core/storage: hijack connections for notification listeners (#4806)

pkg/storage/postgres/backend.go

@@ -74,42 +74,9 @@ func New(dsn string, options ...Option) *Backend {
 		return nil
 	}, backend.cfg.registryTTL/2)
 
-	// listen for changes and broadcast them via signals
-	for _, row := range []struct {
-		signal  *signal.Signal
-		channel string
-	}{
-		{backend.onRecordChange, recordChangeNotifyName},
-		{backend.onServiceChange, serviceChangeNotifyName},
-	} {
-		sig, ch := row.signal, row.channel
 	go backend.doPeriodically(func(ctx context.Context) error {
-			_, pool, err := backend.init(backend.closeCtx)
-			if err != nil {
-				return err
-			}
-
-			conn, err := pool.Acquire(ctx)
-			if err != nil {
-				return err
-			}
-			defer conn.Release()
-
-			_, err = conn.Exec(ctx, `LISTEN `+ch)
-			if err != nil {
-				return err
-			}
-
-			_, err = conn.Conn().WaitForNotification(ctx)
-			if err != nil {
-				return err
-			}
-
-			sig.Broadcast(ctx)
-
-			return nil
+		return backend.listenForNotifications(ctx)
 	}, time.Millisecond*100)
-	}
 
 	return backend
 }
@@ -433,6 +400,46 @@ func (backend *Backend) doPeriodically(f func(ctx context.Context) error, dur ti
 	}
 }
 
+func (backend *Backend) listenForNotifications(ctx context.Context) error {
+	_, pool, err := backend.init(ctx)
+	if err != nil {
+		return fmt.Errorf("error initializing pool for notifications: %w", err)
+	}
+
+	poolConn, err := pool.Acquire(ctx)
+	if err != nil {
+		return fmt.Errorf("error acquiring connection from pool for notifications: %w", err)
+	}
+
+	// hijack the connection so the pool can be left for short-lived queries
+	// and so that LISTENs don't leak to other queries
+	conn := poolConn.Hijack()
+	defer conn.Close(ctx)
+
+	for _, ch := range []string{recordChangeNotifyName, serviceChangeNotifyName} {
+		_, err = conn.Exec(ctx, `LISTEN `+ch)
+		if err != nil {
+			return fmt.Errorf("error listening on channel %s for notifications: %w", ch, err)
+		}
+	}
+
+	// for each notification broadcast the signal
+	for {
+		n, err := conn.WaitForNotification(ctx)
+		if err != nil {
+			// on error we'll close the connection to stop listening
+			return fmt.Errorf("error receiving notification: %w", err)
+		}
+
+		switch n.Channel {
+		case recordChangeNotifyName:
+			backend.onRecordChange.Broadcast(ctx)
+		case serviceChangeNotifyName:
+			backend.onServiceChange.Broadcast(ctx)
+		}
+	}
+}
+
 // ParseConfig parses a DSN into a pgxpool.Config.
 func ParseConfig(dsn string) (*pgxpool.Config, error) {
 	config, err := pgxpool.ParseConfig(dsn)

pkg/storage/postgres/backend_test.go

@@ -195,6 +195,9 @@ func TestBackend(t *testing.T) {
 			storagetest.TestBackendPatch(t, ctx, backend)
 		})
 
+		assert.Equal(t, int32(0), backend.pool.Stat().AcquiredConns(),
+			"acquired connections should be released")
+
 		return nil
 	}))
 }