runtime: runtime.test gets DefaultGODEBUG with panicnil=1,asynctimerchan=1

[kevinburke]

Go version

go version go1.27-devel_f65692ea56 Sun Feb 15 07:19:01 2026 -0800 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE='off'
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/kevin/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/kevin/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/gh/xgxt3m8d5kd3353q6wyl6c1m0000gn/T/go-build544050156=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD=''
GOMODCACHE='/Users/kevin/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/kevin'
GOPRIVATE=''
GOPROXY='direct'
GOROOT='/Users/kevin/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/kevin/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/kevin/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.27-devel_f65692ea56 Sun Feb 15 07:19:01 2026 -0800'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Compile Go tip using 1.25.7 as GOROOT_BOOTSTRAP.

Run the tests with:

GOENV=off GOCACHE=/tmp/go-build-cache ./bin/go test ./src/runtime -count=1

What did you see happen?

I see the following error(s), in addition to others:

--- FAIL: TestSynctestCondSignalFromNoBubble (0.03s)
    --- FAIL: TestSynctestCondSignalFromNoBubble/SynctestCond/signal/no_bubble (0.01s)
        crash_test.go:1258: /var/folders/gh/xgxt3m8d5kd3353q6wyl6c1m0000gn/T/go-build2397586704/testprog.exe SynctestCond/signal/no_bubble: exit status 2
        crash_test.go:1261: output:
            panic: synctest.Run not supported with asynctimerchan!=0 [recovered, repanicked]

            goroutine 6 [running]:
            sync.(*WaitGroup).Go.func1.1()
            	/Users/kevin/go/src/sync/waitgroup.go:251 +0x48
            panic({0x1045138f8?, 0x1044f0530?})
            	/Users/kevin/go/src/runtime/panic.go:860 +0x134
            internal/synctest.Run(0x2e2e71c32220)
            	/Users/kevin/go/src/runtime/synctest.go:173 +0x318
            main.synctestCond.func1()
            	/Users/kevin/go/src/runtime/testdata/testprog/synctest.go:44 +0x88
            sync.(*WaitGroup).Go.func1()
            	/Users/kevin/go/src/sync/waitgroup.go:258 +0x48
            created by sync.(*WaitGroup).Go in goroutine 1
            	/Users/kevin/go/src/sync/waitgroup.go:238 +0x74


            want output containing: fatal error: semaphore wake of synctest goroutine from outside bubble
    --- FAIL: TestSynctestCondSignalFromNoBubble/SynctestCond/broadcast/no_bubble (0.01s)
        crash_test.go:1258: /var/folders/gh/xgxt3m8d5kd3353q6wyl6c1m0000gn/T/go-build2397586704/testprog.exe SynctestCond/broadcast/no_bubble: exit status 2
        crash_test.go:1261: output:
            panic: synctest.Run not supported with asynctimerchan!=0 [recovered, repanicked]

            goroutine 18 [running]:
            sync.(*WaitGroup).Go.func1.1()
            	/Users/kevin/go/src/sync/waitgroup.go:251 +0x48
            panic({0x100cfb8f8?, 0x100cd8530?})
            	/Users/kevin/go/src/runtime/panic.go:860 +0x134
            internal/synctest.Run(0x2b75475321e0)
            	/Users/kevin/go/src/runtime/synctest.go:173 +0x318
            main.synctestCond.func1()
            	/Users/kevin/go/src/runtime/testdata/testprog/synctest.go:44 +0x88
            sync.(*WaitGroup).Go.func1()
            	/Users/kevin/go/src/sync/waitgroup.go:258 +0x48
            created by sync.(*WaitGroup).Go in goroutine 1
            	/Users/kevin/go/src/sync/waitgroup.go:238 +0x74


            want output containing: fatal error: semaphore wake of synctest goroutine from outside bubble

What did you expect to see?

I expected the tests to pass or get marked as 'Skip' if the test cannot pass in the current environment and the behavior is expected.

[gabyhelp]

Related Issues

• testing/synctest: panic with `time.NewTimer` #70741 (closed)
• time: TestTimerGC fails on Mac #76948 (closed)
• runtime: arm64 64k PAGE_SIZE compile all.bash testcase error. #11933 (closed)
• cmd/cgo/internal/testsanitizers: occasional timeout panic #72996
• sync: TestCondBroadcast failures #61874 (closed)
• runtime/pprof: panic: test timed out after 9m0s #52916 (closed)
• testing/synctest: Repeated sync.WaitGroup.Add appears flaky under synctest #74386 (closed)
• runtime: TestCgo* consistently failing on windows/arm64 #49188 (closed)
• runtime: signal 27 received on thread with no signal stack #77588
• runtime: TestRWMutex is flaky? #20903 (closed)

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[prattmic]

I agree that those tests should probably skip if GODEBUG=asynctimerchan=1, but I am confused how this test got GODEBUG=asynctimerchan=1 given that you haven't set GODEBUG.

I haven't been able to reproduce this problem using 1.25.7 as GOROOT_BOOTSTRAP, though I am doing so on linux-amd64.

Does the issue occur when running the test binary directly? And, if so, does go version show the DefaultGODEBUG?

$ cd src/runtime
$ ../../bin/go test -c runtime
$ ./runtime.test -test.run TestSynctestCondSignalFromNoBubble
$ ../../bin/go version -m ./runtime.test

[kevinburke]

I set GO111MODULE=off, which triggered asynctimerchan=1, whoops.