cmd/go: test caching is broken with sub-packages when using testing.T.Chdir

[mvdan]

Go version

go version go1.27-devel_a3f569adee 2026-04-28 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='v4'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/mvdan/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/mvdan/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2037079810=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/mvdan/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/mvdan/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/mvdan/tip'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/mvdan/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/home/mvdan/tip/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.27-devel_a3f569adee 2026-04-28 14:51:38 -0700'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Run a package whose tests use testing.T.Chdir, alongside another test in the same package that reads a file via a relative path. The relative-path file is then modified between go test invocations.

# We sleep for more than 2s to exceed cmd/go's modTimeCutoff,
# otherwise it refuses to save anything into the test cache.
exec sleep 3

# First run: populates the test cache with a PASS.
go test ./pkg
stdout '^ok'
! stdout 'cached'

# Verify the cache hits when nothing changes.
go test ./pkg
stdout 'cached'

# Modify the testdata file that TestB reads.
cp pkg/testdata/input.txt.modified pkg/testdata/input.txt

# We should now get a cache miss again.
go test ./pkg
! stdout 'cached'

-- go.mod --
module test

go 1.24
-- pkg/subdir/.keep --
-- pkg/testdata/input.txt --
original
-- pkg/testdata/input.txt.modified --
MODIFIED
-- pkg/repro_test.go --
package pkg

import (
	"os"
	"testing"
)

func TestA(t *testing.T) { t.Chdir("subdir") }
func TestB(t *testing.T) { os.ReadFile("testdata/input.txt") }

What did you see happen?

The third go test ./pkg reports (cached) even though pkg/testdata/input.txt was modified between the second and third runs.

# First run: populates the test cache with a PASS. (0.149s)
> go test ./pkg
[stdout]
ok  	test/pkg	0.002s

> stdout '^ok'
> ! stdout 'cached'
# Verify the cache hits when nothing changes. (0.044s)
> go test ./pkg
[stdout]
ok  	test/pkg	(cached)

> stdout 'cached'
# Modify the testdata file that TestB reads. (0.000s)
> cp pkg/testdata/input.txt.modified pkg/testdata/input.txt
# We should now get a cache miss again. (0.043s)
> go test ./pkg
[stdout]
ok  	test/pkg	(cached)

> ! stdout 'cached'
FAIL: repro-tchdir-cache.txtar:19: unexpected match for `cached` found in stdout: cached

What did you expect to see?

The third run should be a cache miss and re-run TestB, since TestB reads testdata/input.txt and that file changed.

[mvdan]

cc @golang/go-command I will send a fix for this shortly, but wanted to raise an issue first.

[gabyhelp]

Related Issues

• go test cache check wrong opened file when testing if user do `os.Chdir` #47305 (closed)
• cmd/go: test cache not invalidated when modifying testdata subdirectory #53053 (closed)
• cmd/go: cache not invalidated if testdata files are changed while the test is running #26790
• cmd/go: make test caching aware of testdata directories #22583 (closed)
• cmd/go: test cache hash inputs include ModTime, often causing cache misses on CI #58571
• cmd/go: unexpected test cache hit of "PASS" when test should "FAIL" #30313 (closed)
• cmd/go: 'go test' fails to cache test result due to modified `.git` directory #52573 (closed)
• cmd/go: invoking go run from go test can corrupt build cache #69566
• cmd/go: test -fullpath prevents test caching #64638 (closed)
• cmd/go: test reported as cached, but not actually cached #41593

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

[gopherbot]

Change https://go.dev/cl/771960 mentions this issue: os: notify testlog from (*File).Chdir

[prattmic]

cc @golang/command-line @rsc @ianlancetaylor @bradfitz @griesemer

[prattmic]

It sounds like this may warrant a backport? Failing to rerun a changed test is pretty bad.

[mvdan]

Agreed that a backport would be good. This has bit me a handful of times, where tests passed locally but then failed in CI. It took me until today to make the connection that it was the test caching playing games on me :)

[prattmic]

@gopherbot Please backport to 1.25 and 1.26. This bug causes tests that should execute to report cached results instead.

[gopherbot]

Backport issue(s) opened: #79038 (for 1.25), #79039 (for 1.26).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.