internal/testenv: add HasSymlink/MustHaveSymlink

os package and path/filepath package have duplicated code for
checking symlink supports in test code.
This CL tries to simplify such test code.

Change-Id: I0371488337f5e951eca699852daab9ccb16ddd62
Reviewed-on: https://go-review.googlesource.com/27331
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/deps_test.go

@@ -180,7 +180,7 @@ var pkgDeps = map[string][]string{
 	"testing":          {"L2", "flag", "fmt", "os", "runtime/debug", "runtime/pprof", "runtime/trace", "time"},
 	"testing/iotest":   {"L2", "log"},
 	"testing/quick":    {"L2", "flag", "fmt", "reflect"},
-	"internal/testenv": {"L2", "OS", "flag", "testing"},
+	"internal/testenv": {"L2", "OS", "flag", "testing", "syscall"},
 
 	// L4 is defined as L3+fmt+log+time, because in general once
 	// you're using L3 packages, use of fmt, log, or time is not a big deal.

src/internal/testenv/testenv.go

@@ -127,6 +127,19 @@ func MustHaveExternalNetwork(t *testing.T) {
 	}
 }
 
+// HasSymlink reports whether the current system can use os.Symlink.
+func HasSymlink() bool {
+	return hasSymlink()
+}
+
+// MustHaveSymlink reports whether the current system can use os.Symlink.
+// If not, MustHaveSymlink calls t.Skip with an explanation.
+func MustHaveSymlink(t *testing.T) {
+	if !HasSymlink() {
+		t.Skipf("skipping test: cannot make symlinks on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
 var flaky = flag.Bool("flaky", false, "run known-flaky tests too")
 
 func SkipFlaky(t *testing.T, issue int) {

src/internal/testenv/testenv_notwin.go

@@ -0,0 +1,20 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows
+
+package testenv
+
+import (
+	"runtime"
+)
+
+func hasSymlink() bool {
+	switch runtime.GOOS {
+	case "android", "nacl", "plan9":
+		return false
+	}
+
+	return true
+}

src/internal/testenv/testenv_windows.go

@@ -0,0 +1,40 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testenv
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sync"
+	"syscall"
+)
+
+var symlinkOnce sync.Once
+var winHasSymlink = true
+
+func initWinHasSymlink() {
+	tmpdir, err := ioutil.TempDir("", "symtest")
+	if err != nil {
+		panic("failed to create temp directory: " + err.Error())
+	}
+	defer os.RemoveAll(tmpdir)
+
+	err = os.Symlink("target", filepath.Join(tmpdir, "symlink"))
+	if err != nil {
+		err = err.(*os.LinkError).Err
+		switch err {
+		case syscall.EWINDOWS, syscall.ERROR_PRIVILEGE_NOT_HELD:
+			winHasSymlink = false
+		}
+	}
+	os.Remove("target")
+}
+
+func hasSymlink() bool {
+	symlinkOnce.Do(initWinHasSymlink)
+
+	return winHasSymlink
+}