testing/fstest, os: TestFS fails on os.DirFS if it contains any symlinks to directories

[LukeShu]

What version of Go are you using (go version)?

$ go version
go version go1.17.5 linux/amd64

I also tried with:

$ PATH=$HOME/.local/go1.18beta1/bin:$PATH
$ go version
go version go1.18beta1 linux/amd64

Does this issue reproduce with the latest release?

Yes, 1.17.5 is the latest release. It also reproduces with go1.18beta1.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/lukeshu/.cache/go-build"
GOENV="/home/lukeshu/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/lukeshu/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/lukeshu/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.5"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/run/user/1000/tmpdir/go-build139097469=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I ran this test (closely based on testing/fstest's TestSymlink):

package x_test

import (
	"os"
	"path/filepath"
	"testing"
	"testing/fstest"
)

func TestSymlinkDir(t *testing.T) {
	tmp := t.TempDir()
	tmpfs := os.DirFS(tmp)

	if err := os.Mkdir(filepath.Join(tmp, "subdir"), 0755); err != nil {
		t.Fatal(err)
	}

	if err := os.Symlink(filepath.Join(tmp, "subdir"), filepath.Join(tmp, "subdir.link")); err != nil {
		t.Fatal(err)
	}

	if err := fstest.TestFS(tmpfs, "subdir", "subdir.link"); err != nil {
		t.Fatal(err)
	}
}

What did you expect to see?

I expected the test to pass.

What did you see instead?

$ go test -v .
=== RUN   TestSymlinkDir
    x_test.go:27: TestFS found errors:
        subdir.link: Open+ReadAll: read /run/user/1000/tmpdir/TestSymlinkDir3706588029/001/subdir.link: is a directory
--- FAIL: TestSymlinkDir (0.00s)

I am not sure whether the problem is with os.DirFS or with fstest.TestFS. See also:

• testing/fstest: TestFS fails on valid FS if it contains any symlinks #44113 (similar ticket for symlinks to regular files)
• io/fs: document how hard and symbolic links in a fs.FS should work #45470 (ticket asking for docs on what the correct behavior of an fs.FS is in this situation)

[cagedmantis]

/cc @mpvl