os: Implement symlink support for Windows

Fixes #5750.

https://code.google.com/p/go/issues/detail?id=5750

os: Separate windows from posix. Implement windows support.
path/filepath: Use the same implementation as other platforms
syscall: Add/rework new APIs for Windows

LGTM=alex.brainman
R=golang-codereviews, alex.brainman, gobot, rsc, minux
CC=golang-codereviews
https://golang.org/cl/86160044

6 files changed, 57 insertions, 13 deletions

diff --git a/src/pkg/path/filepath/match_test.go b/src/pkg/path/filepath/match_test.go
index 382692eaa4..20ec5aa2a1 100644
--- a/src/pkg/path/filepath/match_test.go
+++ b/src/pkg/path/filepath/match_test.go
@@ -167,8 +167,13 @@ var globSymlinkTests = []struct {
 
 func TestGlobSymlink(t *testing.T) {
 	switch runtime.GOOS {
-	case "nacl", "plan9", "windows":
+	case "nacl", "plan9":
 		t.Skipf("skipping on %s", runtime.GOOS)
+	case "windows":
+		if !supportsSymlinks {
+			t.Skipf("skipping on %s", runtime.GOOS)
+		}
+
 	}
 
 	tmpDir, err := ioutil.TempDir("", "globsymlink")
diff --git a/src/pkg/path/filepath/path_test.go b/src/pkg/path/filepath/path_test.go
index 819bd217cc..17b53bdf92 100644
--- a/src/pkg/path/filepath/path_test.go
+++ b/src/pkg/path/filepath/path_test.go
@@ -15,6 +15,8 @@ import (
 	"testing"
 )
 
+var supportsSymlinks = true
+
 type PathTest struct {
 	path, result string
 }
@@ -716,7 +718,7 @@ func TestEvalSymlinks(t *testing.T) {
 		if d.dest == "" {
 			err = os.Mkdir(path, 0755)
 		} else {
-			if runtime.GOOS != "windows" {
+			if supportsSymlinks {
 				err = os.Symlink(d.dest, path)
 			}
 		}
@@ -726,7 +728,9 @@ func TestEvalSymlinks(t *testing.T) {
 	}
 
 	var tests []EvalSymlinksTest
-	if runtime.GOOS == "windows" {
+	if supportsSymlinks {
+		tests = EvalSymlinksTests
+	} else {
 		for _, d := range EvalSymlinksTests {
 			if d.path == d.dest {
 				// will test only real files and directories
@@ -739,15 +743,13 @@ func TestEvalSymlinks(t *testing.T) {
 				tests = append(tests, d2)
 			}
 		}
-	} else {
-		tests = EvalSymlinksTests
 	}
 
 	// Evaluate the symlink farm.
 	for _, d := range tests {
 		path := simpleJoin(tmpDir, d.path)
 		dest := simpleJoin(tmpDir, d.dest)
-		if filepath.IsAbs(d.dest) {
+		if filepath.IsAbs(d.dest) || os.IsPathSeparator(d.dest[0]) {
 			dest = d.dest
 		}
 		if p, err := filepath.EvalSymlinks(path); err != nil {
diff --git a/src/pkg/path/filepath/path_windows_test.go b/src/pkg/path/filepath/path_windows_test.go
index 8a9be8e896..100cf30a45 100644
--- a/src/pkg/path/filepath/path_windows_test.go
+++ b/src/pkg/path/filepath/path_windows_test.go
@@ -10,9 +10,29 @@ import (
 	"os/exec"
 	"path/filepath"
 	"reflect"
+	"syscall"
 	"testing"
 )
 
+func init() {
+	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 {
+		return
+	}
+
+	err = err.(*os.LinkError).Err
+	switch err {
+	case syscall.EWINDOWS, syscall.ERROR_PRIVILEGE_NOT_HELD:
+		supportsSymlinks = false
+	}
+}
+
 func TestWinSplitListTestsAreValid(t *testing.T) {
 	comspec := os.Getenv("ComSpec")
 	if comspec == "" {
diff --git a/src/pkg/path/filepath/symlink.go b/src/pkg/path/filepath/symlink.go
index 307dd0f8fe..df0a9e0c2b 100644
--- a/src/pkg/path/filepath/symlink.go
+++ b/src/pkg/path/filepath/symlink.go
@@ -2,18 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !windows
-
 package filepath
 
 import (
 	"bytes"
 	"errors"
 	"os"
-	"strings"
 )
 
-func evalSymlinks(path string) (string, error) {
+const utf8RuneSelf = 0x80
+
+func walkSymlinks(path string) (string, error) {
 	const maxIter = 255
 	originalPath := path
 	// consume path by taking each frontmost path element,
@@ -25,7 +24,13 @@ func evalSymlinks(path string) (string, error) {
 		}
 
 		// find next path component, p
-		i := strings.IndexRune(path, Separator)
+		var i = -1
+		for j, c := range path {
+			if c < utf8RuneSelf && os.IsPathSeparator(uint8(c)) {
+				i = j
+				break
+			}
+		}
 		var p string
 		if i == -1 {
 			p, path = path, ""
@@ -47,7 +52,7 @@ func evalSymlinks(path string) (string, error) {
 		}
 		if fi.Mode()&os.ModeSymlink == 0 {
 			b.WriteString(p)
-			if path != "" {
+			if path != "" || (b.Len() == 2 && len(p) == 2 && p[1] == ':') {
 				b.WriteRune(Separator)
 			}
 			continue
@@ -58,7 +63,7 @@ func evalSymlinks(path string) (string, error) {
 		if err != nil {
 			return "", err
 		}
-		if IsAbs(dest) {
+		if IsAbs(dest) || os.IsPathSeparator(dest[0]) {
 			b.Reset()
 		}
 		path = dest + string(Separator) + path
diff --git a/src/pkg/path/filepath/symlink_unix.go b/src/pkg/path/filepath/symlink_unix.go
new file mode 100644
index 0000000000..d20e63a987
--- /dev/null
+++ b/src/pkg/path/filepath/symlink_unix.go
@@ -0,0 +1,7 @@
+// +build !windows
+
+package filepath
+
+func evalSymlinks(path string) (string, error) {
+	return walkSymlinks(path)
+}
diff --git a/src/pkg/path/filepath/symlink_windows.go b/src/pkg/path/filepath/symlink_windows.go
index 9adc8a48af..327c2c89a3 100644
--- a/src/pkg/path/filepath/symlink_windows.go
+++ b/src/pkg/path/filepath/symlink_windows.go
@@ -50,6 +50,11 @@ func toLong(path string) (string, error) {
 }
 
 func evalSymlinks(path string) (string, error) {
+	path, err := walkSymlinks(path)
+	if err != nil {
+		return "", err
+	}
+
 	p, err := toShort(path)
 	if err != nil {
 		return "", err