internal/testenv: make MustHaveSymlink message friendly

Change-Id: If6e12ebc41152bc0534d3d383df80e960efe97f0
Reviewed-on: https://go-review.googlesource.com/27577Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/testenv/testenv.go

@@ -129,14 +129,16 @@ func MustHaveExternalNetwork(t *testing.T) {
 
 // HasSymlink reports whether the current system can use os.Symlink.
 func HasSymlink() bool {
-	return hasSymlink()
+	ok, _ := hasSymlink()
+	return ok
 }
 
 // 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)
+	ok, reason := hasSymlink()
+	if !ok {
+		t.Skipf("skipping test: cannot make symlinks on %s/%s%s", runtime.GOOS, runtime.GOARCH, reason)
 	}
 }
 

src/internal/testenv/testenv_notwin.go

@@ -10,11 +10,11 @@ import (
 	"runtime"
 )
 
-func hasSymlink() bool {
+func hasSymlink() (ok bool, reason string) {
 	switch runtime.GOOS {
 	case "android", "nacl", "plan9":
-		return false
+		return false, ""
 	}
 
-	return true
+	return true, ""
 }

src/internal/testenv/testenv_windows.go

@@ -13,7 +13,7 @@ import (
 )
 
 var symlinkOnce sync.Once
-var winHasSymlink = true
+var winSymlinkErr error
 
 func initWinHasSymlink() {
 	tmpdir, err := ioutil.TempDir("", "symtest")
@@ -27,14 +27,23 @@ func initWinHasSymlink() {
 		err = err.(*os.LinkError).Err
 		switch err {
 		case syscall.EWINDOWS, syscall.ERROR_PRIVILEGE_NOT_HELD:
-			winHasSymlink = false
+			winSymlinkErr = err
 		}
 	}
 	os.Remove("target")
 }
 
-func hasSymlink() bool {
+func hasSymlink() (ok bool, reason string) {
 	symlinkOnce.Do(initWinHasSymlink)
 
-	return winHasSymlink
+	switch winSymlinkErr {
+	case nil:
+		return true, ""
+	case syscall.EWINDOWS:
+		return false, ": symlinks are not supported on your version of Windows"
+	case syscall.ERROR_PRIVILEGE_NOT_HELD:
+		return false, ": you don't have enough privileges to create symlinks"
+	}
+
+	return false, ""
 }