Fix potential race condition in the sync manager - gossip syncer registration #9891

ziggie1984 · 2025-06-03T15:35:20Z

Fix a potential race condition when adding/removing the gossip_syncer to the sync_manager described below ...

coderabbitai · 2025-06-03T15:35:27Z

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ziggie1984 · 2025-06-03T15:38:46Z

Addresses issues found in: #9886

yyforyongyu · 2025-06-03T15:44:52Z

peer/brontide.go

+	//
+	// NOTE: We need to do this synchronously to make sure a gossip peer
+	// is registered before we signal that the peer is active because
+	// otherwise the is a race which can happen that we try to remove the


what is the race?

what guarantees that the peer does not flake and we shutdown before the gossiper is registered:

Currently we try to register the syncer async here:

lnd/peer/brontide.go

Line 2031 in 7e44422

p.initGossipSync()

However we could already hit the disconnect case here:

because we signal ready here:

lnd/server.go

Line 4597 in 7e44422

close(ready)

and could hit the removal of the gossiper here:

lnd/server.go

Line 4653 in 7e44422

s.authGossiper.PruneSyncState(p.PubKey())

no guarantee that the addition is before the removal or ?

yea make sense - the only thing is whether this initGossipSync will ever be blocking or not?

by blocking you mean blocking for a short period of time ? I mean depends on the busyness of the sync_manager or, I think that is why we need to make it sync in the first place because we cannot be sure that it registers in time before the peer might go offline.

blocking as in whether it will delay the startup or even create a deadlock somewhere - previously it's async so I think it's fine, now it's sync, and I've noticed a few places where the initGossipSync will wait for done signal.

and what happens before if the registration is not finished before the removal of the gossiper? i think it will be a noop? will the new registration still register the syncer afterwards?

ok will double check.

lightninglabs-deploy · 2025-06-11T15:19:46Z

@ziggie1984, remember to re-request review from reviewers when ready

ziggie1984 · 2025-06-16T16:14:37Z

My first analysis was wrong I made here:
#9891 (comment)

We always make sure before we remove the gossiper here:

lnd/server.go

Line 4661 in a5c4a7c

s.authGossiper.PruneSyncState(p.PubKey())

that the peer and therefore the waitGroupCounter has reached zero:

lnd/server.go

Line 4640 in a5c4a7c

p.WaitForDisconnect(ready)

especially here:

lnd/peer/brontide.go

Line 1588 in a5c4a7c

p.cg.WgWait()

so it would never be the case that we remove a gossiper before it was added.

Thanks for your careful analysis @yyforyongyu 🙏

ziggie1984 self-assigned this Jun 3, 2025

yyforyongyu reviewed Jun 3, 2025

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saubyk added this to the v0.19.1 milestone Jun 3, 2025

saubyk linked an issue Jun 3, 2025 that may be closed by this pull request

[bug]: Possible Memory Leak Gossip context.AfterFunc() / v0.19.0-beta #9886

Closed

saubyk modified the milestones: v0.19.1, v0.19.2 Jun 3, 2025

ziggie1984 changed the title ~~memory leak in peer connection~~ Fix potential race condition in the sync manager - gossip syncer registration Jun 3, 2025

brontide: register syncer synchronously with the sync manager

a20c4b3

ziggie1984 force-pushed the fix-memory-leak branch from 40c6278 to a20c4b3 Compare June 3, 2025 19:48

ziggie1984 marked this pull request as ready for review June 3, 2025 19:48

guggero removed a link to an issue Jun 4, 2025

[bug]: Possible Memory Leak Gossip context.AfterFunc() / v0.19.0-beta #9886

Closed

saubyk modified the milestones: v0.19.2, v0.20.0 Jun 5, 2025

saubyk added this to lnd v0.20 Jun 5, 2025

saubyk moved this to In review in lnd v0.20 Jun 5, 2025

ziggie1984 closed this Jun 16, 2025

github-project-automation bot moved this from In review to Done in lnd v0.20 Jun 16, 2025

ziggie1984 removed this from the v0.20.0 milestone Jun 17, 2025

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Fix potential race condition in the sync manager - gossip syncer registration #9891

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