Make a timed batch write on every tick (closes #192)

The prefix, AS and peer handlers skipped a timed write unless one
interval had passed since the last write ended. The next tick came a
little less than one interval after that, so every other tick was
skipped. Each tick now writes. A full batch's write restarts the timer,
so the next timed write comes one interval after that write began.
Comparing clock readings would not do: two ticks one interval apart can
read a hair under it. flushBatch no longer reports whether it wrote a
batch, since nothing reads that. A new test runs the loop on a fake
clock and fails on the old code.

Model: opus-5-5

TODO.md

@@ -33,6 +33,10 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: the prefix, AS and peer handlers make a timed write on every tick
+  of their timers, every 1, 2 and 2 seconds. They used to skip every other tick.
+  A full batch's write restarts the timer, so the next timed write comes one
+  interval after that write began (closes #192)
 - 2026-10-27: once the stop deadline has passed, the AS, peer and peering
   handlers start no write either. A full batch or a tick of the timer that
   waited while a write ran past the deadline could still start one. The flush

internal/routewatch/ashandler.go

@@ -202,11 +202,11 @@ func (h *ASHandler) takeBatch() []asnOp {
 }
 
 // flushBatch takes the pending batch and writes it to the database, without
-// holding the mutex during the write. It reports whether there was a batch.
-func (h *ASHandler) flushBatch() bool {
+// holding the mutex during the write.
+func (h *ASHandler) flushBatch() {
 	batch := h.takeBatch()
 	if len(batch) == 0 {
-		return false
+		return
 	}
 
 	// The earliest and the latest time the batch saw each AS. The batch holds
@@ -229,6 +229,4 @@ func (h *ASHandler) flushBatch() bool {
 	h.mu.Lock()
 	h.writing = 0
 	h.mu.Unlock()
-
-	return true
 }

internal/routewatch/batchwrite.go

@@ -6,27 +6,26 @@ import (
 )
 
 // flushLoop is the flush goroutine of the prefix, AS and peer handlers. It is
-// the only goroutine that writes a handler's batch. flushBatch takes the batch,
-// writes it, and reports whether there was one.
+// the only goroutine that writes a handler's batch. flushBatch takes the batch
+// and writes it.
 //
-// flushLoop calls flushBatch when batchFullCh wakes it. It also calls it every
-// batchTimeout, unless it wrote a batch less than batchTimeout ago. When stopCh
-// hands it Stop's context, it calls flushBatch once more, unless the stop
-// deadline has passed. Then it closes done and returns.
+// flushLoop calls flushBatch on each tick of a timer, every batchTimeout. It
+// also calls it at once when batchFullCh wakes it. A full batch's write
+// restarts the timer, so the next tick comes batchTimeout after that write
+// began. When stopCh hands it Stop's context, it calls flushBatch once more,
+// unless the stop deadline has passed. Then it closes done and returns.
 func flushLoop(
 	batchTimeout time.Duration,
 	batchFullCh <-chan struct{},
 	stopCh <-chan context.Context,
 	done chan<- struct{},
-	flushBatch func() bool,
+	flushBatch func(),
 ) {
 	defer close(done)
 
 	ticker := time.NewTicker(batchTimeout)
 	defer ticker.Stop()
 
-	lastFlush := time.Now()
-
 	for {
 		// Stop goes first. A write can run past the stop deadline while a full
 		// batch's wake-up or a tick waits beside Stop's context. The select
@@ -45,13 +44,12 @@ func flushLoop(
 
 		select {
 		case <-ticker.C:
-			if time.Since(lastFlush) >= batchTimeout && flushBatch() {
-				lastFlush = time.Now()
-			}
+			flushBatch()
 		case <-batchFullCh:
-			if flushBatch() {
-				lastFlush = time.Now()
-			}
+			// Reset also drops a tick that waits in ticker.C, so no timed
+			// write follows this one at once.
+			ticker.Reset(batchTimeout)
+			flushBatch()
 		case ctx := <-stopCh:
 			// Final flush, unless the stop deadline has passed
 			if ctx.Err() == nil {

internal/routewatch/batchwrite_internal_test.go

@@ -9,6 +9,7 @@ import (
 	"slices"
 	"sync"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	"git.eeqj.de/sneak/routewatch/internal/database"
@@ -27,6 +28,58 @@ const (
 	stillWaitingAfter = 100 * time.Millisecond
 )
 
+// TestFlushLoopWritesOnEveryTick runs flushLoop in a synctest bubble, whose
+// clock is fake: time.Sleep there moves the clock on and returns at once. Each
+// write takes a tenth of the interval. Two timed writes must start one interval
+// apart. A full batch's write, which starts as the second timed write ends,
+// must be followed by a timed write one interval after it began.
+func TestFlushLoopWritesOnEveryTick(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		const (
+			interval  = time.Second
+			writeTime = 100 * time.Millisecond
+		)
+
+		writeStarts := make(chan time.Time)
+		batchFullCh := make(chan struct{}, 1)
+		stopCh := make(chan context.Context, 1)
+		done := make(chan struct{})
+
+		go func() {
+			flushLoop(interval, batchFullCh, stopCh, done, func() {
+				writeStarts <- time.Now()
+
+				time.Sleep(writeTime)
+			})
+		}()
+
+		first := <-writeStarts
+		second := <-writeStarts
+
+		if gap := second.Sub(first); gap != interval {
+			t.Errorf("the second timed write started %s after the first, "+
+				"want %s", gap, interval)
+		}
+
+		batchFullCh <- struct{}{}
+
+		full := <-writeStarts
+		timed := <-writeStarts
+
+		if gap := timed.Sub(full); gap != interval {
+			t.Errorf("the timed write after a full batch's started %s after "+
+				"it, want %s", gap, interval)
+		}
+
+		stopCh <- t.Context()
+
+		<-writeStarts // Stop's last write
+		<-done
+	})
+}
+
 // blockingStore is a mockStore whose batch writes for the AS, peer and prefix
 // handlers, delete of a peer's routes and peering write wait until the test
 // calls releaseWrites. writeStarted is closed when the first of them starts. It

internal/routewatch/peerhandler.go

@@ -200,11 +200,11 @@ func (h *PeerHandler) takeBatch() []peerUpdate {
 }
 
 // flushBatch takes the pending batch and writes it to the database, without
-// holding the mutex during the write. It reports whether there was a batch.
-func (h *PeerHandler) flushBatch() bool {
+// holding the mutex during the write.
+func (h *PeerHandler) flushBatch() {
 	batch := h.takeBatch()
 	if len(batch) == 0 {
-		return false
+		return
 	}
 
 	// The earliest and the latest time the batch saw each peer, with the peer
@@ -234,6 +234,4 @@ func (h *PeerHandler) flushBatch() bool {
 	h.mu.Lock()
 	h.writing = 0
 	h.mu.Unlock()
-
-	return true
 }

internal/routewatch/prefixhandler.go

@@ -352,13 +352,12 @@ func (h *PrefixHandler) takeBatch() ([]prefixUpdate, []string) {
 
 // flushBatch writes the routing table's changes, then takes the pending batch
 // and writes it to the database, without holding the mutex during the writes.
-// It reports whether there was a batch, or a peer down to delete the routes of.
-func (h *PrefixHandler) flushBatch() bool {
+func (h *PrefixHandler) flushBatch() {
 	h.writeTableChanges()
 
 	batch, peersDown := h.takeBatch()
 	if len(batch) == 0 && len(peersDown) == 0 {
-		return false
+		return
 	}
 
 	startTime := time.Now()
@@ -441,8 +440,6 @@ func (h *PrefixHandler) flushBatch() bool {
 		"success", successCount,
 		"duration_ms", elapsed.Milliseconds(),
 	)
-
-	return true
 }
 
 // applyToTable applies update to the routing table in memory: as an