fix: use a unique channel name per latency measurement (#27040)

coderd/database/pubsub/latency.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	"github.com/google/uuid"
@@ -18,6 +19,10 @@ type LatencyMeasurer struct {
 	// Create unique pubsub channel names so that multiple coderd replicas do not clash when performing latency measurements.
 	channel uuid.UUID
 	logger  slog.Logger
+	// seq distinguishes consecutive measurements from each other so that a
+	// subscription whose teardown is still in flight cannot receive (and
+	// count) the next measurement's message.
+	seq atomic.Int64
 }
 
 // LatencyMessageLength is the length of a UUIDv4 encoded to hex.
@@ -40,7 +45,8 @@ func (lm *LatencyMeasurer) Measure(ctx context.Context, p Pubsub) (send, recv ti
 	msg := []byte(uuid.New().String())
 	lm.logger.Debug(ctx, "performing measurement", slog.F("msg", msg))
 
-	cancel, err := p.Subscribe(lm.latencyChannelName(), func(ctx context.Context, in []byte) {
+	channel := lm.nextChannelName()
+	cancel, err := p.Subscribe(channel, func(ctx context.Context, in []byte) {
 		if !bytes.Equal(in, msg) {
 			lm.logger.Warn(ctx, "received unexpected message", slog.F("got", in), slog.F("expected", msg))
 			return
@@ -54,7 +60,7 @@ func (lm *LatencyMeasurer) Measure(ctx context.Context, p Pubsub) (send, recv ti
 	defer cancel()
 
 	start = time.Now()
-	err = p.Publish(lm.latencyChannelName(), msg)
+	err = p.Publish(channel, msg)
 	if err != nil {
 		return -1, -1, xerrors.Errorf("failed to publish: %w", err)
 	}
@@ -69,6 +75,12 @@ func (lm *LatencyMeasurer) Measure(ctx context.Context, p Pubsub) (send, recv ti
 	}
 }
 
-func (lm *LatencyMeasurer) latencyChannelName() string {
-	return fmt.Sprintf("latency-measure:%s", lm.channel)
+// nextChannelName returns a channel name unique to this measurement.
+// Uniqueness across replicas comes from the channel UUID; uniqueness
+// across consecutive measurements of the same replica comes from the
+// sequence number. The name must stay within Postgres's 63-byte
+// identifier limit: 16 (prefix) + 36 (UUID) + 1 (dot) leaves 10 digits
+// for the sequence.
+func (lm *LatencyMeasurer) nextChannelName() string {
+	return fmt.Sprintf("latency-measure:%s.%d", lm.channel, lm.seq.Add(1))
 }