Start Channel Arbitrators concurrently by ziggie1984 · Pull Request #9262 · lightningnetwork/lnd

ziggie1984 · 2024-11-12T22:18:28Z

Followup of #9253. This makes sure we start the channel arbitrators concurrently with a maximum timeout of 5 min.

coderabbitai · 2024-11-12T22:18:35Z

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guggero

Nice! Thanks a lot for the fix 🎉

I think we might want to make it configurable, just in case.
Here's my proposal to add a config value (hidden, so we don't have to add it to the sample-lnd.conf file to avoid people messing with it when not necessary):
arb.diff.txt

guggero · 2024-11-13T10:23:59Z


+const (
+	// chainArbTimeout is the timeout for the chain arbitrator to start
+	// the channel arbitrators for each channel.


We could add some rationale here for why this number is rather high. Something along the lines of: "Because starting arbitrators might depend on resolutions that are done by an external component (e.g. aux component hooks), this might require other lnd subsystems to be fully started. So we need to give everything enough time."

guggero · 2024-11-13T10:26:21Z

 			stopAndLog()
+
+			// In case we encounter an error we need to cancel the
+			// context to ensure all goroutines are cleaned up.


nit: we don't actually clean up the goroutines yet (or I mean we don't interrupt the arb.Start() yet, we just don't wait for it anymore). So can perhaps add a TODO here that we'll want to eventually add a context to Start() so things will clean up properly on cancel?

Now we clean them up, I think adding a context ist not really the way to go because looking into the startup functionality it does not look like a context might help here.

ziggie1984 · 2024-11-13T12:04:01Z

Ok I think this approach is not really good because you were raising a really good point here @guggero:

go func(arb *ChannelArbitrator) {
			defer wgChannelArb.Done()

			select {
			case channelArbErrs <- arb.Start(startState):

			case <-ctx.Done():
				channelArbErrs <- ctx.Err()

			case <-c.quit:
				channelArbErrs <- ErrChainArbExiting
			}
		}(arbitrator)

This call does block forever when we launch the arb.Start prior timeout, which we very likely will, the starting is not the problem but more the finishing.
Tho looking into the start() function I don't really see an easy way to spawn goroutines which could cancel because of a timeout. I think what we should garantee is, that the call to the external units should be timing out if the call blocks. So I think this change makes no real sense.

ziggie1984 · 2024-11-13T12:06:38Z

By easy way, I don't think it is appropriate design, cancels imo make only sense if we know we spawn goroutines in that function but our problem arises because a function call to a normal subsystem does not timeout, so I think we need to improve there.

ziggie1984 · 2024-11-13T12:15:47Z

what we could do which is the only way it could work is something like this:

      // Create a new goroutine for the actual Start call
        startErrChan := make(chan error, 1)
        go func() {
            startErrChan <- arb.Start(startState)
        }()

        // Wait for either the start to complete or context to be done
        select {
        case err := <-startErrChan:
            channelArbErrs <- err

        case <-ctx.Done():
            // Don't wait for Start to complete if timeout occurs
            channelArbErrs <- ctx.Err()

        case <-c.quit:
            channelArbErrs <- ErrChainArbExiting

basically starting a goroutine in a goroutine. Only then can we really abort after the timeout.

ziggie1984 · 2024-11-13T12:48:52Z

Ok I mean spawning goroutines might not be the best solution here because we need for each startup 2, but I implemented it nonetheless because maybe it will take a while until we have the hooks for external subsystems configurable with timeouts.

ziggie1984 · 2024-11-15T12:12:21Z

@Roasbeef this works as well and resolves the deadlock.

When starting the channel arbitrators we make sure they are started concurrently.

The channel arbitrator startups have now a configurable timeout but this config value is hidden.

ziggie1984 · 2024-11-19T12:00:59Z

-		}
+		arb := arbitrator // Create new variable for closure
+		eg.Go(func() error {
+			// Create a non-blocking goroutine for the actual Start


we might also check the context in the beginning before starting arbitrator.

if ctx.Err() != nil { return ctx.Err() }

ziggie1984 · 2024-11-19T19:11:16Z

closing in favour of #9253

ziggie1984 force-pushed the fix-channel-arbitrator-deadlock branch 2 times, most recently from 98664a1 to d68ce0c Compare November 12, 2024 22:29

ziggie1984 marked this pull request as ready for review November 12, 2024 22:44

guggero approved these changes Nov 13, 2024

View reviewed changes

ziggie1984 force-pushed the fix-channel-arbitrator-deadlock branch from 1579cc6 to af4df94 Compare November 13, 2024 12:59

ziggie1984 added 4 commits November 15, 2024 13:42

contractcourt: make channel arb starts async.

0db9138

When starting the channel arbitrators we make sure they are started concurrently.

multi: make timeout configurable

1ca2373

The channel arbitrator startups have now a configurable timeout but this config value is hidden.

contractcourt: use concurrent safe waitGroup.

ae120e3

docs: add release-notes

b0ee865

ziggie1984 force-pushed the fix-channel-arbitrator-deadlock branch from 02ef00d to b0ee865 Compare November 15, 2024 12:43

ziggie1984 requested a review from Roasbeef November 15, 2024 12:43

ziggie1984 commented Nov 19, 2024

View reviewed changes

ziggie1984 closed this Nov 19, 2024

guggero mentioned this pull request Nov 29, 2024

[custom channels]: channel arbitrator getting stuck on startup #9323

Closed

ziggie1984 mentioned this pull request Nov 29, 2024

Don't block on channel arbitator startup #9324

Merged

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