Take every down peer out of the table in one walk (closes #225)

The prefix handler ran one PeerDown per peer, one after another. When
the sessions of several peers went down together, their walks queued up.
A returning full-table peer's messages stayed held back while the walks
ahead of its own ran, and could reach the limit of 20,000.

The table's PeersDown now takes several peers out of every prefix in one
walk. One walk runs at a time. The next one takes every peer whose
session went down since the previous one began. The doc of
maxHeldBackMessages says how long a peer's messages are held back. The
limit is now 150,000 messages, about 150 MiB. The status page shows how
many messages each handler holds back.

Model: opus-5-5

README.md

@@ -445,8 +445,9 @@ the routing table, and everything else from the database.
         - 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. The handler
-          holds back the peer's later updates until the peer has left every
-          prefix, then applies them in order.
+          holds back the peer's later updates. It applies them in order after
+          the peer has left every prefix and after the held-back updates of
+          every peer whose session went down before it.
 
         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
@@ -651,10 +652,11 @@ What happens at each limit:
     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.
+  peer's session goes down. It applies them after the peer has left every prefix
+  and after the held-back messages of every peer whose session went down before
+  it. It holds at most 150,000 such messages (`maxHeldBackMessages`), about 150
+  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 +35,14 @@ tested.
 
 # Completed Steps
 
+- 2026-11-02: when the sessions of several peers go down, the prefix handler
+  takes the peers out of the routing table together. The table's `PeersDown`
+  takes several peers out of every prefix in one walk. One walk runs at a time.
+  The next one takes every peer whose session went down since the previous one
+  began. Each peer used to have a walk of its own. A peer's messages stayed held
+  back while the walks queued ahead of its own ran. The handler holds back at
+  most 150,000 messages, about 150 MiB. It used to hold back at most 20,000. The
+  status page shows how many messages the handler holds back (closes #225)
 - 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`

internal/database/database_internal_test.go

@@ -273,7 +273,7 @@ func TestVacuumReturnsFreePages(t *testing.T) {
 	writeTableChanges(t, db, table)
 
 	// The peer going down removes every prefix, and so every row.
-	table.PeerDown(peerA)
+	table.PeersDown([]string{peerA})
 	writeTableChanges(t, db, table)
 
 	var before int

internal/database/routes_internal_test.go

@@ -107,7 +107,7 @@ func TestReadRouteTableGivesBackWhatWasWritten(t *testing.T) {
 
 	// A peer moves to another origin, one goes down and a new one comes.
 	announceTo(source, tablePrefixV4, asB, peerA, 60)
-	source.PeerDown(peerC)
+	source.PeersDown([]string{peerC})
 	announceTo(source, tablePrefixV6, asB, peerE, 70)
 
 	writeTableChanges(t, db, source)
@@ -133,7 +133,7 @@ func TestReadRouteTableGivesATableThatTakesLaterUpdates(t *testing.T) {
 	// another one goes down, and a new peer comes.
 	for _, table := range []*routetable.Table{source, read} {
 		table.Withdraw(netip.MustParsePrefix(tablePrefixV4), peerA)
-		table.PeerDown(peerB)
+		table.PeersDown([]string{peerB})
 		announceTo(table, tablePrefixShortV4, asB, peerD, 50)
 	}
 

internal/routetable/reads_test.go

@@ -227,7 +227,7 @@ func TestCounts(t *testing.T) {
 	})
 
 	// peerC was the last peer of the IPv6 prefix. It keeps its number.
-	table.PeerDown(peerC)
+	table.PeersDown([]string{peerC})
 	checkCounts(t, table, routetable.Counts{
 		Pairs:                2,
 		Peers:                3,

internal/routetable/routetable.go

@@ -28,7 +28,7 @@ const (
 	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.
+	// one hold of the lock. Overview and PeersDown each walk the whole table.
 	// Callers waiting for the lock get it between batches.
 	walkBatch = 10000
 
@@ -205,20 +205,29 @@ 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 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) {
+// PeersDown applies the sessions of peers going down: each of the peers leaves
+// every peer set, and every route of each is dropped. When any of them carries
+// a prefix, PeersDown visits every prefix in the table once, 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
+// one of the peers applied during the walk may be dropped too. So the caller
+// applies those peers' later updates only once PeersDown has returned.
+func (t *Table) PeersDown(peers []string) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
-	number, known := t.peerNumbers[peer]
-	if !known || t.peerPrefixes[number] == 0 {
+	// numbers holds the number of each of the peers that carries a prefix.
+	numbers := make([]uint, 0, len(peers))
+
+	for _, peer := range peers {
+		number, known := t.peerNumbers[peer]
+		if known && t.peerPrefixes[number] > 0 {
+			numbers = append(numbers, number)
+		}
+	}
+
+	if len(numbers) == 0 {
 		return
 	}
 
@@ -228,9 +237,15 @@ func (t *Table) PeerDown(peer string) {
 	for _, f := range []*family{&t.ipv4, &t.ipv6} {
 		for _, prefixes := range f.byLength {
 			for prefix, origins := range prefixes {
-				o := originWithPeer(origins, number)
-				if o != nil {
-					t.leave(prefix, o, number)
+				for _, number := range numbers {
+					o := originWithPeer(origins, number)
+					if o != nil {
+						t.leave(prefix, o, number)
+
+						// leave may have removed an origin of the
+						// prefix, or the prefix itself.
+						origins = prefixes[prefix]
+					}
 				}
 
 				// Go allows the map to change before the next step of

internal/routetable/routetable_internal_test.go

@@ -7,11 +7,11 @@ import (
 	"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) {
+// TestPeersDownLetsReadsIn reads the table's counts again and again while a
+// peer that carries more than two batches of prefixes goes down. PeersDown 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 TestPeersDownLetsReadsIn(t *testing.T) {
 	t.Parallel()
 
 	const (
@@ -32,8 +32,8 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 		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
+	// Hold a read lock until PeersDown waits for the lock, so that the first
+	// read below starts once PeersDown holds it. TryRLock fails once a writer
 	// waits.
 	table.mu.RLock()
 
@@ -42,7 +42,7 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 	go func() {
 		defer close(peerDownDone)
 
-		table.PeerDown(peer)
+		table.PeersDown([]string{peer})
 	}()
 
 	for table.mu.TryRLock() {
@@ -60,7 +60,7 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 
 		select {
 		case <-peerDownDone:
-			t.Fatal("no read got in while PeerDown ran")
+			t.Fatal("no read got in while PeersDown ran")
 		default:
 		}
 	}

internal/routetable/routetable_test.go

@@ -69,10 +69,10 @@ func withdraw(peer string) step {
 	}
 }
 
-// peerDown is the step of peer's session going down.
-func peerDown(peer string) step {
+// peersDown is the step of the sessions of peers going down.
+func peersDown(peers ...string) step {
 	return func(table *routetable.Table, _ netip.Prefix) {
-		table.PeerDown(peer)
+		table.PeersDown(peers)
 	}
 }
 
@@ -272,21 +272,28 @@ func TestWithdrawalsAndPeerDowns(t *testing.T) {
 		{
 			name: "a peer going down leaves the prefix",
 			steps: []step{announce(peerA, asA, 10), announce(peerB, asA, 20),
-				peerDown(peerB)},
+				peersDown(peerB)},
 			want: []routetable.Origin{{ASN: asA, Peers: []string{peerA},
 				Routes: []routetable.Route{routeOf(peerA, asA, 10)}}},
 		},
 		{
 			name: "a peer going down after it withdrew changes nothing",
 			steps: []step{announce(peerA, asA, 10), announce(peerB, asA, 20),
-				withdraw(peerB), peerDown(peerB)},
+				withdraw(peerB), peersDown(peerB)},
 			want: []routetable.Origin{{ASN: asA, Peers: []string{peerA},
 				Routes: []routetable.Route{routeOf(peerA, asA, 10)}}},
 		},
+		{
+			name: "peers going down together each leave the prefix",
+			steps: []step{announce(peerA, asA, 10), announce(peerB, asB, 20),
+				announce(peerC, asB, 30), peersDown(peerA, peerB)},
+			want: []routetable.Origin{{ASN: asB, Peers: []string{peerC},
+				Routes: []routetable.Route{routeOf(peerC, asB, 30)}}},
+		},
 	})
 }
 
-func TestPeerDownLeavesEveryPrefix(t *testing.T) {
+func TestPeersDownLeavesEveryPrefix(t *testing.T) {
 	t.Parallel()
 
 	table := routetable.New(routetable.Params{})
@@ -303,7 +310,7 @@ func TestPeerDownLeavesEveryPrefix(t *testing.T) {
 	}
 
 	table.Announce(prefixes[0], asB, routeOf(peerB, asB, 20))
-	table.PeerDown(peerA)
+	table.PeersDown([]string{peerA})
 
 	want := []routetable.Origin{{ASN: asB, Peers: []string{peerB},
 		Routes: []routetable.Route{routeOf(peerB, asB, 20)}}}
@@ -333,6 +340,32 @@ func TestPeerDownLeavesEveryPrefix(t *testing.T) {
 	}
 }
 
+// TestPeersDownTakesOutPeersOfDifferentPrefixes takes peerA and peerB out in
+// one PeersDown. peerA carries only prefixV4, and peerB only prefixV6. So
+// peerA's leaving removes prefixV4 before PeersDown looks for peerB there. Both
+// prefixes must then be gone.
+func TestPeersDownTakesOutPeersOfDifferentPrefixes(t *testing.T) {
+	t.Parallel()
+
+	table := routetable.New(routetable.Params{})
+	prefixes := []netip.Prefix{
+		netip.MustParsePrefix(prefixV4),
+		netip.MustParsePrefix(prefixV6),
+	}
+
+	table.Announce(prefixes[0], asA, routeOf(peerA, asA, 10))
+	table.Announce(prefixes[1], asB, routeOf(peerB, asB, 20))
+	table.PeersDown([]string{peerA, peerB})
+
+	for _, prefix := range prefixes {
+		got := table.Origins(prefix)
+		if got != nil {
+			t.Errorf("Origins(%s) = %+v after its only peer went down, want nil",
+				prefix, got)
+		}
+	}
+}
+
 func TestCountsOriginsWithoutKeptRoute(t *testing.T) {
 	t.Parallel()
 
@@ -358,7 +391,7 @@ func TestCountsOriginsWithoutKeptRoute(t *testing.T) {
 		{announce(peerC, asA, 30), 0},
 		{withdraw(peerB), 0},
 		{withdraw(peerC), 1},
-		{peerDown(peerA), 0},
+		{peersDown(peerA), 0},
 	}
 
 	for i, s := range steps {
@@ -464,7 +497,7 @@ func TestTakeChangesHoldsOnlyPairsThatChanged(t *testing.T) {
 	checkChanges(t, table, []change{}, map[string]uint{})
 
 	// peerA going down changes v4, the one prefix it carries.
-	table.PeerDown(peerA)
+	table.PeersDown([]string{peerA})
 	checkChanges(t, table, []change{
 		{Prefix: prefixV4, OriginASN: asA, Peers: []uint{1, 2},
 			Routes: []routetable.Route{
@@ -473,7 +506,7 @@ func TestTakeChangesHoldsOnlyPairsThatChanged(t *testing.T) {
 	}, map[string]uint{})
 
 	// peerA, which carries nothing now, goes down again.
-	table.PeerDown(peerA)
+	table.PeersDown([]string{peerA})
 	checkChanges(t, table, []change{}, map[string]uint{})
 }
 
@@ -617,7 +650,7 @@ func TestConcurrentUse(t *testing.T) {
 		func(round int) { table.Announce(prefix, asA, routeOf(peerA, asA, round)) },
 		func(round int) { table.Announce(prefix, asB, routeOf(peerB, asB, round)) },
 		func(int) { table.Withdraw(prefix, peerA) },
-		func(int) { table.PeerDown(peerB) },
+		func(int) { table.PeersDown([]string{peerB}) },
 		func(int) { table.PutBackChanges(table.TakeChanges(math.MaxInt)) },
 		func(int) { table.ChangeCount() },
 		func(int) { table.Lookup(prefix.Addr()) },

internal/routewatch/prefixhandler.go

@@ -46,10 +46,19 @@ const (
 	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
+	// peers whose session went down. A peer's messages are held back from
+	// the moment its session goes down until peerDown has applied them. If
+	// peerDown is running at that moment, the peer waits for the rest of
+	// peerDown's current PeersDown call. The peer also waits while peerDown
+	// applies the messages held back from that call's peers, those that
+	// arrive meanwhile included. The next PeersDown call takes the peer out.
+	// Once that call returns, peerDown applies the messages held back from
+	// that call's peers, one peer after another, in the order the call lists
+	// them. A held-back message that says the peer's session went down again
+	// makes the peer's later messages wait for the next call. A message takes
+	// about 1 KiB, so 150,000 take about 150 MiB (README "Memory"). At this
+	// number HandleMessage waits until one of them is applied.
+	maxHeldBackMessages = 150000
 )
 
 // PrefixHandler keeps the routing table in memory current from the feed. It
@@ -58,14 +67,16 @@ 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.
+// The table's PeersDown applies sessions going down. It visits every prefix,
+// which takes seconds on a full table. So the handler runs PeersDown on a
+// separate goroutine and goes on taking messages meanwhile. One PeersDown runs
+// at a time. The next one takes every peer whose session went down since the
+// previous one began. PeersDown lets the table's lock go about half of the
+// time, so the handler applies other peers' messages while PeersDown runs. The
+// handler holds back a down peer's later messages until peerDown applies them,
+// as the doc of maxHeldBackMessages describes. It 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.
@@ -103,20 +114,25 @@ type PrefixHandler struct {
 	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.
+	// Peers whose session went down. heldBack, under mu, holds each such
+	// peer's later messages, in order, until the peer has left every prefix
+	// 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
+	// be applied; its L is &mu.
+	heldBack  map[string][]*ristypes.RISMessage
+	heldCount int
+	heldTaken sync.Cond
+	// downPeers, under mu, lists the peers whose session went down and that
+	// the next PeersDown takes. peerDownRunning, under mu, says whether
+	// peerDown is running. peerDowns counts the peerDown goroutines that have
+	// not returned.
+	downPeers       []string
+	peerDownRunning bool
+	peerDowns       sync.WaitGroup
+	// tablePeersDown is the table's PeersDown. Only peerDown calls it, so a
+	// test may replace it before it hands the handler a message.
+	tablePeersDown func(peers []string)
 }
 
 // seenPrefix is what the batch keeps of an update: the prefix it names and its
@@ -181,6 +197,7 @@ func newPrefixHandler(
 		stopCh:               make(chan context.Context, 1),
 		done:                 make(chan struct{}),
 		heldBack:             make(map[string][]*ristypes.RISMessage),
+		tablePeersDown:       table.PeersDown,
 	}
 	h.batchTaken.L = &h.mu
 	h.heldTaken.L = &h.mu
@@ -212,7 +229,7 @@ func (h *PrefixHandler) QueueCapacity() int {
 }
 
 // HandleMessage processes a message to track prefix information. A message
-// from a peer whose PeerDown has not returned is held back until it has.
+// from a peer whose session went down is held back until peerDown applies it.
 func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 	h.mu.Lock()
 	defer h.mu.Unlock()
@@ -235,7 +252,16 @@ func (h *PrefixHandler) HandleMessage(msg *ristypes.RISMessage) {
 	h.applyMessage(msg)
 }
 
-// Stop waits for every PeerDown the handler started and for the messages held
+// HeldBackCount returns how many messages the handler holds back from peers
+// whose session went down. The status page shows it.
+func (h *PrefixHandler) HeldBackCount() int {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	return h.heldCount
+}
+
+// Stop waits for every PeersDown 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
@@ -290,8 +316,8 @@ func (h *PrefixHandler) Stop(ctx context.Context) {
 }
 
 // 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.
+// has the peer leave every prefix. 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 {
@@ -370,34 +396,64 @@ func (h *PrefixHandler) applyMessage(msg *ristypes.RISMessage) {
 	}
 }
 
-// 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.
+// startPeerDown adds peer to downPeers. It starts peerDown on a new goroutine
+// unless peerDown is running. The caller holds mu. The peer's messages are
+// held back until peerDown has applied them. Messages held back for an earlier
+// session of the peer going down, 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.downPeers = append(h.downPeers, peer)
+
+	if h.peerDownRunning {
+		return
+	}
+
+	h.peerDownRunning = true
 	h.peerDowns.Add(1)
 
-	go h.peerDown(peer)
+	go h.peerDown()
 }
 
-// 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) {
+// peerDown takes every peer in downPeers and runs the table's PeersDown for
+// them. Then it applies the messages held back from each of those peers. It
+// repeats this until downPeers is empty.
+func (h *PrefixHandler) peerDown() {
 	defer h.peerDowns.Done()
 
-	h.peerDownMu.Lock()
-	h.table.PeerDown(peer)
-	h.peerDownMu.Unlock()
+	for {
+		h.mu.Lock()
 
+		peers := h.downPeers
+		h.downPeers = nil
+
+		if len(peers) == 0 {
+			h.peerDownRunning = false
+			h.mu.Unlock()
+
+			return
+		}
+
+		h.mu.Unlock()
+
+		h.tablePeersDown(peers)
+
+		for _, peer := range peers {
+			h.applyHeldBack(peer)
+		}
+	}
+}
+
+// applyHeldBack applies the messages held back from 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 adds the
+// peer to downPeers. The rest stay held back until the next PeersDown has
+// returned.
+func (h *PrefixHandler) applyHeldBack(peer string) {
 	for {
 		h.mu.Lock()
 

internal/routewatch/prefixhandler_internal_test.go

@@ -238,7 +238,7 @@ func TestPrefixHandlerStopLogsNothingWhenItWroteEverything(t *testing.T) {
 }
 
 // TestPrefixHandlerStopLogsTheMessagesHeldBack stops a prefix handler with a
-// deadline that has passed, while a peer's PeerDown is held up and one message
+// deadline that has passed, while a peer's PeersDown 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()
@@ -249,8 +249,8 @@ func TestPrefixHandlerStopLogsTheMessagesHeldBack(t *testing.T) {
 		routetable.New(routetable.Params{}), longerThanAnyTest,
 		maxTableChangesPerWrite)
 
-	h.peerDownMu.Lock()
-	defer h.peerDownMu.Unlock()
+	release := holdUpPeersDown(h)
+	defer close(release)
 
 	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
 	h.HandleMessage(sessionDown())
@@ -268,8 +268,8 @@ func TestPrefixHandlerStopLogsTheMessagesHeldBack(t *testing.T) {
 }
 
 // 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
+// while a peer's PeersDown is held up and an announcement of the peer is held
+// back. Stop must wait. Once the PeersDown is let go and Stop returns, the
 // announcement must be in the database.
 func TestPrefixHandlerStopWaitsForTheMessagesHeldBack(t *testing.T) {
 	t.Parallel()
@@ -278,7 +278,7 @@ func TestPrefixHandlerStopWaitsForTheMessagesHeldBack(t *testing.T) {
 	h := newPrefixHandler(db, logger.New(), routetable.New(routetable.Params{}),
 		longerThanAnyTest, maxTableChangesPerWrite)
 
-	h.peerDownMu.Lock()
+	release := holdUpPeersDown(h)
 	h.HandleMessage(sessionDown())
 	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
 
@@ -291,16 +291,16 @@ func TestPrefixHandlerStopWaitsForTheMessagesHeldBack(t *testing.T) {
 
 	select {
 	case <-stopped:
-		t.Error("Stop returned while a PeerDown was held up")
+		t.Error("Stop returned while a PeersDown was held up")
 	case <-time.After(stillWaitingAfter):
 	}
 
-	h.peerDownMu.Unlock()
+	close(release)
 
 	select {
 	case <-stopped:
 	case <-time.After(stepTimeout):
-		t.Fatalf("Stop did not return within %s of the PeerDown's release",
+		t.Fatalf("Stop did not return within %s of the PeersDown's release",
 			stepTimeout)
 	}
 
@@ -324,16 +324,32 @@ const longerThanAnyTest = time.Hour
 // applies the updates to an empty routing table. It is built without
 // NewPrefixHandler's flush goroutine, so the test flushes itself.
 func prefixHandlerWithoutTimer(store database.Store) *PrefixHandler {
+	table := routetable.New(routetable.Params{})
+
 	return &PrefixHandler{
 		db:                   store,
 		logger:               logger.New(),
-		table:                routetable.New(routetable.Params{}),
+		table:                table,
 		batchMaxSize:         prefixBatchMaxSize,
 		tableChangesPerWrite: maxTableChangesPerWrite,
 		heldBack:             make(map[string][]*ristypes.RISMessage),
+		tablePeersDown:       table.PeersDown,
 	}
 }
 
+// holdUpPeersDown makes each PeersDown of h wait until the returned channel is
+// closed, as another peer's long PeersDown would hold it up. The test calls it
+// before it hands h a message.
+func holdUpPeersDown(h *PrefixHandler) chan struct{} {
+	release := make(chan struct{})
+	h.tablePeersDown = func(peers []string) {
+		<-release
+		h.table.PeersDown(peers)
+	}
+
+	return release
+}
+
 // newTestDatabase opens a database in a new temporary directory. It is closed
 // when the test ends.
 func newTestDatabase(t *testing.T) *database.Database {

internal/routewatch/prefixhandler_table_internal_test.go

@@ -103,23 +103,24 @@ 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
+// TestPrefixHandlerTakesMessagesWhileAPeerGoesDown holds up the PeersDown of a
+// peer whose session went down, as another peer's long PeersDown 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.
+// HeldBackCount must count them. Once the PeersDown calls 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))
+	const heldMessages = 3
 
-	h.peerDownMu.Lock()
+	h := prefixHandlerWithoutTimer(newMockStore())
+	release := holdUpPeersDown(h)
+	h.HandleMessage(peerAnnouncement(testPeerIP, testV4Prefix))
 
 	handled := returnsInTime(func() {
 		for _, msg := range []*ristypes.RISMessage{
@@ -144,10 +145,15 @@ func TestPrefixHandlerTakesMessagesWhileAPeerGoesDown(t *testing.T) {
 		setV4Prefix:   {},
 	})
 
-	h.peerDownMu.Unlock()
+	if h.HeldBackCount() != heldMessages {
+		t.Errorf("HeldBackCount() = %d, want %d", h.HeldBackCount(),
+			heldMessages)
+	}
+
+	close(release)
 
 	if !returnsInTime(h.peerDowns.Wait) {
-		t.Fatalf("the PeerDowns did not return within %s", stepTimeout)
+		t.Fatalf("the PeersDown calls did not return within %s", stepTimeout)
 	}
 
 	checkTableOrigins(t, h.table, map[string]map[int][]string{
@@ -158,21 +164,85 @@ func TestPrefixHandlerTakesMessagesWhileAPeerGoesDown(t *testing.T) {
 	})
 }
 
-// TestPrefixHandlerAppliesMessagesWhilePeerDownRuns gives a peer 50,000
+// TestPrefixHandlerTakesPeersDownTogether has four peers announce testV4Prefix.
+// It holds up the PeersDown of the first peer, whose session goes down.
+// Meanwhile the sessions of the three others go down, and each of them
+// announces otherV4Prefix. Once the first PeersDown is let go, the next one
+// must take the three others together. No peer may then carry testV4Prefix,
+// and the three others must carry otherV4Prefix.
+func TestPrefixHandlerTakesPeersDownTogether(t *testing.T) {
+	t.Parallel()
+
+	others := []string{secondPeerIP, thirdPeerIP, fourthPeerIP}
+	h := prefixHandlerWithoutTimer(newMockStore())
+
+	// calls receives the peers of each PeersDown, with room for one call
+	// for each peer.
+	calls := make(chan []string, len(others)+1)
+	release := make(chan struct{})
+	h.tablePeersDown = func(peers []string) {
+		calls <- slices.Clone(peers)
+
+		<-release
+		h.table.PeersDown(peers)
+	}
+
+	for _, peer := range append([]string{testPeerIP}, others...) {
+		h.HandleMessage(peerAnnouncement(peer, testV4Prefix))
+	}
+
+	h.HandleMessage(sessionDown())
+
+	select {
+	case <-calls:
+	case <-time.After(stepTimeout):
+		t.Fatalf("PeersDown did not begin within %s", stepTimeout)
+	}
+
+	for _, peer := range others {
+		h.HandleMessage(sessionDownOf(peer))
+		h.HandleMessage(peerAnnouncement(peer, otherV4Prefix))
+	}
+
+	close(release)
+
+	if !returnsInTime(h.peerDowns.Wait) {
+		t.Fatalf("the PeersDown calls did not return within %s", stepTimeout)
+	}
+
+	close(calls)
+
+	later := make([][]string, 0, len(calls))
+	for peers := range calls {
+		later = append(later, peers)
+	}
+
+	if !reflect.DeepEqual(later, [][]string{others}) {
+		t.Errorf("the PeersDown calls after the first took %v, want %v "+
+			"together", later, others)
+	}
+
+	checkTableOrigins(t, h.table, map[string]map[int][]string{
+		testV4Prefix:  {},
+		otherV4Prefix: {testASNB: others},
+	})
+}
+
+// TestPrefixHandlerAppliesMessagesWhilePeersDownRuns 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
+// table's PeersDown then visits the prefixes in five batches. Once PeersDown
+// has begun, the handler is handed 1,000 announcements of another peer. The
+// handler must apply them all before PeersDown ends, while the down peer still
 // carries some of its prefixes.
-func TestPrefixHandlerAppliesMessagesWhilePeerDownRuns(t *testing.T) {
+func TestPrefixHandlerAppliesMessagesWhilePeersDownRuns(t *testing.T) {
 	t.Parallel()
 
 	const (
-		// downPeerPrefixes is five times the 10,000 prefixes PeerDown visits
+		// downPeerPrefixes is five times the 10,000 prefixes PeersDown 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
+		// while PeersDown runs, if PeersDown 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.
@@ -184,7 +254,7 @@ func TestPrefixHandlerAppliesMessagesWhilePeerDownRuns(t *testing.T) {
 
 	h.HandleMessage(sessionDown())
 
-	// Wait until PeerDown has begun: the down peer has left some of its
+	// Wait until PeersDown 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 {
@@ -207,15 +277,76 @@ func TestPrefixHandlerAppliesMessagesWhilePeerDownRuns(t *testing.T) {
 	}
 
 	if counts.IPv4Routes == 0 {
-		t.Errorf("PeerDown ended before the handler applied the other peer's "+
+		t.Errorf("PeersDown 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)
+		t.Fatalf("the PeersDown did not return within %s", stepTimeout)
 	}
 }
 
+// TestPrefixHandlerTakesMessagesWhileAReturningPeerSendsItsTable gives a peer
+// 50,000 prefixes and hands the prefix handler that peer's session going down.
+// Once PeersDown has begun, the peer comes back and sends more announcements
+// than the handler's queue holds, as a peer that sends its whole table again
+// does. The handler must hold them back and go on taking messages. Another
+// peer's announcement, handed next, must be applied before PeersDown ends.
+func TestPrefixHandlerTakesMessagesWhileAReturningPeerSendsItsTable(t *testing.T) {
+	t.Parallel()
+
+	const (
+		// downPeerPrefixes is five times the 10,000 prefixes PeersDown visits
+		// in one hold of the table's lock.
+		downPeerPrefixes  = 50000
+		returningMessages = prefixHandlerQueueSize + 1
+	)
+
+	table := routetable.New(routetable.Params{})
+	announceManyPrefixes(table, downPeerPrefixes)
+
+	h := newPrefixHandler(newMockStore(), logger.New(), table,
+		longerThanAnyTest, maxTableChangesPerWrite)
+	defer stopInTime(t, h)
+
+	h.HandleMessage(sessionDown())
+
+	// Wait until PeersDown has begun: the down peer has left some of its
+	// prefixes, which are all IPv4.
+	for h.table.Counts().IPv4Routes == downPeerPrefixes {
+		runtime.Gosched()
+	}
+
+	returning := peerAnnouncement(testPeerIP, testV6Prefix)
+
+	if !returnsInTime(func() {
+		for range returningMessages {
+			h.HandleMessage(returning)
+		}
+
+		h.HandleMessage(peerAnnouncement(secondPeerIP, testV6Prefix))
+	}) {
+		t.Fatalf("the handler did not take the messages within %s", stepTimeout)
+	}
+
+	if h.table.Counts().IPv4Routes == 0 {
+		t.Error("PeersDown ended before the handler took the other peer's " +
+			"announcement")
+	}
+
+	checkTableOrigins(t, h.table, map[string]map[int][]string{
+		testV6Prefix: {testASNB: {secondPeerIP}},
+	})
+
+	if !returnsInTime(h.peerDowns.Wait) {
+		t.Fatalf("the PeersDown did not return within %s", stepTimeout)
+	}
+
+	checkTableOrigins(t, h.table, map[string]map[int][]string{
+		testV6Prefix: {testASNB: {testPeerIP, secondPeerIP}},
+	})
+}
+
 // 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
@@ -237,14 +368,14 @@ func TestPrefixHandlerTakesMessagesWhileItAppliesHeldBackOnes(t *testing.T) {
 
 	defer stopInTime(t, h)
 
-	h.peerDownMu.Lock()
+	release := holdUpPeersDown(h)
 	h.HandleMessage(sessionDown())
 
 	for range heldMessages {
 		h.HandleMessage(peerAnnouncement(testPeerIP, badPrefix))
 	}
 
-	h.peerDownMu.Unlock()
+	close(release)
 
 	select {
 	case <-logs.firstWrite:
@@ -268,10 +399,10 @@ func TestPrefixHandlerTakesMessagesWhileItAppliesHeldBackOnes(t *testing.T) {
 	}
 }
 
-// TestPrefixHandlerWaitsAtTheMaximumHeldBack holds up the PeerDown of a peer
+// TestPrefixHandlerWaitsAtTheMaximumHeldBack holds up the PeersDown 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.
+// message, until the PeersDown returns and a held-back message is applied.
 func TestPrefixHandlerWaitsAtTheMaximumHeldBack(t *testing.T) {
 	t.Parallel()
 
@@ -280,7 +411,7 @@ func TestPrefixHandlerWaitsAtTheMaximumHeldBack(t *testing.T) {
 		maxTableChangesPerWrite)
 	defer stopInTime(t, h)
 
-	h.peerDownMu.Lock()
+	release := holdUpPeersDown(h)
 	h.HandleMessage(sessionDown())
 
 	held := peerAnnouncement(testPeerIP, testV6Prefix)
@@ -302,12 +433,12 @@ func TestPrefixHandlerWaitsAtTheMaximumHeldBack(t *testing.T) {
 	case <-time.After(stillWaitingAfter):
 	}
 
-	h.peerDownMu.Unlock()
+	close(release)
 
 	select {
 	case <-returned:
 	case <-time.After(stepTimeout):
-		t.Fatal("HandleMessage did not return after the PeerDown returned")
+		t.Fatal("HandleMessage did not return after the PeersDown returned")
 	}
 }
 
@@ -424,7 +555,7 @@ func TestPrefixHandlerKeepsThePathOfAPathEndingInASet(t *testing.T) {
 }
 
 // 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
+// checks the handler's routing table once the handler's PeersDown calls have
 // returned, with no flush.
 func checkTableCases(t *testing.T, tests []tableCase) {
 	t.Helper()

internal/server/handlers.go

@@ -486,6 +486,7 @@ type HandlerStatsInfo struct {
 	QueueHighWaterMark int     `json:"queue_high_water_mark"`
 	ProcessedCount     uint64  `json:"processed_count"`
 	DroppedCount       uint64  `json:"dropped_count"`
+	HeldBackCount      int     `json:"held_back_count"`
 	AvgProcessTimeMs   float64 `json:"avg_process_time_ms"`
 	MinProcessTimeMs   float64 `json:"min_process_time_ms"`
 	MaxProcessTimeMs   float64 `json:"max_process_time_ms"`
@@ -570,6 +571,7 @@ func handlerStatsInfo(handlerStats []streamer.HandlerStats) []HandlerStatsInfo {
 			QueueHighWaterMark: hs.QueueHighWaterMark,
 			ProcessedCount:     hs.ProcessedCount,
 			DroppedCount:       hs.DroppedCount,
+			HeldBackCount:      hs.HeldBackCount,
 			AvgProcessTimeMs: float64(hs.AvgProcessTime.Microseconds()) /
 				microsecondsPerMillisecond,
 			MinProcessTimeMs: float64(hs.MinProcessTime.Microseconds()) /

internal/server/handlers_internal_test.go

@@ -295,6 +295,43 @@ func TestStatsAnswerCarriesChurnPerMinute(t *testing.T) {
 	}
 }
 
+// heldBackMessages is how many messages heldBackHandler holds back.
+const heldBackMessages = 7
+
+// heldBackHandler is a stream handler that wants no message and holds back
+// heldBackMessages messages.
+type heldBackHandler struct{}
+
+func (heldBackHandler) WantsMessage(string) bool           { return false }
+func (heldBackHandler) HandleMessage(*ristypes.RISMessage) {}
+func (heldBackHandler) QueueCapacity() int                 { return 1 }
+func (heldBackHandler) HeldBackCount() int                 { return heldBackMessages }
+
+// TestStatsAnswerGivesHowManyMessagesAHandlerHoldsBack registers a handler
+// that holds back messages. Its figures in /api/v1/stats, which the status
+// page shows, must give how many.
+func TestStatsAnswerGivesHowManyMessagesAHandlerHoldsBack(t *testing.T) {
+	t.Parallel()
+
+	s := newServerWithTestRoutes(t)
+	s.streamer.RegisterHandler(heldBackHandler{})
+
+	var body struct {
+		Data StatsResponse `json:"data"`
+	}
+
+	err := json.Unmarshal(get(t, s, statsPath).Body.Bytes(), &body)
+	if err != nil {
+		t.Fatalf("decoding the answer: %v", err)
+	}
+
+	stats := body.Data.HandlerStats
+	if len(stats) != 1 || stats[0].HeldBackCount != heldBackMessages {
+		t.Errorf("handler figures %+v, want one handler holding back %d "+
+			"messages", stats, heldBackMessages)
+	}
+}
+
 // fixedIntervalASNFetcher is a WHOIS fetcher that does nothing and reports
 // interval as its wait between fetches.
 type fixedIntervalASNFetcher struct {

internal/streamer/streamer.go

@@ -351,6 +351,10 @@ type HandlerStats struct {
 	AvgProcessTime     time.Duration
 	MinProcessTime     time.Duration
 	MaxProcessTime     time.Duration
+	// HeldBackCount is how many messages the handler holds back to apply
+	// later: what its HeldBackCount method returns, or 0 for a handler
+	// without one.
+	HeldBackCount int
 }
 
 // GetHandlerStats returns a snapshot of performance statistics for all registered
@@ -392,6 +396,11 @@ func (s *Streamer) GetHandlerStats() []HandlerStats {
 
 		info.metrics.mu.Unlock()
 
+		holder, holdsBack := info.handler.(interface{ HeldBackCount() int })
+		if holdsBack {
+			hs.HeldBackCount = holder.HeldBackCount()
+		}
+
 		stats = append(stats, hs)
 	}
 

internal/templates/status.html

@@ -541,6 +541,10 @@
                         <span class="metric-label">Dropped</span>
                         <span class="metric-value ${handler.dropped_count > 0 ? 'disconnected' : ''}">${formatNumber(handler.dropped_count)}</span>
                     </div>
+                    <div class="metric">
+                        <span class="metric-label">Held Back</span>
+                        <span class="metric-value">${formatNumber(handler.held_back_count)}</span>
+                    </div>
                     <div class="metric">
                         <span class="metric-label">Avg Time</span>
                         <span class="metric-value">${formatProcessingTime(handler.avg_process_time_ms)}</span>