sweeper: avoid deadlock on shutdown

sweep/sweeper.go

@@ -154,6 +154,7 @@ type inputCluster struct {
 // attempting to sweep.
 type pendingSweepsReq struct {
 	respChan chan map[wire.OutPoint]*PendingInput
+	errChan  chan error
 }
 
 // PendingInput contains information about an input that is currently being
@@ -381,6 +382,33 @@ func (s *UtxoSweeper) Start() error {
 		defer s.wg.Done()
 
 		s.collector(blockEpochs.Epochs)
+
+		// The collector exited and won't longer handle incoming
+		// requests. This can happen on shutdown, when the block
+		// notifier shuts down before the sweeper and its clients. In
+		// order to not deadlock the clients waiting for their requests
+		// being handled, we handle them here and immediately return an
+		// error. When the sweeper finally is shut down we can exit as
+		// the clients will be notified.
+		for {
+			select {
+			case inp := <-s.newInputs:
+				inp.resultChan <- Result{
+					Err: ErrSweeperShuttingDown,
+				}
+
+			case req := <-s.pendingSweepsReqs:
+				req.errChan <- ErrSweeperShuttingDown
+
+			case req := <-s.updateReqs:
+				req.responseChan <- &updateResp{
+					err: ErrSweeperShuttingDown,
+				}
+
+			case <-s.quit:
+				return
+			}
+		}
 	}()
 
 	return nil
@@ -1290,9 +1318,11 @@ func (s *UtxoSweeper) waitForSpend(outpoint wire.OutPoint,
 // attempting to sweep.
 func (s *UtxoSweeper) PendingInputs() (map[wire.OutPoint]*PendingInput, error) {
 	respChan := make(chan map[wire.OutPoint]*PendingInput, 1)
+	errChan := make(chan error, 1)
 	select {
 	case s.pendingSweepsReqs <- &pendingSweepsReq{
 		respChan: respChan,
+		errChan:  errChan,
 	}:
 	case <-s.quit:
 		return nil, ErrSweeperShuttingDown
@@ -1301,6 +1331,8 @@ func (s *UtxoSweeper) PendingInputs() (map[wire.OutPoint]*PendingInput, error) {
 	select {
 	case pendingSweeps := <-respChan:
 		return pendingSweeps, nil
+	case err := <-errChan:
+		return nil, err
 	case <-s.quit:
 		return nil, ErrSweeperShuttingDown
 	}

sweep/sweeper_test.go

@@ -2218,3 +2218,44 @@ func TestRequiredTxOuts(t *testing.T) {
 		})
 	}
 }
+
+// TestSweeperShutdownHandling tests that we notify callers when the sweeper
+// cannot handle requests since it's in the process of shutting down.
+func TestSweeperShutdownHandling(t *testing.T) {
+	ctx := createSweeperTestContext(t)
+
+	// Make the backing notifier break down. This is what happens during
+	// lnd shut down, since the notifier is stopped before the sweeper.
+	require.Len(t, ctx.notifier.epochChan, 1)
+	for epochChan := range ctx.notifier.epochChan {
+		close(epochChan)
+	}
+
+	// Give the collector some time to exit.
+	time.Sleep(50 * time.Millisecond)
+
+	// Now trying to sweep inputs should return an error on the error
+	// channel.
+	resultChan, err := ctx.sweeper.SweepInput(
+		spendableInputs[0], defaultFeePref,
+	)
+	require.NoError(t, err)
+
+	select {
+	case res := <-resultChan:
+		require.Equal(t, ErrSweeperShuttingDown, res.Err)
+
+	case <-time.After(defaultTestTimeout):
+		t.Fatalf("no result arrived")
+	}
+
+	// Stop the sweeper properly.
+	err = ctx.sweeper.Stop()
+	require.NoError(t, err)
+
+	// Now attempting to sweep an input should error out immediately.
+	_, err = ctx.sweeper.SweepInput(
+		spendableInputs[0], defaultFeePref,
+	)
+	require.Error(t, err)
+}