testing/synctest: Cleanup not run on deadlock

[firelizzard18]

Go version

go version go1.27-devel_c43088c7a6

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='$HOME/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='$HOME/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2845624490=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='$HOME/src/game/loki/go.mod'
GOMODCACHE='$HOME/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='$HOME/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='$HOME/src/golang/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='$HOME/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='$HOME/src/golang/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.27-devel_c43088c7a6 Wed Mar 11 15:09:09 2026 -0700'
GOWORK='$HOME/src/game/loki/go.work'
PKG_CONFIG='pkg-config'

What did you do?

func TestSynctestDeadlockCleanup(t *testing.T) {
	synctest.Test(t, func(t *testing.T) {
		t.Cleanup(func() {
			t.Log("cleanup ran")
		})

		ch := make(chan struct{})
		<-ch // deadlock
	})
}

What did you see happen?

Cleanup did not run.

What did you expect to see?

Cleanup to run.

[gabyhelp]

Related Issues

• testing/synctest: allow `runtime.AddCleanup` functions in bubble #75113
• testing/synctest: test timeout with no runnable goroutines #75134 (closed)
• testing/synctest: be more explicit about goroutine leaks #75052 (closed)
• testing/synctest: bubble not terminating #74837 (closed)
• testing/synctest: hangs with dataloaders simpler usage (works fine without synctest) #75626 (closed)
• testing/synctest: *persistConn causing infinite hang #78492 (closed)
• testing/synctest: runtime.SetBlockProfileRate breaks promise with synctest.Wait #76286
• testing/synctest: race when logging in goroutine in bubble #76615
• testing/synctest: `synctest` does not respect `t.Helper()` #74199 (closed)
• testing/synctest: GOEXPERIMENT=synctest blocks indefinitely with context.WithTimeout in tight loop #73130 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[cagedmantis]

cc @adonovan @neild

[neild]

This is tricky, because cleanup functions run inside the bubble. If the bubble is deadlocked, then by definition no goroutine in the bubble can make progress, and the cleanup functions therefore cannot run. (The panic occurs outside the bubble.)

It might be possible to make this work with some changes to how we start bubbles and detect deadlocks. We'd want to be careful to preserve the property that we print stacks for all the goroutines in the deadlocked bubble at the time of deadlock--it's not very useful if we print stacks after a cleanup function has resolved the deadlock.

I'm not certain if it's worth putting effort into trying to change the current behavior, though, given that it only applies to cases when a test is encountering a bug and failing. There are plenty of other ways that a failing test can skip running cleanup functions. Deadlocking outside of a synctest bubble, for example--the difference in the synctest case is just that we detect the deadlock and report it.

[firelizzard18]

My use case is printing diagnostic information/writing log files if a test fails. When tests pass I don't need that data, but when a test fails I'd like to see the trace I captured of messages exchanged that lead to the deadlock. My tests are simulating a distributed system so a deadlock most often amounts to "the event I expected to happen did not happen". At the very least I think the synctest/Cleanup situation deserves being documented.