testing: t.TempDir() failing if test name exceeds 255 chars

[Luap99]

Go version

go version go1.24.0 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='/root/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/root/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build206958557=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/root/test/go.mod'
GOMODCACHE='/root/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/root/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/root/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

When migrating some test to using t.TempDir() over os.MkdirTemp() I all of the sudden got failures, turns out we have some quite long test names and t.TempDir() uses the name as basis for the temp directory. That fails if the path component exceeds 255 chars of course.

package main_test

import (
	"strings"
	"testing"
)

func TestTempDir(t *testing.T) {
	// "TestTempDir" -> length 11
	// max unit32 is used as pattern suffix -> max length 10 (random of course)
	// + the name from t.Run() pushes the path component above 255 chars
	t.Run(strings.Repeat("A", 255), func(t *testing.T) {
		t.TempDir()
	})
}

https://go.dev/play/p/K3vSiE14faS

What did you see happen?

t.TempDir() will fail to create the directory if the test name is to long.

Given the path suffix is a random unit32 (with no zero padding) one might even see flakes if the total test name length is between 246-254 which makes it even worse.

What did you expect to see?

t.TempDir() should work regardless of the test name, maybe it should trim the path to 255 chars if necessary.

[gabyhelp]

Related Issues

• testing: surprising interaction of subtests with TempDir #46624 (closed)
• testing: `T.TempDir` does not clean illegal characters correctly on Go 1.16.13 #50644 (closed)
• testing: TempDir fails when TMPDIR is a relative directory and os.Chdir() is called #67954 (closed)
• cmd/go: go test doesn't like long paths on Windows #10651 (closed)
• testing: new T.TempDir doesn't work with subtests #38465 (closed)
• io/ioutil: TempDir should support patterns like TempFile does #33805 (closed)
• testing: test failes if leaving files without write permission in T.TempDir #40853
• testing: document that directories returned by TB.TempDir should not be accessed after cleanup #43547
• testing: TB.TempDir paths can be too long for Unix socket paths #62614
• DOS shell can't handle deep directory structures with go test #6300 (closed)

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

[adonovan]

A test name of 255 bytes seems excessive, but I agree that if t.Run accepts the name, then t.TempDir within the subtest should work.

[Luap99]

Yeah it is not a common thing for sure, in our specific case we could use another name I suppose but we actually use the test name to contain all the test input data with several nested loops, that way we cover all combinations
https://github.com/containers/common/blob/1e4c9bf42bd56b0500948287da1d710f85d29352/libimage/manifest_list_test.go#L459-L505
and then I got a failure like

manifest_list_test.go:385: TempDir: mkdir /tmp/TestAddArtifactsartifactType=applicationx-something-else,configType=applicationvnd.oci.image.config.v1+json,configLength=9,layerType=applicationvnd.oci.image.layer.v1.tar+gzip,excludeTitles=true,artifactSubject=blobs-only,indexSubject=config-only2880673117: file name too long

Considering someone needs to know which iteration failed with which inputs I think adding them all in the name makes sense. I guess the other option would be to t.Logf() instead of the name.

[stephentth]

That makes sense, it's not something expected. Working on this one. I accidentally stumbled upon this bug too.

[seankhliao]

the bug should be fixed, but you could also have setup your tests with a subtest per level of nested for loop
you're not limited to one level of subtests

for a := range ...
  t.Run(a, func{
     for b := range ...
       t.Run(b, func{
          for c := range ...
             t.Run(c, func{

[gopherbot]

Change https://go.dev/cl/649915 mentions this issue: testing: fix TempDir failure when test name too long

[gopherbot]

Change https://go.dev/cl/671577 mentions this issue: testing: limit TempDir name length