os: enable symlink creation on Windows 10

src/internal/syscall/windows/symlink_windows.go

@@ -0,0 +1,14 @@
+// Copyright 2018 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 windows
+
+import "syscall"
+
+const (
+	ERROR_INVALID_PARAMETER syscall.Errno = 87
+
+	// symlink support for CreateSymbolicLink() starting with Windows 10 (1703, v10.0.14972)
+	SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE = 0x2
+)

src/os/file_windows.go

@@ -373,11 +373,20 @@ func Symlink(oldname, newname string) error {
 		return &LinkError{"symlink", oldname, newname, err}
 	}
 
-	var flags uint32
+	var flags uint32 = windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
 	if isdir {
 		flags |= syscall.SYMBOLIC_LINK_FLAG_DIRECTORY
 	}
 	err = syscall.CreateSymbolicLink(n, o, flags)
+
+	if err != nil {
+		// the unprivileged create flag is unsupported
+		// below Windows 10 (1703, v10.0.14972). retry without it.
+		flags &^= windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+
+		err = syscall.CreateSymbolicLink(n, o, flags)
+	}
+
 	if err != nil {
 		return &LinkError{"symlink", oldname, newname, err}
 	}

src/os/os_windows_test.go

@@ -1011,3 +1011,47 @@ func TestWindowsDevNullFile(t *testing.T) {
 		t.Errorf(`"NUL" and "nul" are not the same file`)
 	}
 }
+
+// TestSymlinkCreation verifies that creating a symbolic link
+// works on Windows when developer mode is active.
+// This is supported starting Windows 10 (1703, v10.0.14972).
+func TestSymlinkCreation(t *testing.T) {
+	if !isWindowsDeveloperModeActive() {
+		t.Skip("Windows developer mode is not active")
+	}
+
+	temp, err := ioutil.TempDir("", "TestSymlinkCreation")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(temp)
+
+	dummyFile := filepath.Join(temp, "file")
+	err = ioutil.WriteFile(dummyFile, []byte(""), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	linkFile := filepath.Join(temp, "link")
+	err = os.Symlink(dummyFile, linkFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+// isWindowsDeveloperModeActive checks whether or not the developer mode is active on Windows 10.
+// Returns false for prior Windows versions.
+// see https://docs.microsoft.com/en-us/windows/uwp/get-started/enable-your-device-for-development
+func isWindowsDeveloperModeActive() bool {
+	key, err := registry.OpenKey(registry.LOCAL_MACHINE, "SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\AppModelUnlock", registry.READ)
+	if err != nil {
+		return false
+	}
+
+	val, _, err := key.GetIntegerValue("AllowDevelopmentWithoutDevLicense")
+	if err != nil {
+		return false
+	}
+
+	return val != 0
+}