sweeper: avoid deadlock on shutdown

We risked deadlocking on shutdown if a client (in our case a contract
resolver) attempted to schedule a sweep of an input after the
ChainNotifier had been shut down. This would cause the `collector`
goroutine to exit, and not handle incoming requests, causing a deadlock
(since the ChainArbitrator is being stopped before the Sweeper in the
server).

To fix this we could change the order these subsystems are stopped, but
this doesn't ensure there aren't other clients that could end up in the
same deadlock scenario. So instead we keep handling the incoming
requests even after the collector has exited (immediatly returning an
error), until the sweeper is signalled to shutdown.

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)
+}