lndclient: block until chain notifier is ready

lnd_services.go

@@ -143,6 +143,11 @@ type LndServicesConfig struct {
 	// block download is still in progress.
 	BlockUntilChainSynced bool
 
+	// BlockUntilChainNotifier indicates that the client should wait until
+	// the ChainNotifier RPC is accepting subscriptions. This requires lnd
+	// to be built with the "chainrpc" tag.
+	BlockUntilChainNotifier bool
+
 	// BlockUntilUnlocked denotes that the NewLndServices function should
 	// block until lnd is unlocked.
 	BlockUntilUnlocked bool
@@ -453,6 +458,25 @@ func NewLndServices(cfg *LndServicesConfig) (*GrpcLndServices, error) {
 		log.Infof("lnd is now fully synced to its chain backend")
 	}
 
+	// If requested, wait until the chain notifier RPC is ready before we
+	// return. This ensures sub-servers relying on the notifier don't fail
+	// during startup.
+	if cfg.BlockUntilChainNotifier {
+		log.Infof("Waiting for chain notifier RPC to be ready")
+
+		err := services.waitForChainNotifier(
+			cfg.CallerCtx, timeout, cfg.ChainSyncPollInterval,
+		)
+		if err != nil {
+			cleanup()
+
+			return nil, fmt.Errorf("error waiting for chain "+
+				"notifier readiness: %w", err)
+		}
+
+		log.Infof("Chain notifier RPC is ready")
+	}
+
 	return services, nil
 }
 
@@ -533,6 +557,77 @@ func (s *GrpcLndServices) waitForChainSync(ctx context.Context,
 	return <-update
 }
 
+// waitForChainNotifier blocks until the ChainNotifier RPC accepts block epoch
+// subscriptions and delivers at least one block height.
+func (s *GrpcLndServices) waitForChainNotifier(ctx context.Context,
+	timeout, pollInterval time.Duration) error {
+
+	register := s.ChainNotifier.RegisterBlockEpochNtfn
+
+	var errRetry = errors.New("retry RegisterBlockEpochNtfn")
+
+	// attempt is a single attempt to make a RegisterBlockEpochNtfn call.
+	// It returns nil on success, errRetry if another retry is needed and
+	// other error in case of a final error.
+	attempt := func() error {
+		subCtx, cancel := context.WithTimeout(ctx, timeout)
+		defer cancel()
+
+		// Make new RegisterBlockEpochNtfn call.
+		blockChan, errChan, err := register(subCtx)
+		if err != nil {
+			return fmt.Errorf("register block epoch ntfn: %w", err)
+		}
+
+		// Wait for block height notification, which indicates success.
+		select {
+		case <-subCtx.Done():
+			return subCtx.Err()
+
+		case err := <-errChan:
+			// If chainNotifier is not ready yet, retry.
+			if isChainNotifierStartingErr(err) {
+				select {
+				case <-time.After(pollInterval):
+					return errRetry
+
+				case <-ctx.Done():
+					return ctx.Err()
+				}
+			}
+
+			return err
+
+		// We got a block height. Success!
+		case <-blockChan:
+			return nil
+		}
+	}
+
+	// Main retry loop.
+	for {
+		log.Info("Trying to make RegisterBlockEpochNtfn and receive " +
+			"the current height...")
+		err := attempt()
+
+		if errors.Is(err, errRetry) {
+			log.Info("ChainNotifier is not ready yet. Will retry")
+
+			continue
+		} else if err != nil {
+			log.Info("RegisterBlockEpochNtfn returned unexpected "+
+				"error %v. LND client failed to start!", err)
+
+			return err
+		}
+
+		log.Info("RegisterBlockEpochNtfn returned a height. Success!")
+		break
+	}
+
+	return nil
+}
+
 // getLndInfo queries lnd for information about the node it is connected to.
 // If the waitForUnlocked boolean is set, it will examine any errors returned
 // and back off if the failure is due to lnd currently being locked. Otherwise,
@@ -676,6 +771,37 @@ func IsUnlockError(err error) bool {
 	return false
 }
 
+// chainNotifierStartupMessage matches the error string returned by lnd
+// v0.20.0-rc3+ when a ChainNotifier RPC is invoked before the sub-server
+// finishes initialization.
+const chainNotifierStartupMessage = "chain notifier RPC is still in the " +
+	"process of starting"
+
+// isChainNotifierStartingErr reports whether err is due to the lnd
+// ChainNotifier sub-server still starting up. Starting with lnd v0.20.0-rc3
+// the notifier is initialised later in the daemon lifecycle, and the RPC layer
+// surfaces this as an Unknown gRPC status that contains the message defined in
+// chainNotifierStartupMessage. There is a PR in LND to return code Unavailable
+// instead of Unknown: https://github.com/lightningnetwork/lnd/pull/10352
+func isChainNotifierStartingErr(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	// gRPC code Unavailable means "the server can't handle this request
+	// now, retry later". LND's chain notifier returns this error when
+	// the server is starting.
+	// See https://github.com/lightningnetwork/lnd/pull/10352
+	st, ok := status.FromError(err)
+	if ok && st.Code() == codes.Unavailable {
+		return true
+	}
+
+	// TODO(ln-v0.20.0) remove the string fallback once lndclient depends on
+	// a version of lnd that returns codes.Unavailable for this condition.
+	return strings.Contains(err.Error(), chainNotifierStartupMessage)
+}
+
 // checkLndCompatibility makes sure the connected lnd instance is running on the
 // correct network, has the version RPC implemented, is the correct minimal
 // version and supports all required build tags/subservers.

lnd_services_test.go

@@ -361,3 +361,27 @@ func TestCustomMacaroonHex(t *testing.T) {
 	_, err = NewLndServices(testCfg)
 	require.Error(t, err, "must set only one")
 }
+
+// TestIsChainNotifierStartingErr ensures we correctly detect the startup lag
+// error returned by lnd v0.20.0-rc3+.
+func TestIsChainNotifierStartingErr(t *testing.T) {
+	t.Parallel()
+
+	require.True(t, isChainNotifierStartingErr(
+		status.Error(codes.Unavailable, chainNotifierStartupMessage),
+	))
+
+	require.True(t, isChainNotifierStartingErr(
+		status.Error(codes.Unknown, chainNotifierStartupMessage),
+	))
+
+	require.True(t, isChainNotifierStartingErr(
+		status.Error(codes.Unavailable, "some other error"),
+	))
+
+	require.False(t, isChainNotifierStartingErr(nil))
+
+	require.False(t, isChainNotifierStartingErr(
+		status.Error(codes.Unknown, "some other error"),
+	))
+}