htlcswitch: extend ChannelLink interface with ShutdownIfChannelClean

This allows a caller to ensure to optimistically shut down the link
if the channel is clean. If the channel is not clean, an error is
returned and the link continues functioning as normal. The caller
should also call RemoveLink to ensure that the link isn't seen as
usable within the switch.

htlcswitch/link_test.go

@@ -6532,6 +6532,91 @@ func TestPendingCommitTicker(t *testing.T) {
 	}
 }
 
+// TestShutdownIfChannelClean tests that a link will exit the htlcManager loop
+// if and only if the underlying channel state is clean.
+func TestShutdownIfChannelClean(t *testing.T) {
+	t.Parallel()
+
+	const chanAmt = btcutil.SatoshiPerBitcoin * 5
+	const chanReserve = btcutil.SatoshiPerBitcoin * 1
+	aliceLink, bobChannel, batchTicker, start, cleanUp, _, err :=
+		newSingleLinkTestHarness(chanAmt, chanReserve)
+	require.NoError(t, err)
+
+	var (
+		coreLink  = aliceLink.(*channelLink)
+		aliceMsgs = coreLink.cfg.Peer.(*mockPeer).sentMsgs
+	)
+
+	shutdownAssert := func(expectedErr error) {
+		err = aliceLink.ShutdownIfChannelClean()
+		if expectedErr != nil {
+			require.Error(t, err, expectedErr)
+		} else {
+			require.NoError(t, err)
+		}
+	}
+
+	err = start()
+	require.NoError(t, err)
+	defer cleanUp()
+
+	ctx := linkTestContext{
+		t:          t,
+		aliceLink:  aliceLink,
+		bobChannel: bobChannel,
+		aliceMsgs:  aliceMsgs,
+	}
+
+	// First send an HTLC from Bob to Alice and assert that the link can't
+	// be shutdown while the update is outstanding.
+	htlc := generateHtlc(t, coreLink, 0)
+
+	// <---add-----
+	ctx.sendHtlcBobToAlice(htlc)
+	// <---sig-----
+	ctx.sendCommitSigBobToAlice(1)
+	// ----rev---->
+	ctx.receiveRevAndAckAliceToBob()
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// ----sig---->
+	ctx.receiveCommitSigAliceToBob(1)
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// <---rev-----
+	ctx.sendRevAndAckBobToAlice()
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// ---settle-->
+	ctx.receiveSettleAliceToBob()
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// ----sig---->
+	ctx.receiveCommitSigAliceToBob(0)
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// <---rev-----
+	ctx.sendRevAndAckBobToAlice()
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// <---sig-----
+	ctx.sendCommitSigBobToAlice(0)
+	shutdownAssert(ErrLinkFailedShutdown)
+
+	// ----rev---->
+	ctx.receiveRevAndAckAliceToBob()
+	shutdownAssert(nil)
+
+	// Now that the link has exited the htlcManager loop, attempt to
+	// trigger the batch ticker. It should not be possible.
+	select {
+	case batchTicker <- time.Now():
+		t.Fatalf("expected batch ticker to be inactive")
+	case <-time.After(5 * time.Second):
+	}
+}
+
 // assertFailureCode asserts that an error is of type ClearTextError and that
 // the failure code is as expected.
 func assertFailureCode(t *testing.T, err error, code lnwire.FailCode) {