testing/synctest: panic with time.NewTimer

[codekitchen]

Go version

go version devel go1.24-312f7c1bd3 Fri Dec 6 21:15:30 2024 +0000 X:synctest 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=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/brianp/Library/Caches/go-build'
GODEBUG=''
GOENV='/Users/brianp/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='synctest'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/n6/9stkk73j37n0zj9sqfsq63240000gn/T/go-build2814904191=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/brianp/p/test/go.mod'
GOMODCACHE='/Users/brianp/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/brianp/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/brianp/p/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/brianp/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/brianp/p/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='devel go1.24-312f7c1bd3 Fri Dec 6 21:15:30 2024 +0000 X:synctest'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

I've been kicking the tires on the experimental testing/synctest feature. I have converted a few tests to use it successfully. But I am running into a panic issue: any test case that uses time.NewTimer or time.After and then time.Sleep panics, for example

func TestNewTimerCrashes(t *testing.T) {
	synctest.Run(func() {
		t1 := time.NewTimer(time.Second)
		time.Sleep(2 * time.Second)
		<-t1.C
	})
}

What did you see happen?

panic: timer moved between synctest groups

What did you expect to see?

I would expect this test to succeed, pretty much immediately due to synctest stubbing out the Sleep.

If I use time.NewTicker instead, then it does succeed, as long as I Stop() that ticker (see this other issue)

[gabyhelp]

Related Issues

• testing: "panic: Log in goroutine after Test..." is unreliable due to lack of synchronization on t.done #67701
• runtime: new timer causes racy use of timer #35394 (closed)
• sync: apparent deadlock in TestWaitGroupMisuse3 #35774 (closed)
• runtime/pprof: panic: test timed out after 9m0s #52916 (closed)
• cmd/internal/test2json: test timeout panic trace is attributed to the running test, even if the test just passed #57305
• testing: inconsistent behaviors between running tests directly and after compiling the code first #59879 (closed)
• sync: test timeout with -cpu=10 #19476 (closed)
• runtime: "panic holding locks" when adding a timer #25619 (closed)
• time: TestTicker timeouts after CL 232298 #42424 (closed)
• testing: stack unavailable when a parallel test times out #18042 (closed)

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[ianlancetaylor]

CC @neild

[gopherbot]

Change https://go.dev/cl/634955 mentions this issue: runtime: avoid panic in expired synctest timer chan read

[neild]

Thanks for the report! Should be fixed now.