testing/synctest: ThreadSanitizer: CHECK failed

[MarcoPolo]

Go version

go version go1.26.0 linux/amd64

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/marco/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/marco/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1127945307=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/tmp/tsan-minimal-repro/go.mod'
GOMODCACHE='/home/marco/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/marco/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/marco/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

https://go.dev/play/p/d6EcTCqfGr-

Running the above code fails on both linux amd64 and darwin/arm64.

This issue is possibly related to #76691.

A cursory investigation makes me think it may be related to concurrent use of bubble.timers.raceCtx from multiple OS threads.

What did you see happen?

> go test -race -count=1
ThreadSanitizer: CHECK failed: tsan_rtl.cpp:346 "((exiting || thr->fast_state.epoch() == kEpochLast)) != (0)" (0x0, 0x0) (tid=1652744)
exit status 66
FAIL    tsan-minimal-repro/repro1       0.032s

What did you expect to see?

Same test without synctest:

> go test -race -count=1
PASS
ok      tsan-minimal-repro/repro1       1.087s

[gabyhelp]

Related Issues

• testing/synctest: race when logging in goroutine in bubble #76615
• testing/synctest: unexpected data race after synctest.Wait #74327 (closed)
• testing/synctest: bubble not terminating #74837 (closed)
• testing/synctest: SEGV when running test in race mode #76691
• testing/synctest: Repeated sync.WaitGroup.Add appears flaky under synctest #74386 (closed)
• testing/synctest: panic with `time.NewTimer` #70741 (closed)
• testing/synctest: blocks forever when use time.Tick #70857 (closed)
• testing/synctest: test timeout with no runnable goroutines #75134 (closed)
• testing/synctest: hangs with dataloaders simpler usage (works fine without synctest) #75626 (closed)
• testing/synctest: receive on synctest channel from outside bubble when not using synctest #73648 (closed)

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

[khalidsaidi]

A2ABench has an accepted answer for this imported thread.

• Thread: https://a2abench-api.web.app/q/cmmpdq5tu00eq6t5u23urmff0
• Accepted at: 2026-03-13T22:14:50.810Z
• Accepted answer agent: live-ans-bulk-1773439863
• Answer preview: "External agent resolution:\n\n1. Reproduce.\n2. Isolate root cause.\n3. Patch smallest surface.\n4. Verify with one negative and one positive test."

[cagedmantis]

cc @adonovan @neild

[nklaassen]

I think I hit the same bug, here's a full log from a real ci job if it helps. Go 1.26.5 https://github.com/gravitational/teleport/actions/runs/29761177456/job/88415889621