a lookup asks: IANA first, then the RIR IANA refers it
@@ -131,6 +132,151 @@ func TestFetchAndUpdateKeepsTheRowWhenTheLookupFails(t *testing.T) {
 	}
 }
 
+// TestFetchAndUpdateWaitsOutAPause looks up an AS at RIPE while RIPE refuses
+// the query. The test sets the WHOIS client's clock. The next lookup comes a
+// second before whois.PauseTime has passed. It sends RIPE nothing and leaves
+// the AS never looked up. It counts as neither a success nor a failure. A
+// lookup at whois.PauseTime asks RIPE again.
+func TestFetchAndUpdateWaitsOutAPause(t *testing.T) {
+	t.Parallel()
+
+	f, db := newFetcherWithWHOISAnswers(t, map[string]string{
+		ianaWHOISServer: ianaRefersToRIPE,
+		ripeWHOISServer: ripeAccessDenied,
+	})
+
+	ripeAsks := 0
+	dial := f.whoisClient.DialContext
+	f.whoisClient.DialContext = func(
+		ctx context.Context, network, address string,
+	) (net.Conn, error) {
+		if address == ripeWHOISServer {
+			ripeAsks++
+		}
+
+		return dial(ctx, network, address)
+	}
+
+	start := time.Date(2026, 11, 2, 0, 0, 0, 0, time.UTC)
+	now := start
+	f.whoisClient.Now = func() time.Time { return now }
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	now = start.Add(whois.PauseTime - time.Second)
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	if ripeAsks != 1 {
+		t.Errorf("RIPE was asked %d times during its pause, want 1", ripeAsks)
+	}
+
+	stats, err := db.GetWHOISStats(t.Context(), whoisStaleThreshold)
+	if err != nil {
+		t.Fatalf("GetWHOISStats: %v", err)
+	}
+
+	if stats.NeverFetched != 1 {
+		t.Errorf("never looked up = %d, want 1", stats.NeverFetched)
+	}
+
+	got := f.GetStats()
+	if got.ConsecutiveFails != 1 || got.ErrorsLastHour != 1 ||
+		got.SuccessesLastHour != 0 {
+		t.Errorf("%d failures in a row, %d errors, %d successes; want 1, 1, 0",
+			got.ConsecutiveFails, got.ErrorsLastHour, got.SuccessesLastHour)
+	}
+
+	now = start.Add(whois.PauseTime)
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	if ripeAsks != 2 {
+		t.Errorf("RIPE was asked %d times after its pause, want 2", ripeAsks)
+	}
+}
+
+// TestFetchAndUpdateBacksOffDuringAPause looks up an AS at RIPE while RIPE
+// refuses the query. The refusal pauses RIPE for an hour, so every later
+// lookup in the test is skipped. The test sets the WHOIS client's clock and
+// looks the AS up again each time the interval has passed. Like a failure,
+// each skip must make the interval 1.5 times as long, up to one minute. The
+// skips must not count as failures.
+func TestFetchAndUpdateBacksOffDuringAPause(t *testing.T) {
+	t.Parallel()
+
+	f, _ := newFetcherWithWHOISAnswers(t, map[string]string{
+		ianaWHOISServer: ianaRefersToRIPE,
+		ripeWHOISServer: ripeAccessDenied,
+	})
+
+	now := time.Date(2026, 11, 2, 0, 0, 0, 0, time.UTC)
+	f.whoisClient.Now = func() time.Time { return now }
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	for i, want := range []time.Duration{
+		33750 * time.Millisecond,
+		50625 * time.Millisecond,
+		time.Minute,
+		time.Minute,
+	} {
+		now = now.Add(f.getInterval())
+
+		f.fetchAndUpdate(t.Context(), testASNA)
+
+		if got := f.getInterval(); got != want {
+			t.Errorf("interval after %d skipped lookups = %v, want %v",
+				i+1, got, want)
+		}
+	}
+
+	got := f.GetStats()
+	if got.ConsecutiveFails != 1 || got.ErrorsLastHour != 1 {
+		t.Errorf("%d failures in a row and %d errors, want 1 and 1",
+			got.ConsecutiveFails, got.ErrorsLastHour)
+	}
+}
+
+// TestFetchAndUpdateKeepsAskingIANA looks up an AS while IANA's server
+// refuses the connection. The test sets the WHOIS client's clock. The next
+// lookup comes a second before whois.PauseTime has passed. IANA is never
+// paused, so that lookup asks IANA again. Each refusal counts as a failure.
+func TestFetchAndUpdateKeepsAskingIANA(t *testing.T) {
+	t.Parallel()
+
+	// With no answers, every server refuses the connection.
+	f, _ := newFetcherWithWHOISAnswers(t, map[string]string{})
+
+	ianaAsks := 0
+	dial := f.whoisClient.DialContext
+	f.whoisClient.DialContext = func(
+		ctx context.Context, network, address string,
+	) (net.Conn, error) {
+		if address == ianaWHOISServer {
+			ianaAsks++
+		}
+
+		return dial(ctx, network, address)
+	}
+
+	start := time.Date(2026, 11, 2, 0, 0, 0, 0, time.UTC)
+	now := start
+	f.whoisClient.Now = func() time.Time { return now }
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	now = start.Add(whois.PauseTime - time.Second)
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	got := f.GetStats()
+	if ianaAsks != 2 || got.ConsecutiveFails != 2 {
+		t.Errorf("IANA was asked %d times, with %d failures in a row; "+
+			"want 2 and 2", ianaAsks, got.ConsecutiveFails)
+	}
+}
+
 // TestRecordStepsTheInterval has the fetcher fail from its base interval of
 // 15 seconds, then succeed. Each failure makes the interval 1.5 times as
 // long, up to one minute, and each success halves it, down to one second.
@@ -224,7 +370,7 @@ func TestASNFetcherStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-lookupStarted:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the fetcher did not start the lookup")
 	}
 
@@ -240,22 +386,22 @@ func TestASNFetcherStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-f.done:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the fetcher goroutine did not return after the lookup ended")
 	}
 }
 
 // TestRecordKeepsOnlyTheStatsWindow records a success and a failure every
-// second, by a clock the test sets, far longer than statsWindow, and never
+// second, by a clock the test sets, for three times statsWindow, and never
 // asks for the stats. The fetcher must keep only the times within the last
 // statsWindow.
 func TestRecordKeepsOnlyTheStatsWindow(t *testing.T) {
 	t.Parallel()
 
 	const (
-		records = 100_000
 		step    = time.Second
 		want    = int(statsWindow / step)
+		records = 3 * want
 	)
 
 	clock := time.Now()

internal/routewatch/batchwrite_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"log/slog"
 	"slices"
 	"sync"
@@ -19,9 +20,15 @@ import (
 )
 
 const (
-	// handleMessageTimeout is how long a test waits for HandleMessage to
-	// return, or for a write to start, before it fails.
-	handleMessageTimeout = 2 * time.Second
+	// stepTimeout is how long a test waits for a step it expects, such as
+	// HandleMessage returning, a write starting or Shutdown returning, before
+	// it fails. Only a step that never comes should reach it. On a loaded
+	// host, under the race detector, HandleMessage takes seconds to apply a
+	// full prefix batch. The helpers below bound every wait with it, Stop
+	// included. A test that hangs in one of them waits out at most two of
+	// these bounds, its step's and Stop's, and then fails with its own
+	// message. That leaves time before go test's 90-second timeout.
+	stepTimeout = 15 * time.Second
 
 	// stillWaitingAfter is how long a test watches a HandleMessage that must
 	// wait. One that does not wait returns well within it.
@@ -171,12 +178,14 @@ type stopLog struct {
 	Msg          string `json:"msg"`
 	Updates      int    `json:"updates"`
 	TableChanges int    `json:"table_changes"`
+	HeldMessages int    `json:"held_messages"`
 	Paths        int    `json:"paths"`
+	BatchEntries int    `json:"batch_entries"`
 }
 
 // newErrorLogger returns a logger that writes only errors, as JSON lines, to
 // logs.
-func newErrorLogger(logs *bytes.Buffer) *logger.Logger {
+func newErrorLogger(logs io.Writer) *logger.Logger {
 	return &logger.Logger{Logger: slog.New(slog.NewJSONHandler(logs,
 		&slog.HandlerOptions{Level: slog.LevelError}))}
 }
@@ -198,7 +207,7 @@ func lastLogged(t *testing.T, logs *bytes.Buffer) stopLog {
 	return logged
 }
 
-// returnsInTime reports whether f returns within handleMessageTimeout.
+// returnsInTime reports whether f returns within stepTimeout.
 func returnsInTime(f func()) bool {
 	done := make(chan struct{})
 
@@ -210,11 +219,22 @@ func returnsInTime(f func()) bool {
 	select {
 	case <-done:
 		return true
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		return false
 	}
 }
 
+// stopInTime stops h, with no deadline, and fails the test if Stop does not
+// return within stepTimeout. It only marks the test failed, so it can be
+// deferred.
+func stopInTime(t *testing.T, h interface{ Stop(ctx context.Context) }) {
+	t.Helper()
+
+	if !returnsInTime(func() { h.Stop(t.Context()) }) {
+		t.Errorf("Stop did not return within %s", stepTimeout)
+	}
+}
+
 // startBlockedWrite hands h the messages in fill and waits until h starts to
 // write them to store. The prefix, AS and peer handlers start when fill fills
 // their batch, and the peering handler on its timer. store holds the write
@@ -240,7 +260,7 @@ func startBlockedWrite(
 
 	select {
 	case <-store.writeStarted:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the handler did not start to write the messages")
 	}
 }
@@ -261,7 +281,7 @@ func handleWhileAWriteIsBlocked(
 ) {
 	t.Helper()
 
-	defer h.Stop(t.Context())
+	defer stopInTime(t, h)
 	defer store.releaseWrites()
 
 	startBlockedWrite(t, store, h, fill)
@@ -275,7 +295,9 @@ func handleWhileAWriteIsBlocked(
 // while store holds the write, hands h the messages in toMax, which bring its
 // next batch to its maximum size, and the message extra. HandleMessage for
 // extra must wait until the test lets the write finish, and return after. Then
-// it stops h, which writes what it holds.
+// it stops h, which writes what it holds. The tests lower h's maximum batch
+// size to one batch, so toMax is one batch: the real maximum of ten batches is
+// slow to reach under the race detector.
 func handleAtMaxBatchSize(
 	t *testing.T,
 	store *blockingStore,
@@ -288,13 +310,18 @@ func handleAtMaxBatchSize(
 ) {
 	t.Helper()
 
-	defer h.Stop(t.Context())
+	defer stopInTime(t, h)
 	defer store.releaseWrites()
 
 	startBlockedWrite(t, store, h, fill)
 
-	for _, msg := range toMax {
-		h.HandleMessage(msg)
+	if !returnsInTime(func() {
+		for _, msg := range toMax {
+			h.HandleMessage(msg)
+		}
+	}) {
+		t.Fatal("HandleMessage did not return while the batch grew to its " +
+			"maximum size")
 	}
 
 	returned := make(chan struct{})
@@ -315,7 +342,7 @@ func handleAtMaxBatchSize(
 
 	select {
 	case <-returned:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("HandleMessage did not return after the database write finished")
 	}
 }
@@ -345,8 +372,12 @@ func stopWhileAWriteIsBlocked(
 
 	startBlockedWrite(t, store, h, fill)
 
-	for _, msg := range extra {
-		h.HandleMessage(msg)
+	if !returnsInTime(func() {
+		for _, msg := range extra {
+			h.HandleMessage(msg)
+		}
+	}) {
+		t.Fatal("HandleMessage did not return while a database write was blocked")
 	}
 
 	ctx, cancel := context.WithTimeout(t.Context(), stopDeadline)
@@ -361,7 +392,7 @@ func stopWhileAWriteIsBlocked(
 
 	select {
 	case <-done:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the flush goroutine did not return after the write finished")
 	}
 }
@@ -380,7 +411,8 @@ func TestPrefixHandlerSkipsItsLastWriteAfterTheStopDeadline(t *testing.T) {
 
 	store := newBlockingStore()
 	h := newPrefixHandler(store, newErrorLogger(&logs),
-		routetable.New(routetable.Params{}), longerThanAnyTest)
+		routetable.New(routetable.Params{}), longerThanAnyTest,
+		maxTableChangesPerWrite)
 	fill := slices.Repeat([]string{testV4Prefix}, prefixBatchSize)
 	stopWhileAWriteIsBlocked(t, store, h, h.done,
 		[]*ristypes.RISMessage{prefixAnnouncement(fill)},
@@ -466,20 +498,20 @@ func TestPrefixHandlerStopCountsTheTableChangesOfAWriteInProgress(t *testing.T)
 	}
 }
 
+// TestASHandlerSkipsItsLastWriteAfterTheStopDeadline also checks what Stop
+// logs: the asnBatchSize entries of the held write and the entry for testASNB,
+// handled since, are not written.
 func TestASHandlerSkipsItsLastWriteAfterTheStopDeadline(t *testing.T) {
 	t.Parallel()
 
-	// Each message gives an update for its origin AS and one for each AS
-	// in its path: fill asnBatchSize+1, extra 2.
-	const wantUpdates = asnBatchSize + 1 + 2
+	const wantEntries = asnBatchSize + 1
 
 	var logs bytes.Buffer
 
 	store := newBlockingStore()
 	h := newASHandler(store, newErrorLogger(&logs), longerThanAnyTest)
-	fill := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchSize)}
 	stopWhileAWriteIsBlocked(t, store, h, h.done,
-		[]*ristypes.RISMessage{{Path: fill}},
+		[]*ristypes.RISMessage{{Path: pathOfASes(asnBatchSize)}},
 		&ristypes.RISMessage{Path: ristypes.ASPath{Sequence: []int{testASNB}}})
 
 	store.mu.Lock()
@@ -489,26 +521,27 @@ func TestASHandlerSkipsItsLastWriteAfterTheStopDeadline(t *testing.T) {
 		t.Errorf("AS%d was written after the stop deadline", testASNB)
 	}
 
-	if logged := lastLogged(t, &logs); logged.Updates != wantUpdates {
-		t.Errorf("Stop logged %d updates not written, want %d",
-			logged.Updates, wantUpdates)
+	if logged := lastLogged(t, &logs); logged.BatchEntries != wantEntries {
+		t.Errorf("Stop logged %d batch entries not written, want %d",
+			logged.BatchEntries, wantEntries)
 	}
 }
 
 // TestASHandlerSkipsAFullBatchAfterTheStopDeadline hands the AS handler a full
 // batch while a write is held, so the batch's wake-up of the flush goroutine
-// waits. The stop deadline passes meanwhile. When the write ends, the flush
-// goroutine must stop instead of writing the batch.
+// waits. The full batch holds the fill's ASes again, and testASNB. The stop
+// deadline passes meanwhile. When the write ends, the flush goroutine must stop
+// instead of writing the batch.
 func TestASHandlerSkipsAFullBatchAfterTheStopDeadline(t *testing.T) {
 	t.Parallel()
 
 	store := newBlockingStore()
 	h := newASHandler(store, logger.New(), longerThanAnyTest)
-	fill := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchSize)}
-	full := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNB}, asnBatchSize)}
+	fill := &ristypes.RISMessage{Path: pathOfASes(asnBatchSize)}
 	stopWhileAWriteIsBlocked(t, store, h, h.done,
-		[]*ristypes.RISMessage{{Path: fill}},
-		&ristypes.RISMessage{Path: full})
+		[]*ristypes.RISMessage{fill},
+		fill,
+		&ristypes.RISMessage{Path: ristypes.ASPath{Sequence: []int{testASNB}}})
 
 	store.mu.Lock()
 	defer store.mu.Unlock()
@@ -597,7 +630,7 @@ func TestPeeringHandlerStopCountsTheWriteInProgress(t *testing.T) {
 
 	select {
 	case <-h.done:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the processing goroutine did not return after the write " +
 			"finished")
 	}
@@ -618,7 +651,7 @@ func TestPeeringHandlerStartsNoWriteAfterTheStopDeadline(t *testing.T) {
 	const processInterval = 10 * time.Millisecond
 
 	store := newBlockingStore()
-	h := newPeeringHandler(store, logger.New(), processInterval)
+	h := newPeeringHandler(store, logger.New(), processInterval, maxTrackedPaths)
 	stopWhileAWriteIsBlocked(t, store, h, h.done,
 		[]*ristypes.RISMessage{
 			{Path: ristypes.ASPath{Sequence: []int{testASNA, testASNB}}},
@@ -674,9 +707,8 @@ func TestASHandlerHandlesMessagesWhileAWriteIsBlocked(t *testing.T) {
 
 	store := newBlockingStore()
 	h := newASHandler(store, logger.New(), longerThanAnyTest)
-	fill := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchSize)}
 	handleWhileAWriteIsBlocked(t, store, h,
-		[]*ristypes.RISMessage{{Path: fill}},
+		[]*ristypes.RISMessage{{Path: pathOfASes(asnBatchSize)}},
 		&ristypes.RISMessage{Path: ristypes.ASPath{Sequence: []int{testASNB}}})
 
 	store.mu.Lock()
@@ -714,9 +746,11 @@ func TestPrefixHandlerWaitsAtTheMaximumBatchSize(t *testing.T) {
 	const extraPrefix = "198.51.100.0/24"
 
 	store := newBlockingStore()
-	h := NewPrefixHandler(store, logger.New(), routetable.New(routetable.Params{}))
+	h := newPrefixHandler(store, logger.New(), routetable.New(routetable.Params{}),
+		longerThanAnyTest, maxTableChangesPerWrite)
+	h.batchMaxSize = prefixBatchSize
 	fill := slices.Repeat([]string{testV4Prefix}, prefixBatchSize)
-	toMax := slices.Repeat([]string{testV4Prefix}, prefixBatchMaxSize)
+	toMax := slices.Repeat([]string{testV4Prefix}, prefixBatchSize)
 	handleAtMaxBatchSize(t, store, h,
 		[]*ristypes.RISMessage{prefixAnnouncement(fill)},
 		[]*ristypes.RISMessage{prefixAnnouncement(toMax)},
@@ -731,31 +765,48 @@ func TestPrefixHandlerWaitsAtTheMaximumBatchSize(t *testing.T) {
 	}
 }
 
-func TestASHandlerWaitsAtTheMaximumBatchSize(t *testing.T) {
+// TestASHandlerNeverWaitsForAWrite starts a write that the store holds. Then it
+// hands the AS handler twenty messages whose paths each hold one AS
+// asnBatchSize times. HandleMessage must return each time, and the batch must
+// hold one entry, for that AS.
+func TestASHandlerNeverWaitsForAWrite(t *testing.T) {
 	t.Parallel()
 
+	const messages = 20
+
 	store := newBlockingStore()
 	h := newASHandler(store, logger.New(), longerThanAnyTest)
-	fill := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchSize)}
-	toMax := ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, asnBatchMaxSize)}
-	handleAtMaxBatchSize(t, store, h,
-		[]*ristypes.RISMessage{{Path: fill}},
-		[]*ristypes.RISMessage{{Path: toMax}},
-		&ristypes.RISMessage{Path: ristypes.ASPath{Sequence: []int{testASNB}}})
 
-	store.mu.Lock()
-	defer store.mu.Unlock()
+	defer stopInTime(t, h)
+	defer store.releaseWrites()
 
-	if !store.asns[testASNB] {
-		t.Errorf("Stop did not write AS%d, handled after the batch reached its "+
-			"maximum size", testASNB)
+	startBlockedWrite(t, store, h,
+		[]*ristypes.RISMessage{{Path: pathOfASes(asnBatchSize)}})
+
+	oneAS := &ristypes.RISMessage{
+		Path: ristypes.ASPath{Sequence: slices.Repeat([]int{testASNB}, asnBatchSize)},
+	}
+
+	if !returnsInTime(func() {
+		for range messages {
+			h.HandleMessage(oneAS)
+		}
+	}) {
+		t.Fatal("HandleMessage did not return while a database write was blocked")
+	}
+
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	if len(h.batch) != 1 {
+		t.Errorf("the batch holds %d entries for one AS, want 1", len(h.batch))
 	}
 }
 
 // TestPeerHandlerWaitsAtTheMaximumBatchSize must not have its flush timer fire.
 // A timer flush during the fill would take only part of it. The batch would
 // then reach its maximum size before the test's last message, and the test
-// would hang.
+// would fail.
 func TestPeerHandlerWaitsAtTheMaximumBatchSize(t *testing.T) {
 	t.Parallel()
 
@@ -763,10 +814,11 @@ func TestPeerHandlerWaitsAtTheMaximumBatchSize(t *testing.T) {
 
 	store := newBlockingStore()
 	h := newPeerHandler(store, logger.New(), longerThanAnyTest)
+	h.batchMaxSize = peerBatchSize
 	message := []*ristypes.RISMessage{{Peer: testPeerIP}}
 	handleAtMaxBatchSize(t, store, h,
 		slices.Repeat(message, peerBatchSize),
-		slices.Repeat(message, peerBatchMaxSize),
+		slices.Repeat(message, peerBatchSize),
 		&ristypes.RISMessage{Peer: otherPeerIP})
 
 	store.mu.Lock()
@@ -796,8 +848,9 @@ func TestHandlersWakeTheFlushGoroutineWhenTheBatchIsFull(t *testing.T) {
 			batchSize: prefixBatchSize,
 			handle: func(n int) chan struct{} {
 				h := &PrefixHandler{
-					batchFullCh: make(chan struct{}, 1),
-					table:       routetable.New(routetable.Params{}),
+					batchMaxSize: prefixBatchMaxSize,
+					batchFullCh:  make(chan struct{}, 1),
+					table:        routetable.New(routetable.Params{}),
 				}
 				h.HandleMessage(prefixAnnouncement(
 					slices.Repeat([]string{testV4Prefix}, n)))
@@ -809,11 +862,12 @@ func TestHandlersWakeTheFlushGoroutineWhenTheBatchIsFull(t *testing.T) {
 			name:      "AS",
 			batchSize: asnBatchSize,
 			handle: func(n int) chan struct{} {
-				h := &ASHandler{batchFullCh: make(chan struct{}, 1)}
-				// The origin AS and each AS in the path are one update each.
-				h.HandleMessage(&ristypes.RISMessage{
-					Path: ristypes.ASPath{Sequence: slices.Repeat([]int{testASNA}, n-1)},
-				})
+				h := &ASHandler{
+					batch:       make(map[int]database.SeenTimes),
+					batchFullCh: make(chan struct{}, 1),
+				}
+				// Each of the path's n ASes is one entry of the batch.
+				h.HandleMessage(&ristypes.RISMessage{Path: pathOfASes(n)})
 
 				return h.batchFullCh
 			},
@@ -822,7 +876,10 @@ func TestHandlersWakeTheFlushGoroutineWhenTheBatchIsFull(t *testing.T) {
 			name:      "peer",
 			batchSize: peerBatchSize,
 			handle: func(n int) chan struct{} {
-				h := &PeerHandler{batchFullCh: make(chan struct{}, 1)}
+				h := &PeerHandler{
+					batchMaxSize: peerBatchMaxSize,
+					batchFullCh:  make(chan struct{}, 1),
+				}
 				for range n {
 					h.HandleMessage(&ristypes.RISMessage{Peer: testPeerIP})
 				}

internal/routewatch/dbmaintainer_internal_test.go

@@ -97,7 +97,7 @@ func TestDBMaintainerStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-store.started:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the maintainer did not start the checkpoint")
 	}
 
@@ -113,7 +113,7 @@ func TestDBMaintainerStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-m.done:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the maintenance goroutine did not return after the " +
 			"checkpoint ended")
 	}

internal/routewatch/peerhandler.go

@@ -43,10 +43,14 @@ type PeerHandler struct {
 	db     database.Store
 	logger *logger.Logger
 
+	// batchMaxSize is peerBatchMaxSize. Only HandleMessage reads it, so a test
+	// may lower it before it hands the handler a message.
+	batchMaxSize int
+
 	// Batching. mu guards peerBatch and writing only: the flush goroutine
 	// takes the batch under mu and writes it after releasing mu, so
 	// HandleMessage waits for a database write only while peerBatch is at
-	// peerBatchMaxSize.
+	// batchMaxSize.
 	mu        sync.Mutex
 	peerBatch []peerUpdate
 	// writing is how many updates the flush goroutine took from peerBatch
@@ -84,12 +88,13 @@ func newPeerHandler(
 	db database.Store, logger *logger.Logger, batchTimeout time.Duration,
 ) *PeerHandler {
 	h := &PeerHandler{
-		db:          db,
-		logger:      logger,
-		peerBatch:   make([]peerUpdate, 0, peerBatchSize),
-		batchFullCh: make(chan struct{}, 1),
-		stopCh:      make(chan context.Context, 1),
-		done:        make(chan struct{}),
+		db:           db,
+		logger:       logger,
+		batchMaxSize: peerBatchMaxSize,
+		peerBatch:    make([]peerUpdate, 0, peerBatchSize),
+		batchFullCh:  make(chan struct{}, 1),
+		stopCh:       make(chan context.Context, 1),
+		done:         make(chan struct{}),
 	}
 	h.batchTaken.L = &h.mu
 
@@ -135,7 +140,7 @@ func (h *PeerHandler) HandleMessage(msg *ristypes.RISMessage) {
 	// While the batch is at its maximum size, wait until the flush goroutine
 	// takes it. Meanwhile the handler takes nothing off its queue, which
 	// fills, and the streamer drops messages for it.
-	for len(h.peerBatch) >= peerBatchMaxSize {
+	for len(h.peerBatch) >= h.batchMaxSize {
 		h.batchTaken.Wait()
 	}
 
@@ -149,7 +154,7 @@ func (h *PeerHandler) HandleMessage(msg *ristypes.RISMessage) {
 
 	// A full batch wakes the flush goroutine. If it is writing, the wake-up
 	// waits in batchFullCh and the batch keeps growing until the write ends
-	// or the batch reaches peerBatchMaxSize.
+	// or the batch reaches batchMaxSize.
 	if len(h.peerBatch) >= peerBatchSize {
 		select {
 		case h.batchFullCh <- struct{}{}:

internal/routewatch/peerhandler_internal_test.go

@@ -35,7 +35,7 @@ func TestPeerHandlerWritesTheEarliestAndLatestTimesOfANewPeer(t *testing.T) {
 
 	// Built without NewPeerHandler's flush goroutine, so the test flushes
 	// itself.
-	h := &PeerHandler{db: store, logger: logger.New()}
+	h := &PeerHandler{db: store, logger: logger.New(), batchMaxSize: peerBatchMaxSize}
 
 	first := peerAnnouncement(testPeerIP, testV4Prefix)
 	second := peerAnnouncement(testPeerIP, testV4Prefix)

internal/routewatch/peeringhandler.go

@@ -42,6 +42,10 @@ type PeeringHandler struct {
 	db     database.Store
 	logger *logger.Logger
 
+	// maxPaths is the most paths asPaths holds: maxTrackedPaths, except in
+	// tests that give a smaller one.
+	maxPaths int
+
 	// In-memory AS path tracking: each path, JSON-encoded, with the earliest
 	// and the latest time an update carried it.
 	mu           sync.Mutex
@@ -62,21 +66,24 @@ type PeeringHandler struct {
 // periodically processes accumulated AS paths into peering records. The
 // handler begins processing immediately upon creation.
 func NewPeeringHandler(db database.Store, logger *logger.Logger) *PeeringHandler {
-	return newPeeringHandler(db, logger, peeringProcessInterval)
+	return newPeeringHandler(db, logger, peeringProcessInterval, maxTrackedPaths)
 }
 
 // newPeeringHandler is NewPeeringHandler with the processing timer's interval,
-// processInterval, in place of peeringProcessInterval. A test gives a short
-// one to have the timer start a write.
+// processInterval, in place of peeringProcessInterval, and the most paths it
+// holds, maxPaths, in place of maxTrackedPaths. A test gives a short interval to
+// have the timer start a write, and a small maxPaths to fill the map quickly.
 func newPeeringHandler(
 	db database.Store, logger *logger.Logger, processInterval time.Duration,
+	maxPaths int,
 ) *PeeringHandler {
 	h := &PeeringHandler{
-		db:      db,
-		logger:  logger,
-		asPaths: make(map[string]database.SeenTimes),
-		stopCh:  make(chan context.Context, 1),
-		done:    make(chan struct{}),
+		db:       db,
+		logger:   logger,
+		asPaths:  make(map[string]database.SeenTimes),
+		maxPaths: maxPaths,
+		stopCh:   make(chan context.Context, 1),
+		done:     make(chan struct{}),
 	}
 
 	// Start the periodic processing goroutine
@@ -131,7 +138,7 @@ func (h *PeeringHandler) HandleMessage(msg *ristypes.RISMessage) {
 	defer h.mu.Unlock()
 
 	seen, exists := h.asPaths[key]
-	if !exists && len(h.asPaths) >= maxTrackedPaths {
+	if !exists && len(h.asPaths) >= h.maxPaths {
 		// Map is full; drop this new path and count it.
 		h.droppedPaths++
 

internal/routewatch/peeringhandler_internal_test.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"maps"
 	"slices"
-	"strconv"
 	"sync"
 	"testing"
 	"time"
@@ -20,6 +19,10 @@ const (
 	testASNB = 64501
 	testASNC = 64502
 	testASND = 64503
+
+	// testMaxTrackedPaths is a limit of the path map that a test fills
+	// quickly, in place of maxTrackedPaths.
+	testMaxTrackedPaths = 10
 )
 
 // recordingStore wraps mockStore to count every RecordPeering call, so a
@@ -52,9 +55,10 @@ func (r *recordingStore) callCount() int {
 // processing goroutine, so tests drive processing explicitly.
 func newTestHandler(db database.Store) *PeeringHandler {
 	return &PeeringHandler{
-		db:      db,
-		logger:  logger.New(),
-		asPaths: make(map[string]database.SeenTimes),
+		db:       db,
+		logger:   logger.New(),
+		asPaths:  make(map[string]database.SeenTimes),
+		maxPaths: maxTrackedPaths,
 	}
 }
 
@@ -244,25 +248,28 @@ func TestPeeringHandlerStopWritesTheRemainingPaths(t *testing.T) {
 
 // TestPeeringHandlerCapDropsAndCounts verifies that a full map drops new paths
 // and counts them, while a path already tracked is refreshed rather than
-// dropped.
+// dropped. The handler's limit is small, and its timer does not fire.
 func TestPeeringHandlerCapDropsAndCounts(t *testing.T) {
 	t.Parallel()
 
 	store := &recordingStore{mockStore: newMockStore()}
-	h := newTestHandler(store)
+	h := newPeeringHandler(store, logger.New(), longerThanAnyTest,
+		testMaxTrackedPaths)
+
+	defer stopInTime(t, h)
 
 	now := time.Now().UTC()
 
-	// Fill the map to exactly maxTrackedPaths, including one real path key so
-	// the "already tracked" branch can be exercised. The filler keys are never
-	// processed in this test, so their contents do not matter.
+	// Fill the map to exactly its limit, including one real path key so the
+	// "already tracked" branch can be exercised. The other paths are of one
+	// AS, so they carry no peering.
 	existing := pathKey(t, testASNA, testASNB)
 
 	h.mu.Lock()
 	h.asPaths[existing] = database.SeenTimes{First: now, Last: now}
 
-	for i := 0; len(h.asPaths) < maxTrackedPaths; i++ {
-		h.asPaths[strconv.Itoa(i)] = database.SeenTimes{First: now, Last: now}
+	for asn := 1; len(h.asPaths) < testMaxTrackedPaths; asn++ {
+		h.asPaths[pathKey(t, asn)] = database.SeenTimes{First: now, Last: now}
 	}
 	h.mu.Unlock()
 
@@ -270,8 +277,9 @@ func TestPeeringHandlerCapDropsAndCounts(t *testing.T) {
 	handle(h, now.Add(time.Second), testASNA, testASNC)
 
 	tracked, dropped := snapshot(h)
-	if tracked != maxTrackedPaths {
-		t.Fatalf("want map size %d after drop, got %d", maxTrackedPaths, tracked)
+	if tracked != testMaxTrackedPaths {
+		t.Fatalf("want map size %d after drop, got %d", testMaxTrackedPaths,
+			tracked)
 	}
 
 	if dropped != 1 {
@@ -283,8 +291,9 @@ func TestPeeringHandlerCapDropsAndCounts(t *testing.T) {
 	handle(h, refreshed, testASNA, testASNB)
 
 	tracked, dropped = snapshot(h)
-	if tracked != maxTrackedPaths {
-		t.Fatalf("want map size %d after refresh, got %d", maxTrackedPaths, tracked)
+	if tracked != testMaxTrackedPaths {
+		t.Fatalf("want map size %d after refresh, got %d", testMaxTrackedPaths,
+			tracked)
 	}
 
 	if dropped != 1 {
@@ -303,17 +312,21 @@ func TestPeeringHandlerCapDropsAndCounts(t *testing.T) {
 
 // TestPeeringHandlerRunResetsTheDroppedCount fills the map, has the handler
 // drop one path, and runs the processing. The count of dropped paths is the
-// run's own, so it is 0 again after the run.
+// run's own, so it is 0 again after the run. The handler's limit is small, and
+// its timer does not fire.
 func TestPeeringHandlerRunResetsTheDroppedCount(t *testing.T) {
 	t.Parallel()
 
-	h := newTestHandler(newMockStore())
+	h := newPeeringHandler(newMockStore(), logger.New(), longerThanAnyTest,
+		testMaxTrackedPaths)
+
+	defer stopInTime(t, h)
 
 	now := time.Now().UTC()
 
 	// Paths of one AS carry no peering, so the run records nothing.
 	h.mu.Lock()
-	for asn := 1; len(h.asPaths) < maxTrackedPaths; asn++ {
+	for asn := 1; len(h.asPaths) < testMaxTrackedPaths; asn++ {
 		h.asPaths[pathKey(t, asn)] = database.SeenTimes{First: now, Last: now}
 	}
 	h.mu.Unlock()

internal/routewatch/prefixhandler.go

@@ -44,6 +44,12 @@ const (
 	// most one pair. So the million pairs that a full-table peer's session
 	// going down changes are written in about a minute.
 	maxTableChangesPerWrite = 20000
+
+	// maxHeldBackMessages is the most messages the handler holds back from
+	// peers whose PeerDown has not returned. A message takes about 1 KiB, so
+	// that many take about 20 MiB (README "Memory"). At this number
+	// HandleMessage waits until one of them is applied.
+	maxHeldBackMessages = 20000
 )
 
 // PrefixHandler keeps the routing table in memory current from the feed. It
@@ -52,6 +58,15 @@ const (
 // does an announcement whose AS path is empty or holds AS 0 anywhere. A peer
 // whose BGP session goes down leaves every prefix.
 //
+// The table's PeerDown applies a session going down. It visits every prefix,
+// which takes seconds on a full table. So the handler runs PeerDown on
+// a separate goroutine, one peer at a time, and goes on taking messages
+// meanwhile. PeerDown lets the table's lock go about half of the time, so the
+// handler applies other peers' messages while PeerDown runs. The handler holds
+// back the down peer's later messages until PeerDown returns. Then the handler
+// applies them in order, one at a time, and takes new messages in between. It
+// holds back at most maxHeldBackMessages messages.
+//
 // Each flush writes the table's changes to the database. It also stores each
 // prefix the updates name with the time it was first seen.
 type PrefixHandler struct {
@@ -60,10 +75,17 @@ type PrefixHandler struct {
 	metrics *metrics.Tracker
 	table   *routetable.Table
 
-	// Batching. mu guards batch, writing and writingTableChanges only: the
+	// batchMaxSize is prefixBatchMaxSize. Only HandleMessage reads it, so a
+	// test may lower it before it hands the handler a message.
+	batchMaxSize int
+	// tableChangesPerWrite is maxTableChangesPerWrite, except in tests that
+	// give a smaller one.
+	tableChangesPerWrite int
+
+	// Batching. mu guards batch, writing and writingTableChanges: the
 	// flush goroutine takes them under mu and writes them after releasing
 	// mu, so HandleMessage waits for a database write only while batch is at
-	// prefixBatchMaxSize.
+	// batchMaxSize.
 	mu    sync.Mutex
 	batch []seenPrefix
 	// writing is how many updates the flush goroutine took and has not
@@ -80,6 +102,21 @@ type PrefixHandler struct {
 	// Stop does not wait for a write in progress to hand it over.
 	stopCh chan context.Context
 	done   chan struct{} // closed when the flush goroutine returns
+
+	// Peers whose session went down. peerDownMu lets one PeerDown run at a
+	// time, so the handler's own updates wait for the table's lock behind
+	// at most one. heldBack, under mu, holds each such peer's later
+	// messages, in order, until its PeerDown has returned and they are
+	// applied. A peer with no such messages yet has an empty entry.
+	// heldCount, under mu, is how many messages heldBack holds.
+	// heldTaken is signalled each time a held-back message is taken out to
+	// be applied; its L is &mu. peerDowns counts the peerDown goroutines
+	// that have not returned.
+	peerDownMu sync.Mutex
+	heldBack   map[string][]*ristypes.RISMessage
+	heldCount  int
+	heldTaken  sync.Cond
+	peerDowns  sync.WaitGroup
 }
 
 // seenPrefix is what the batch keeps of an update: the prefix it names and its
@@ -119,26 +156,34 @@ func (u prefixUpdate) announcesRoute() bool {
 func NewPrefixHandler(
 	db database.Store, logger *logger.Logger, table *routetable.Table,
 ) *PrefixHandler {
-	return newPrefixHandler(db, logger, table, prefixBatchTimeout)
+	return newPrefixHandler(db, logger, table, prefixBatchTimeout,
+		maxTableChangesPerWrite)
 }
 
 // newPrefixHandler is NewPrefixHandler with the flush timer's interval,
-// batchTimeout, in place of prefixBatchTimeout. A test that the timer must not
-// disturb gives an interval longer than the test runs.
+// batchTimeout, in place of prefixBatchTimeout, and tableChangesPerWrite in
+// place of maxTableChangesPerWrite. A test that the timer must not disturb
+// gives an interval longer than the test runs. A test of writes split over
+// several flushes gives a small tableChangesPerWrite, so that it exceeds it
+// with few changes.
 func newPrefixHandler(
 	db database.Store, logger *logger.Logger, table *routetable.Table,
-	batchTimeout time.Duration,
+	batchTimeout time.Duration, tableChangesPerWrite int,
 ) *PrefixHandler {
 	h := &PrefixHandler{
-		db:          db,
-		logger:      logger,
-		table:       table,
-		batch:       make([]seenPrefix, 0, prefixBatchSize),
-		batchFullCh: make(chan struct{}, 1),
-		stopCh:      make(chan context.Context, 1),
-		done:        make(chan struct{}),
+		db:                   db,
+		logger:               logger,
+		table:                table,
+		batch:                make([]seenPrefix, 0, prefixBatchSize),
+		batchMaxSize:         prefixBatchMaxSize,
+		tableChangesPerWrite: tableChangesPerWrite,
+		batchFullCh:          make(chan struct{}, 1),
+		stopCh:               make(chan context.Context, 1),
+		done:                 make(chan struct{}),
+		heldBack:             make(map[string][]*ristypes.RISMessage),
 	}
 	h.batchTaken.L = &h.mu
+	h.heldTaken.L = &h.mu
 
 	// Start the flush timer goroutine
 	go flushLoop(batchTimeout, h.batchFullCh, h.stopCh, h.done, h.flushBatch)
@@ -166,11 +211,91 @@ func (h *PrefixHandler) QueueCapacity() int {
 	return prefixHandlerQueueSize
 }
 
-// HandleMessage processes a message to track prefix information
+// HandleMessage processes a message to track prefix information. A message
+// from a peer whose PeerDown has not returned is held back until it has.
 func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	// While maxHeldBackMessages messages are held back, wait until one of them
+	// is taken out to be applied. Meanwhile the handler takes nothing off its
+	// queue, as at batchMaxSize.
+	for h.heldCount >= maxHeldBackMessages {
+		h.heldTaken.Wait()
+	}
+
+	held, goingDown := h.heldBack[msg.Peer]
+	if goingDown {
+		h.heldBack[msg.Peer] = append(held, msg)
+		h.heldCount++
+
+		return
+	}
+
+	h.applyMessage(msg)
+}
+
+// Stop waits for every PeerDown the handler started and for the messages held
+// back meanwhile to be applied. It gives up this wait once ctx ends. Then Stop
+// stops the flush goroutine and waits for it until ctx ends. The flush
+// goroutine makes one last flush before it returns. That flush writes at most
+// tableChangesPerWrite of the routing table's changes, and the prefixes of the
+// updates the handler holds. Once ctx has ended the flush goroutine skips that
+// last flush, and a write still running is lost when the process exits. A
+// failed write of the table's changes is not retried either. Stop logs how
+// many updates and table changes are not written. It also logs how many
+// held-back messages are not applied. The caller calls Stop once its last
+// HandleMessage has returned.
+func (h *PrefixHandler) Stop(ctx context.Context) {
+	peerDownsDone := make(chan struct{})
+
+	go func() {
+		h.peerDowns.Wait()
+		close(peerDownsDone)
+	}()
+
+	select {
+	case <-peerDownsDone:
+	case <-ctx.Done():
+	}
+
+	h.stopCh <- ctx
+
+	select {
+	case <-h.done:
+	case <-ctx.Done():
+	}
+
+	h.mu.Lock()
+	unwritten := h.writing + len(h.batch)
+	tableChanges := h.writingTableChanges + h.table.ChangeCount()
+	heldMessages := h.heldCount
+	h.mu.Unlock()
+
+	if unwritten == 0 && tableChanges == 0 && heldMessages == 0 {
+		return
+	}
+
+	if ctx.Err() != nil {
+		h.logger.Error("Stop deadline passed before the prefix handler wrote "+
+			"its updates", "updates", unwritten, "table_changes", tableChanges,
+			"held_messages", heldMessages)
+
+		return
+	}
+
+	h.logger.Error("The prefix handler stopped before it wrote everything",
+		"updates", unwritten, "table_changes", tableChanges,
+		"held_messages", heldMessages)
+}
+
+// applyMessage applies msg. The caller holds mu. A peer's session going down
+// starts the peer's PeerDown. An update is applied to the routing table, and
+// its prefixes join the batch.
+func (h *PrefixHandler) applyMessage(msg *ristypes.RISMessage) {
 	if msg.Type == ristypes.MessageTypeState {
 		if msg.State == ristypes.StateDown {
-			h.table.PeerDown(msg.Peer)
+			h.startPeerDown(msg.Peer)
 		}
 
 		return
@@ -185,13 +310,10 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 	// the path after the rest of it.
 	path := slices.Concat(msg.Path.Sequence, msg.Path.Set)
 
-	h.mu.Lock()
-	defer h.mu.Unlock()
-
 	// While the batch is at its maximum size, wait until the flush goroutine
 	// takes it. Meanwhile the handler takes nothing off its queue, which
 	// fills, and the streamer drops messages for it.
-	for len(h.batch) >= prefixBatchMaxSize {
+	for len(h.batch) >= h.batchMaxSize {
 		h.batchTaken.Wait()
 	}
 
@@ -239,7 +361,7 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 
 	// A full batch wakes the flush goroutine. If it is writing, the wake-up
 	// waits in batchFullCh and the batch keeps growing until the write ends
-	// or the batch reaches prefixBatchMaxSize.
+	// or the batch reaches batchMaxSize.
 	if len(h.batch) >= prefixBatchSize {
 		select {
 		case h.batchFullCh <- struct{}{}:
@@ -248,39 +370,58 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 	}
 }
 
-// Stop stops the flush goroutine, which makes one last flush before it returns,
-// and waits for it until ctx ends. That flush writes at most
-// maxTableChangesPerWrite of the routing table's changes, and the prefixes of
-// the updates the handler holds. Once ctx has ended the flush goroutine skips
-// that last flush, and a write still running is lost when the process exits. A
-// failed write of the table's changes is not retried either. Stop logs how many
-// updates and how many of the table's changes are not written.
-func (h *PrefixHandler) Stop(ctx context.Context) {
-	h.stopCh <- ctx
-
-	select {
-	case <-h.done:
-	case <-ctx.Done():
+// startPeerDown starts peerDown for peer on a new goroutine. The caller holds
+// mu. The peer's messages are held back until peerDown has applied them.
+// Messages held back for an earlier PeerDown of the peer, and not yet applied,
+// stay held back.
+func (h *PrefixHandler) startPeerDown(peer string) {
+	_, goingDown := h.heldBack[peer]
+	if !goingDown {
+		h.heldBack[peer] = []*ristypes.RISMessage{}
 	}
 
-	h.mu.Lock()
-	unwritten := h.writing + len(h.batch)
-	tableChanges := h.writingTableChanges + h.table.ChangeCount()
-	h.mu.Unlock()
+	h.peerDowns.Add(1)
 
-	if unwritten == 0 && tableChanges == 0 {
-		return
+	go h.peerDown(peer)
+}
+
+// peerDown runs the table's PeerDown for peer, once no other PeerDown is
+// running. Then peerDown applies the messages held back from the peer, in
+// order. It takes mu for one message at a time, so HandleMessage takes new
+// messages in between. The peer's new messages join the end of the held-back
+// ones. A held-back message that says the peer's session went down again
+// starts a new peerDown, which applies the rest.
+func (h *PrefixHandler) peerDown(peer string) {
+	defer h.peerDowns.Done()
+
+	h.peerDownMu.Lock()
+	h.table.PeerDown(peer)
+	h.peerDownMu.Unlock()
+
+	for {
+		h.mu.Lock()
+
+		held := h.heldBack[peer]
+		if len(held) == 0 {
+			delete(h.heldBack, peer)
+			h.mu.Unlock()
+
+			return
+		}
+
+		msg := held[0]
+		held[0] = nil // so that the message is freed once applied
+		h.heldBack[peer] = held[1:]
+		h.heldCount--
+		h.heldTaken.Broadcast()
+		h.applyMessage(msg)
+		h.mu.Unlock()
+
+		if msg.Type == ristypes.MessageTypeState &&
+			msg.State == ristypes.StateDown {
+			return
+		}
 	}
-
-	if ctx.Err() != nil {
-		h.logger.Error("Stop deadline passed before the prefix handler wrote "+
-			"its updates", "updates", unwritten, "table_changes", tableChanges)
-
-		return
-	}
-
-	h.logger.Error("The prefix handler stopped before it wrote everything",
-		"updates", unwritten, "table_changes", tableChanges)
 }
 
 // takeBatch returns the pending updates and leaves an empty batch in their
@@ -378,14 +519,13 @@ func (h *PrefixHandler) applyToTable(update prefixUpdate) {
 	})
 }
 
-// writeTableChanges writes at most maxTableChangesPerWrite changes of the
-// routing table in memory, and every peer the table numbered since the last
-// write.
+// writeTableChanges writes at most tableChangesPerWrite changes of the routing
+// table in memory, and every peer the table numbered since the last write.
 // When the write fails, they go back to the table, so that a later write writes
 // them again, each pair with its content then.
 func (h *PrefixHandler) writeTableChanges() {
 	h.mu.Lock()
-	changes, newPeers := h.table.TakeChanges(maxTableChangesPerWrite)
+	changes, newPeers := h.table.TakeChanges(h.tableChangesPerWrite)
 	h.writingTableChanges = len(changes)
 	h.mu.Unlock()
 

internal/routewatch/prefixhandler_internal_test.go

@@ -237,9 +237,87 @@ func TestPrefixHandlerStopLogsNothingWhenItWroteEverything(t *testing.T) {
 	}
 }
 
-// longerThanAnyTest is a flush timer interval that no test lasts. A test whose
-// prefix, AS or peer handler must flush only when its batch is full and on Stop
-// gives it to newPrefixHandler, newASHandler or newPeerHandler.
+// TestPrefixHandlerStopLogsTheMessagesHeldBack stops a prefix handler with a
+// deadline that has passed, while a peer's PeerDown is held up and one message
+// of the peer is held back. Stop must log that message as not applied.
+func TestPrefixHandlerStopLogsTheMessagesHeldBack(t *testing.T) {
+	t.Parallel()
+
+	var logs bytes.Buffer
+
+	h := newPrefixHandler(newMockStore(), newErrorLogger(&logs),
+		routetable.New(routetable.Params{}), longerThanAnyTest,
+		maxTableChangesPerWrite)
+
+	h.peerDownMu.Lock()
+	defer h.peerDownMu.Unlock()
+
+	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
+	h.HandleMessage(sessionDown())
+	h.HandleMessage(peerAnnouncement(testPeerIP, testV6Prefix))
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	h.Stop(ctx)
+
+	logged := lastLogged(t, &logs)
+	if logged.HeldMessages != 1 {
+		t.Errorf("Stop logged %d held messages not applied, want 1",
+			logged.HeldMessages)
+	}
+}
+
+// TestPrefixHandlerStopWaitsForTheMessagesHeldBack stops a prefix handler
+// while a peer's PeerDown is held up and an announcement of the peer is held
+// back. Stop must wait. Once the PeerDown is let go and Stop returns, the
+// announcement must be in the database.
+func TestPrefixHandlerStopWaitsForTheMessagesHeldBack(t *testing.T) {
+	t.Parallel()
+
+	db := newTestDatabase(t)
+	h := newPrefixHandler(db, logger.New(), routetable.New(routetable.Params{}),
+		longerThanAnyTest, maxTableChangesPerWrite)
+
+	h.peerDownMu.Lock()
+	h.HandleMessage(sessionDown())
+	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
+
+	stopped := make(chan struct{})
+
+	go func() {
+		h.Stop(t.Context())
+		close(stopped)
+	}()
+
+	select {
+	case <-stopped:
+		t.Error("Stop returned while a PeerDown was held up")
+	case <-time.After(stillWaitingAfter):
+	}
+
+	h.peerDownMu.Unlock()
+
+	select {
+	case <-stopped:
+	case <-time.After(stepTimeout):
+		t.Fatalf("Stop did not return within %s of the PeerDown's release",
+			stepTimeout)
+	}
+
+	read, err := db.ReadRouteTable(t.Context())
+	if err != nil {
+		t.Fatalf("ReadRouteTable: %v", err)
+	}
+
+	checkTableOrigins(t, read, map[string]map[int][]string{
+		testV4Prefix: {testASNB: {testPeerIP}},
+	})
+}
+
+// longerThanAnyTest is a timer interval that no test lasts. A test gives it to
+// newPrefixHandler, newASHandler, newPeerHandler or newPeeringHandler when the
+// handler must write only when its batch is full, when the test says, or on
+// Stop.
 const longerThanAnyTest = time.Hour
 
 // prefixHandlerWithoutTimer returns a prefix handler that writes to store and
@@ -247,9 +325,12 @@ const longerThanAnyTest = time.Hour
 // NewPrefixHandler's flush goroutine, so the test flushes itself.
 func prefixHandlerWithoutTimer(store database.Store) *PrefixHandler {
 	return &PrefixHandler{
-		db:     store,
-		logger: logger.New(),
-		table:  routetable.New(routetable.Params{}),
+		db:                   store,
+		logger:               logger.New(),
+		table:                routetable.New(routetable.Params{}),
+		batchMaxSize:         prefixBatchMaxSize,
+		tableChangesPerWrite: maxTableChangesPerWrite,
+		heldBack:             make(map[string][]*ristypes.RISMessage),
 	}
 }
 

internal/routewatch/prefixhandler_table_internal_test.go

@@ -7,8 +7,10 @@ import (
 	"log/slog"
 	"net/netip"
 	"reflect"
+	"runtime"
 	"slices"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -30,11 +32,12 @@ const (
 
 	// firstManyPrefix is the first prefix announceManyPrefixes announces.
 	firstManyPrefix = "10.0.0.0/32"
-)
 
-// tableWriteTimeout is how long a test waits for the flush timer to write the
-// routing table's changes: a few ticks of it.
-const tableWriteTimeout = 5 * prefixBatchTimeout
+	// testMaxTableChangesPerWrite is a limit of one write of the routing
+	// table's changes that a test exceeds quickly, in place of
+	// maxTableChangesPerWrite.
+	testMaxTableChangesPerWrite = 10
+)
 
 // tableCase is one test of the routing table in memory: the messages handed to
 // the prefix handler, and for each prefix the peers that carry it with each
@@ -100,6 +103,228 @@ func TestPrefixHandlerAppliesUpdatesToTheRoutingTable(t *testing.T) {
 	})
 }
 
+// TestPrefixHandlerTakesMessagesWhileAPeerGoesDown holds up the PeerDown of a
+// peer whose session went down, as another peer's long PeerDown would. The
+// handler must go on taking messages. It applies another peer's announcement
+// at once. It holds back these messages of the down peer, in this order:
+//   - an announcement;
+//   - its session going down again;
+//   - a second announcement.
+//
+// Once the PeerDowns have returned, the down peer must carry only the prefix
+// of the second announcement.
+func TestPrefixHandlerTakesMessagesWhileAPeerGoesDown(t *testing.T) {
+	t.Parallel()
+
+	h := prefixHandlerWithoutTimer(newMockStore())
+	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
+
+	h.peerDownMu.Lock()
+
+	handled := returnsInTime(func() {
+		for _, msg := range []*ristypes.RISMessage{
+			sessionDown(),
+			peerAnnouncement(secondPeerIP, otherV4Prefix),
+			peerAnnouncement(testPeerIP, testV6Prefix),
+			sessionDown(),
+			peerAnnouncement(testPeerIP, setV4Prefix),
+		} {
+			h.HandleMessage(msg)
+		}
+	})
+	if !handled {
+		t.Fatalf("the handler did not take the messages after the session "+
+			"went down within %s", stepTimeout)
+	}
+
+	checkTableOrigins(t, h.table, map[string]map[int][]string{
+		testV4Prefix:  {testASNB: {testPeerIP}},
+		otherV4Prefix: {testASNB: {secondPeerIP}},
+		testV6Prefix:  {},
+		setV4Prefix:   {},
+	})
+
+	h.peerDownMu.Unlock()
+
+	if !returnsInTime(h.peerDowns.Wait) {
+		t.Fatalf("the PeerDowns did not return within %s", stepTimeout)
+	}
+
+	checkTableOrigins(t, h.table, map[string]map[int][]string{
+		testV4Prefix:  {},
+		otherV4Prefix: {testASNB: {secondPeerIP}},
+		testV6Prefix:  {},
+		setV4Prefix:   {testASNB: {testPeerIP}},
+	})
+}
+
+// TestPrefixHandlerAppliesMessagesWhilePeerDownRuns gives a peer 50,000
+// prefixes and hands the prefix handler that peer's session going down. The
+// table's PeerDown then visits the prefixes in five batches. Once PeerDown has
+// begun, the handler is handed 1,000 announcements of another peer. The
+// handler must apply them all before PeerDown ends, while the down peer still
+// carries some of its prefixes.
+func TestPrefixHandlerAppliesMessagesWhilePeerDownRuns(t *testing.T) {
+	t.Parallel()
+
+	const (
+		// downPeerPrefixes is five times the 10,000 prefixes PeerDown visits
+		// in one hold of the table's lock.
+		downPeerPrefixes = 50000
+		// otherPeerMessages is far more messages than the handler applies
+		// while PeerDown runs, if PeerDown takes the lock back at once after
+		// each batch. The handler then applies only a few for each batch.
+		otherPeerMessages = 1000
+		// firstOtherAddr is the address of the other peer's first prefix.
+		firstOtherAddr = "2001:db8:2::"
+	)
+
+	h := prefixHandlerWithoutTimer(newMockStore())
+	announceManyPrefixes(h.table, downPeerPrefixes)
+
+	h.HandleMessage(sessionDown())
+
+	// Wait until PeerDown has begun: the down peer has left some of its
+	// prefixes. The down peer's prefixes are IPv4 and the other peer's are
+	// IPv6, so the counts tell them apart.
+	for h.table.Counts().IPv4Routes == downPeerPrefixes {
+		runtime.Gosched()
+	}
+
+	// Each announcement names the /128 of the next address.
+	addr := netip.MustParseAddr(firstOtherAddr)
+	for range otherPeerMessages {
+		h.HandleMessage(peerAnnouncement(secondPeerIP,
+			netip.PrefixFrom(addr, addr.BitLen()).String()))
+
+		addr = addr.Next()
+	}
+
+	counts := h.table.Counts()
+	if counts.IPv6Routes != otherPeerMessages {
+		t.Errorf("the other peer carries %d prefixes, want %d",
+			counts.IPv6Routes, otherPeerMessages)
+	}
+
+	if counts.IPv4Routes == 0 {
+		t.Errorf("PeerDown ended before the handler applied the other peer's "+
+			"%d announcements", otherPeerMessages)
+	}
+
+	if !returnsInTime(h.peerDowns.Wait) {
+		t.Fatalf("the PeerDown did not return within %s", stepTimeout)
+	}
+}
+
+// TestPrefixHandlerTakesMessagesWhileItAppliesHeldBackOnes holds back many
+// messages of a peer whose session went down. Each names a prefix that does
+// not parse, so applying it logs an error. Each log write takes a
+// millisecond. While the handler applies the held-back messages, it must take
+// another peer's announcement.
+func TestPrefixHandlerTakesMessagesWhileItAppliesHeldBackOnes(t *testing.T) {
+	t.Parallel()
+
+	const (
+		// heldMessages take the handler at least a second to apply.
+		heldMessages = 1000
+		badPrefix    = "not a prefix"
+	)
+
+	logs := &slowWriter{firstWrite: make(chan struct{})}
+	h := newPrefixHandler(newMockStore(), newErrorLogger(logs),
+		routetable.New(routetable.Params{}), longerThanAnyTest,
+		maxTableChangesPerWrite)
+
+	defer stopInTime(t, h)
+
+	h.peerDownMu.Lock()
+	h.HandleMessage(sessionDown())
+
+	for range heldMessages {
+		h.HandleMessage(peerAnnouncement(testPeerIP, badPrefix))
+	}
+
+	h.peerDownMu.Unlock()
+
+	select {
+	case <-logs.firstWrite:
+	case <-time.After(stepTimeout):
+		t.Fatal("the handler did not start to apply the held-back messages")
+	}
+
+	if !returnsInTime(func() {
+		h.HandleMessage(peerAnnouncement(secondPeerIP, otherV4Prefix))
+	}) {
+		t.Fatalf("HandleMessage did not return within %s", stepTimeout)
+	}
+
+	h.mu.Lock()
+	left := len(h.heldBack[testPeerIP])
+	h.mu.Unlock()
+
+	if left == 0 {
+		t.Error("the handler took another peer's announcement only once it " +
+			"had applied every held-back message")
+	}
+}
+
+// TestPrefixHandlerWaitsAtTheMaximumHeldBack holds up the PeerDown of a peer
+// whose session went down. It hands the handler maxHeldBackMessages messages
+// of that peer. HandleMessage must then wait, even for another peer's
+// message, until the PeerDown returns and a held-back message is applied.
+func TestPrefixHandlerWaitsAtTheMaximumHeldBack(t *testing.T) {
+	t.Parallel()
+
+	h := newPrefixHandler(newMockStore(), logger.New(),
+		routetable.New(routetable.Params{}), longerThanAnyTest,
+		maxTableChangesPerWrite)
+	defer stopInTime(t, h)
+
+	h.peerDownMu.Lock()
+	h.HandleMessage(sessionDown())
+
+	held := peerAnnouncement(testPeerIP, testV6Prefix)
+	for range maxHeldBackMessages {
+		h.HandleMessage(held)
+	}
+
+	returned := make(chan struct{})
+
+	go func() {
+		h.HandleMessage(peerAnnouncement(secondPeerIP, otherV4Prefix))
+		close(returned)
+	}()
+
+	select {
+	case <-returned:
+		t.Error("HandleMessage returned while maxHeldBackMessages messages " +
+			"were held back")
+	case <-time.After(stillWaitingAfter):
+	}
+
+	h.peerDownMu.Unlock()
+
+	select {
+	case <-returned:
+	case <-time.After(stepTimeout):
+		t.Fatal("HandleMessage did not return after the PeerDown returned")
+	}
+}
+
+// slowWriter takes a millisecond over each write and drops what it is given.
+// It closes firstWrite at the first write.
+type slowWriter struct {
+	firstWrite chan struct{}
+	once       sync.Once
+}
+
+func (w *slowWriter) Write(p []byte) (int, error) {
+	w.once.Do(func() { close(w.firstWrite) })
+	time.Sleep(time.Millisecond)
+
+	return len(p), nil
+}
+
 // TestPrefixHandlerAppliesAPathWithoutAnOriginToTheRoutingTable hands the
 // prefix handler announcements whose AS path gives no origin. One whose path
 // is empty or holds AS 0 removes the peer's route, as a withdrawal does; one
@@ -198,8 +423,9 @@ func TestPrefixHandlerKeepsThePathOfAPathEndingInASet(t *testing.T) {
 	}
 }
 
-// checkTableCases hands the messages of each case to a new prefix handler and
-// checks its routing table right after, with no flush.
+// checkTableCases hands the messages of each case to a new prefix handler. It
+// checks the handler's routing table once the handler's PeerDowns have
+// returned, with no flush.
 func checkTableCases(t *testing.T, tests []tableCase) {
 	t.Helper()
 
@@ -212,17 +438,28 @@ func checkTableCases(t *testing.T, tests []tableCase) {
 				h.HandleMessage(msg)
 			}
 
-			for prefix, want := range tt.want {
-				got := tableOrigins(h.table, prefix)
-				if !reflect.DeepEqual(got, want) {
-					t.Errorf("peers of %s by origin AS = %v, want %v",
-						prefix, got, want)
-				}
-			}
+			h.peerDowns.Wait()
+			checkTableOrigins(t, h.table, tt.want)
 		})
 	}
 }
 
+// checkTableOrigins checks, for each prefix in want, the peers that carry it
+// in table with each origin AS.
+func checkTableOrigins(
+	t *testing.T, table *routetable.Table, want map[string]map[int][]string,
+) {
+	t.Helper()
+
+	for prefix, wantOrigins := range want {
+		got := tableOrigins(table, prefix)
+		if !reflect.DeepEqual(got, wantOrigins) {
+			t.Errorf("peers of %s by origin AS = %v, want %v",
+				prefix, got, wantOrigins)
+		}
+	}
+}
+
 // tableWritesStore is a real database that hands each write of the routing
 // table's changes to written once it is made.
 type tableWritesStore struct {
@@ -276,9 +513,9 @@ func TestPrefixHandlerWritesTheTableChangesEachSecond(t *testing.T) {
 				t.Fatalf("wrote %+v, want the pair of %s and AS%d with "+
 					"removed %t", changes, prefix, testASNB, step.wantRemoved)
 			}
-		case <-time.After(tableWriteTimeout):
+		case <-time.After(stepTimeout):
 			t.Fatalf("the routing table's change was not written within %s",
-				tableWriteTimeout)
+				stepTimeout)
 		}
 	}
 }
@@ -375,25 +612,32 @@ func (s *tableWriteSizesStore) WriteRouteChanges(
 // takes, then flushes three times. The first write must hold as many pairs as
 // one write takes, and the peer. The second must hold the other pairs, and the
 // third flush must write nothing. The database must then hold the table in
-// memory.
+// memory. The handler's limit is small, and its timer does not fire.
 func TestPrefixHandlerWritesManyTableChangesOverSeveralFlushes(t *testing.T) {
 	t.Parallel()
 
 	const (
-		pairs   = maxTableChangesPerWrite * 3 / 2
+		pairs   = testMaxTableChangesPerWrite * 3 / 2
 		flushes = 3
 	)
 
 	db := newTestDatabase(t)
 	store := &tableWriteSizesStore{Database: db}
-	h := prefixHandlerWithoutTimer(store)
-	announceManyPrefixes(h.table, pairs)
+	table := routetable.New(routetable.Params{})
+	announceManyPrefixes(table, pairs)
+
+	h := newPrefixHandler(store, logger.New(), table, longerThanAnyTest,
+		testMaxTableChangesPerWrite)
+
+	defer stopInTime(t, h)
 
 	for range flushes {
 		h.flushBatch()
 	}
 
-	wantChanges := []int{maxTableChangesPerWrite, pairs - maxTableChangesPerWrite}
+	wantChanges := []int{
+		testMaxTableChangesPerWrite, pairs - testMaxTableChangesPerWrite,
+	}
 	wantNewPeers := []int{1, 0}
 
 	if !slices.Equal(store.changes, wantChanges) ||
@@ -415,11 +659,11 @@ func TestPrefixHandlerWritesManyTableChangesOverSeveralFlushes(t *testing.T) {
 // takes. Its last flush writes one write's worth, so Stop must log the pairs
 // that no write took, and must not say that the stop deadline passed. The
 // flush timer must not fire, or its writes could leave no pairs for Stop to
-// log.
+// log. The handler's limit of one write is small.
 func TestPrefixHandlerStopLogsTheTableChangesLeftUnwritten(t *testing.T) {
 	t.Parallel()
 
-	const pairs = maxTableChangesPerWrite + 1
+	const pairs = testMaxTableChangesPerWrite + 1
 
 	var logs bytes.Buffer
 
@@ -427,7 +671,8 @@ func TestPrefixHandlerStopLogsTheTableChangesLeftUnwritten(t *testing.T) {
 	table := routetable.New(routetable.Params{})
 	announceManyPrefixes(table, pairs)
 
-	h := newPrefixHandler(store, newErrorLogger(&logs), table, longerThanAnyTest)
+	h := newPrefixHandler(store, newErrorLogger(&logs), table, longerThanAnyTest,
+		testMaxTableChangesPerWrite)
 	h.Stop(t.Context())
 
 	written := 0
@@ -489,6 +734,7 @@ func TestARestartReadsTheRoutingTableBack(t *testing.T) {
 		h.HandleMessage(msg)
 	}
 
+	h.peerDowns.Wait()
 	h.flushBatch()
 
 	var logs bytes.Buffer
@@ -527,6 +773,7 @@ func TestARestartReadsTheRoutingTableBack(t *testing.T) {
 		restarted.HandleMessage(msg)
 	}
 
+	h.peerDowns.Wait()
 	restarted.Stop(t.Context())
 
 	read, err := db.ReadRouteTable(t.Context())

internal/whois/whois.go

@@ -12,6 +12,7 @@ import (
 	"regexp"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 	"unicode/utf8"
 
@@ -70,12 +71,12 @@ const nicbrHeader = "% Copyright (c) Nic.br"
 // queries to stop the data being taken in bulk.
 const nicbrLimitReached = "Query rate limit exceeded"
 
-// NICbrSkipTime is how long the client stops asking NIC.br after NIC.br
-// answers that the client is over its query limit. NIC.br publishes
-// neither the limit nor how long it lasts. An hour outlasts a limit
-// counted per minute or per hour. The ASes looked up meanwhile get
-// NIC.br's fields when they are next refreshed.
-const NICbrSkipTime = time.Hour
+// PauseTime is how long the client sends an RIR's or NIC.br's server no
+// query after that server refuses a query. queryRegistry says what a
+// refusal is. RIPE refuses an address for good when the address keeps
+// passing RIPE's limit (see noPersonalFlag). An hour outlasts a limit
+// counted per minute or per hour.
+const PauseTime = time.Hour
 
 // RIR identifiers.
 const (
@@ -125,6 +126,10 @@ var (
 	errAnswerTooLarge = errors.New("WHOIS answer reached the size cap")
 )
 
+// ErrPaused is returned when a lookup needs a server that refused a query
+// less than PauseTime ago. The client sends that server nothing meanwhile.
+var ErrPaused = errors.New("WHOIS server paused after refusing a query")
+
 // ASNInfo contains parsed WHOIS information for an ASN.
 type ASNInfo struct {
 	ASN         int
@@ -156,12 +161,12 @@ type Client struct {
 
 	logger *slog.Logger
 
-	// mu guards nicbrSkipUntil.
+	// mu guards pausedUntil.
 	mu sync.Mutex
 
-	// nicbrSkipUntil is when the client next asks NIC.br, after NIC.br
-	// answered that the client is over its query limit.
-	nicbrSkipUntil time.Time
+	// pausedUntil maps a server, given as host:port, to the time the client
+	// may ask it again.
+	pausedUntil map[string]time.Time
 }
 
 // NewClient creates a new WHOIS client that logs to logger.
@@ -172,23 +177,29 @@ func NewClient(logger *slog.Logger) *Client {
 		DialContext: dialer.DialContext,
 		Now:         time.Now,
 		logger:      logger,
+		pausedUntil: make(map[string]time.Time),
 	}
 }
 
 // LookupASN asks IANA which RIR holds the given ASN, asks that RIR, and
 // returns what the RIR's answer says. When LACNIC's answer is NIC.br's, it
 // returns what NIC.br's own server says instead, unless that query fails or
-// is skipped, as askNICbr says.
+// NIC.br is paused, as askNICbr says.
 //
-// It returns ErrNoReferral when IANA refers the AS to no RIR, and another
-// error when the query to IANA or to the RIR fails, when the RIR refuses
-// it, when IANA's answer has neither a referral nor the AS's block, when
-// IANA refers the AS to a server none of the five RIRs run, or when the
-// answer carries none of ASNInfo's fields.
+// It returns ErrNoReferral when IANA refers the AS to no RIR. Its error
+// wraps ErrPaused when the RIR is paused. It returns another error in each
+// of these cases:
+//
+//   - the query to IANA or to the RIR fails
+//   - the RIR refuses the query
+//   - IANA's answer has neither a referral nor the AS's block
+//   - IANA refers the AS to a server none of the five RIRs run
+//   - the answer carries none of ASNInfo's fields
 func (c *Client) LookupASN(ctx context.Context, asn int) (*ASNInfo, error) {
 	// Query IANA first to find the authoritative RIR
 	query := fmt.Sprintf("AS%d", asn)
 
+	// IANA is never paused, because a pause of IANA would stop every lookup.
 	ianaResp, err := c.query(ctx, whoisServerIANA, query)
 	if err != nil {
 		return nil, fmt.Errorf("IANA query failed: %w", err)
@@ -211,7 +222,7 @@ func (c *Client) LookupASN(ctx context.Context, asn int) (*ASNInfo, error) {
 	}
 
 	// Query the authoritative RIR
-	rirResp, err := c.queryRIR(ctx, whoisServer, rirQuery(rir, query))
+	rirResp, err := c.queryRegistry(ctx, whoisServer, rirQuery(rir, query))
 	if err != nil {
 		return nil, fmt.Errorf("RIR query failed: %w", err)
 	}
@@ -241,22 +252,19 @@ func (c *Client) LookupASN(ctx context.Context, asn int) (*ASNInfo, error) {
 // askNICbr returns lacnicInfo, what LACNIC's answer says, and logs a
 // warning when the query fails, when NIC.br sends the reduced answer for a
 // client over its limit, or when NIC.br's answer carries none of ASNInfo's
-// fields. After the reduced answer, it returns lacnicInfo without asking
-// NIC.br until NICbrSkipTime has passed.
+// fields. The reduced answer pauses NIC.br, as a refusal does. While NIC.br
+// is paused, askNICbr returns lacnicInfo without a warning. The ASes looked
+// up meanwhile get NIC.br's fields when they are next refreshed.
 func (c *Client) askNICbr(
 	ctx context.Context, asQuery string, lacnicInfo *ASNInfo,
 ) *ASNInfo {
 	asn := lacnicInfo.ASN
 
-	c.mu.Lock()
-	skipping := c.Now().Before(c.nicbrSkipUntil)
-	c.mu.Unlock()
-
-	if skipping {
+	answer, err := c.queryRegistry(ctx, whoisServerNICbr, asQuery)
+	if errors.Is(err, ErrPaused) {
 		return lacnicInfo
 	}
 
-	answer, err := c.query(ctx, whoisServerNICbr, asQuery)
 	if err != nil {
 		c.logger.Warn("NIC.br query failed, keeping LACNIC's fields",
 			"asn", asn, "error", err)
@@ -265,13 +273,11 @@ func (c *Client) askNICbr(
 	}
 
 	if strings.Contains(answer, nicbrLimitReached) {
-		c.mu.Lock()
-		c.nicbrSkipUntil = c.Now().Add(NICbrSkipTime)
-		c.mu.Unlock()
+		c.pause(whoisServerNICbr)
 
 		c.logger.Warn("NIC.br answered that the client is over its query "+
-			"limit, keeping LACNIC's fields and skipping NIC.br",
-			"asn", asn, "skip_for", NICbrSkipTime)
+			"limit, keeping LACNIC's fields and pausing NIC.br",
+			"asn", asn, "pause_for", PauseTime)
 
 		return lacnicInfo
 	}
@@ -309,25 +315,65 @@ func rirQuery(rir, asQuery string) string {
 	}
 }
 
-// queryRIR sends query to an RIR's server and returns the answer. An answer
-// holding an error line with which the server refuses the query is an
-// error, and the error holds that line.
-func (c *Client) queryRIR(
+// queryRegistry sends query to the server of an RIR or of NIC.br, and
+// returns the answer. The client pauses the server for PauseTime when the
+// server refuses the query. Each of these is a refusal:
+//
+//   - a refused connection
+//   - a reset connection
+//   - an %ERROR:201: or %ERROR:202: line in the answer
+//
+// A refusal is returned as an error. While the server is paused,
+// queryRegistry sends it nothing and returns an error that wraps ErrPaused.
+func (c *Client) queryRegistry(
 	ctx context.Context, server, query string,
 ) (string, error) {
+	c.mu.Lock()
+	pausedUntil := c.pausedUntil[server]
+	c.mu.Unlock()
+
+	if c.Now().Before(pausedUntil) {
+		return "", fmt.Errorf("%w: %s until %s", ErrPaused, server,
+			pausedUntil.UTC().Format(time.RFC3339))
+	}
+
 	answer, err := c.query(ctx, server, query)
+	if err == nil {
+		err = checkRefusal(answer)
+	}
+
+	if errors.Is(err, errRefused) || errors.Is(err, syscall.ECONNREFUSED) ||
+		errors.Is(err, syscall.ECONNRESET) {
+		c.pause(server)
+	}
+
 	if err != nil {
 		return "", err
 	}
 
+	return answer, nil
+}
+
+// checkRefusal returns an error holding the first line of answer that
+// starts with accessDeniedError or accessLimitError. It returns nil when
+// answer has no such line.
+func checkRefusal(answer string) error {
 	for line := range strings.SplitSeq(answer, "\n") {
 		if strings.HasPrefix(line, accessDeniedError) ||
 			strings.HasPrefix(line, accessLimitError) {
-			return "", fmt.Errorf("%w: %s", errRefused, line)
+			return fmt.Errorf("%w: %s", errRefused, line)
 		}
 	}
 
-	return answer, nil
+	return nil
+}
+
+// pause stops queryRegistry from querying server until PauseTime has passed.
+func (c *Client) pause(server string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.pausedUntil[server] = c.Now().Add(PauseTime)
 }
 
 // query performs a raw WHOIS query to the specified server and returns the

internal/whois/whois_test.go

@@ -718,9 +718,10 @@ func TestLookupASNKeepsLACNICsAnswerWhenNICbrFails(t *testing.T) {
 
 // TestLookupASNSkipsNICbrAfterItsLimit looks up an AS that NIC.br
 // registers three times with one client, while NIC.br answers that the
-// client is over its query limit. The first lookup asks NIC.br. The
-// second, a second before NICbrSkipTime has passed, does not. The third,
-// once NICbrSkipTime has passed, asks again. Each keeps LACNIC's fields.
+// client is over its query limit. The test sets the client's clock. The
+// first lookup asks NIC.br. The next comes a second before PauseTime has
+// passed, and does not ask NIC.br. The last comes at PauseTime and asks
+// NIC.br again. Each keeps LACNIC's fields.
 func TestLookupASNSkipsNICbrAfterItsLimit(t *testing.T) {
 	t.Parallel()
 
@@ -740,8 +741,8 @@ func TestLookupASNSkipsNICbrAfterItsLimit(t *testing.T) {
 		wantAsks int
 	}{
 		{after: 0, wantAsks: 1},
-		{after: whois.NICbrSkipTime - time.Second, wantAsks: 1},
-		{after: whois.NICbrSkipTime, wantAsks: 2},
+		{after: whois.PauseTime - time.Second, wantAsks: 1},
+		{after: whois.PauseTime, wantAsks: 2},
 	} {
 		now = start.Add(step.after)
 
@@ -785,6 +786,142 @@ func TestLookupASNReportsARefusal(t *testing.T) {
 	}
 }
 
+// resetServer starts a TCP server on the loopback address and returns the
+// server's address. The server reads one query on each connection, then
+// resets the connection.
+func resetServer(t *testing.T) string {
+	t.Helper()
+
+	var listenConfig net.ListenConfig
+
+	listener, err := listenConfig.Listen(t.Context(), "tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+
+	t.Cleanup(func() { _ = listener.Close() })
+
+	go func() {
+		for {
+			conn, err := listener.Accept()
+			if err != nil {
+				return
+			}
+
+			_, _ = bufio.NewReader(conn).ReadString('\n')
+
+			// Closing a connection whose linger time is zero resets it.
+			if tcpConn, ok := conn.(*net.TCPConn); ok {
+				_ = tcpConn.SetLinger(0)
+			}
+
+			_ = conn.Close()
+		}
+	}()
+
+	return listener.Addr().String()
+}
+
+// newRIPEClient returns a client whose fake servers send answers. The
+// client adds one to *ripeAsks each time it connects to RIPE's server. When
+// reset is true, RIPE's server resets the connection instead of answering.
+func newRIPEClient(
+	t *testing.T, answers map[string]string, reset bool, ripeAsks *int,
+) *whois.Client {
+	t.Helper()
+
+	dial := fakeServers(t, answers)
+	resetAddress := resetServer(t)
+
+	client := whois.NewClient(slog.New(slog.DiscardHandler))
+	client.DialContext = func(
+		ctx context.Context, network, address string,
+	) (net.Conn, error) {
+		if address == ripeServer {
+			*ripeAsks++
+		}
+
+		if address == ripeServer && reset {
+			var dialer net.Dialer
+
+			return dialer.DialContext(ctx, network, resetAddress)
+		}
+
+		return dial(ctx, network, address)
+	}
+
+	return client
+}
+
+// TestLookupASNPausesAServerThatRefuses looks up an AS at RIPE while RIPE
+// refuses the query, in each of the ways a server refuses. The test sets
+// the client's clock. A lookup a second before PauseTime has passed fails
+// with ErrPaused and asks RIPE nothing. A lookup of an AS at APNIC succeeds
+// during the pause. A lookup at PauseTime asks RIPE again.
+func TestLookupASNPausesAServerThatRefuses(t *testing.T) {
+	t.Parallel()
+
+	for name, test := range map[string]struct {
+		// ripeAnswer is RIPE's answer. With none, RIPE's server refuses
+		// the connection, or resets it when reset is true.
+		ripeAnswer string
+		reset      bool
+	}{
+		"access denied":      {ripeAnswer: ripeRefusal},
+		"limit reached":      {ripeAnswer: ripeLimitReached + "\n"},
+		"refused connection": {},
+		"reset connection":   {reset: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			answers := map[string]string{
+				ianaQuery:      ianaReferral,
+				apnicIANAQuery: "refer:        whois.apnic.net\n",
+				apnicQuery:     apnicRecord,
+			}
+			if test.ripeAnswer != "" {
+				answers[ripeQuery] = test.ripeAnswer
+			}
+
+			ripeAsks := 0
+			client := newRIPEClient(t, answers, test.reset, &ripeAsks)
+
+			start := time.Date(2026, 11, 2, 0, 0, 0, 0, time.UTC)
+			now := start
+			client.Now = func() time.Time { return now }
+
+			_, err := client.LookupASN(t.Context(), testASN)
+			if err == nil || errors.Is(err, whois.ErrPaused) {
+				t.Fatalf("LookupASN while RIPE refuses: error %v, want a "+
+					"refusal", err)
+			}
+
+			now = start.Add(whois.PauseTime - time.Second)
+
+			_, err = client.LookupASN(t.Context(), testASN)
+			if !errors.Is(err, whois.ErrPaused) || ripeAsks != 1 {
+				t.Errorf("LookupASN during the pause: error %v after %d "+
+					"queries to RIPE, want ErrPaused after 1", err, ripeAsks)
+			}
+
+			_, err = client.LookupASN(t.Context(), apnicASN)
+			if err != nil {
+				t.Errorf("LookupASN of an AS at APNIC during RIPE's "+
+					"pause: %v", err)
+			}
+
+			now = start.Add(whois.PauseTime)
+
+			_, _ = client.LookupASN(t.Context(), testASN)
+			if ripeAsks != 2 {
+				t.Errorf("RIPE was asked %d times after the pause, want 2",
+					ripeAsks)
+			}
+		})
+	}
+}
+
 // TestLookupASNFailsWhenTheRIRGivesNoAnswer looks up an AS IANA refers to
 // RIPE when RIPE's server refuses the connection, and when it answers only
 // with an error. Neither is a lookup that found the AS.