syscall: Readlink doesn't handle junction on windows

Fixes #9190

Change-Id: I22177687ed834feed165454019d28c11fcbf0fa2
Reviewed-on: https://go-review.googlesource.com/2307Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/os/os_windows_test.go

@@ -3,11 +3,15 @@ package os_test
 import (
 	"io/ioutil"
 	"os"
+	osexec "os/exec"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"testing"
 )
 
+var supportJunctionLinks = true
+
 func init() {
 	tmpdir, err := ioutil.TempDir("", "symtest")
 	if err != nil {
@@ -16,15 +20,19 @@ func init() {
 	defer os.RemoveAll(tmpdir)
 
 	err = os.Symlink("target", filepath.Join(tmpdir, "symlink"))
-	if err == nil {
-		return
-	}
-
+	if err != nil {
 		err = err.(*os.LinkError).Err
 		switch err {
 		case syscall.EWINDOWS, syscall.ERROR_PRIVILEGE_NOT_HELD:
 			supportsSymlinks = false
 		}
+	}
+	defer os.Remove("target")
+
+	b, _ := osexec.Command("cmd", "/c", "mklink", "/?").Output()
+	if !strings.Contains(string(b), " /J ") {
+		supportJunctionLinks = false
+	}
 }
 
 func TestSameWindowsFile(t *testing.T) {
@@ -79,3 +87,33 @@ func TestSameWindowsFile(t *testing.T) {
 		t.Errorf("files should be same")
 	}
 }
+
+func TestStatJunctionLink(t *testing.T) {
+	if !supportJunctionLinks {
+		t.Skip("skipping because junction links are not supported")
+	}
+
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	link := filepath.Join(filepath.Dir(dir), filepath.Base(dir)+"-link")
+
+	output, err := osexec.Command("cmd", "/c", "mklink", "/J", link, dir).CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run mklink %v %v: %v %q", link, dir, err, output)
+	}
+	defer os.Remove(link)
+
+	fi, err := os.Stat(link)
+	if err != nil {
+		t.Fatalf("failed to stat link %v: %v", link, err)
+	}
+	expected := filepath.Base(dir)
+	got := fi.Name()
+	if !fi.IsDir() || expected != got {
+		t.Fatalf("link should point to %v but points to %v instead", expected, got)
+	}
+}

src/syscall/syscall_windows.go

@@ -1003,13 +1003,22 @@ func Readlink(path string, buf []byte) (n int, err error) {
 	}
 
 	rdb := (*reparseDataBuffer)(unsafe.Pointer(&rdbbuf[0]))
-	if uintptr(bytesReturned) < unsafe.Sizeof(*rdb) ||
-		rdb.ReparseTag != IO_REPARSE_TAG_SYMLINK {
-		// the path is not a symlink but another type of reparse point
+	var s string
+	switch rdb.ReparseTag {
+	case IO_REPARSE_TAG_SYMLINK:
+		data := (*symbolicLinkReparseBuffer)(unsafe.Pointer(&rdb.reparseBuffer))
+		p := (*[0xffff]uint16)(unsafe.Pointer(&data.PathBuffer[0]))
+		s = UTF16ToString(p[data.PrintNameOffset/2 : (data.PrintNameLength-data.PrintNameOffset)/2])
+	case _IO_REPARSE_TAG_MOUNT_POINT:
+		data := (*mountPointReparseBuffer)(unsafe.Pointer(&rdb.reparseBuffer))
+		p := (*[0xffff]uint16)(unsafe.Pointer(&data.PathBuffer[0]))
+		s = UTF16ToString(p[data.PrintNameOffset/2 : (data.PrintNameLength-data.PrintNameOffset)/2])
+	default:
+		// the path is not a symlink or junction but another type of reparse
+		// point
 		return -1, ENOENT
 	}
-
-	s := UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rdb.PathBuffer[0]))[:rdb.PrintNameLength/2])
 	n = copy(buf, []byte(s))
+
 	return n, nil
 }

src/syscall/ztypes_windows.go

@@ -1083,12 +1083,7 @@ type TCPKeepalive struct {
 	Interval uint32
 }
 
-type reparseDataBuffer struct {
-	ReparseTag        uint32
-	ReparseDataLength uint16
-	Reserved          uint16
-
-	// SymbolicLinkReparseBuffer
+type symbolicLinkReparseBuffer struct {
 	SubstituteNameOffset uint16
 	SubstituteNameLength uint16
 	PrintNameOffset      uint16
@@ -1097,9 +1092,27 @@ type reparseDataBuffer struct {
 	PathBuffer           [1]uint16
 }
 
+type mountPointReparseBuffer struct {
+	SubstituteNameOffset uint16
+	SubstituteNameLength uint16
+	PrintNameOffset      uint16
+	PrintNameLength      uint16
+	PathBuffer           [1]uint16
+}
+
+type reparseDataBuffer struct {
+	ReparseTag        uint32
+	ReparseDataLength uint16
+	Reserved          uint16
+
+	// GenericReparseBuffer
+	reparseBuffer byte
+}
+
 const (
 	FSCTL_GET_REPARSE_POINT          = 0x900A8
 	MAXIMUM_REPARSE_DATA_BUFFER_SIZE = 16 * 1024
+	_IO_REPARSE_TAG_MOUNT_POINT      = 0xA0000003
 	IO_REPARSE_TAG_SYMLINK           = 0xA000000C
 	SYMBOLIC_LINK_FLAG_DIRECTORY     = 0x1
 )