os: Readlink returns garbage when symlink target is like \\?\Volume{XXXXX}\

[alexbrainman]

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

go version devel +8ee9bca272 Wed Feb 27 08:22:03 2019 +0000 windows/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Alex\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=c:\users\alex\dev
set GOPROXY=
set GORACE=
set GOROOT=c:\users\alex\dev\go
set GOTMPDIR=
set GOTOOLDIR=c:\users\alex\dev\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\Alex\AppData\Local\Temp\go-build403512930=/tmp/go-build -gno-record-gcc-switches

What did you do?

I run go test command against this main_test.go file

package main_test

import (
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"testing"
)

func TestReadlink(t *testing.T) {
	tmpdir, err := ioutil.TempDir("", "TestReadlink")
	if err != nil {
		t.Fatal(err)
	}
	defer os.RemoveAll(tmpdir)

	vol := filepath.VolumeName(tmpdir)
	output, err := exec.Command("cmd", "/c", "mountvol", vol, "/L").CombinedOutput()
	if err != nil {
		t.Fatalf("failed to run mountvol %v /L: %v %q", vol, err, output)
	}
	target := strings.Trim(string(output), " \n\r")

	dirlink := filepath.Join(tmpdir, "dirlink")
	output, err = exec.Command("cmd", "/c", "mklink", "/J", dirlink, target).CombinedOutput()
	if err != nil {
		t.Fatalf("failed to run mklink %v %v: %v %q", dirlink, target, err, output)
	}

	got, err := os.Readlink(dirlink)
	if err != nil {
		t.Fatal(err)
	}
	if want := vol + `\`; got != want {
		t.Errorf(`os.Readlink(%q): got %q, want %q`, dirlink, got, want)
	}
}

What did you expect to see?

I expected test to pass.

What did you see instead?

Test fails with this error:

--- FAIL: TestReadlink (0.17s)
    main_test.go:37: os.Readlink("C:\\Users\\Alex\\AppData\\Local\\Temp\\TestReadlink551162827\\dirlink"): got "Volume{ea961e77-0000-0000-0000-501f00000000}\\", want "C:\\"
FAIL

The problem here is that os.Readlink does not handle links where target looks like this \\?\Volume{XXXXX}\. Volume{ea961e77-0000-0000-0000-501f00000000}\\ that os.Readlink returns is not useful for anything.

We have to handle symlinks, like that, because, for example, windows-arm TMP directory is such link (see #29746 (comment) for details). So it will be quite common at least on windows-arm.

Alex

[gopherbot]

Change https://golang.org/cl/164201 mentions this issue: os: make Readlink work with symlinks with target like \??\Volume{ABCD}\

[alexbrainman]

@jhowardmsft FYI, please, see https://golang.org/cl/164201 - hopefully it will be part of go1.13.

Alex

[lowenna]

@alexbrainman Thanks for pinging me on this. @jiria FYI

[bcmills]

containerd/continuity@8100e75 referred to this issue, but I believe its use of Readlink was (and still is) incorrect; see containerd/continuity#232.