os: add os.ModeSocket support to os.Stat on windows

Unix sockets are identified by the IO_REPARSE_TAG_AF_UNIX reparse tag.

Teach fileStat.Mode() to recognize this tag and set the os.ModeSocket
bit in such case.

Note that there is a bug starting in Windows 19H1 until 20H1 that
makes the IO_REPARSE_TAG_AF_UNIX tag not being set for unix sockets.
This CL doesn't provide a workaround for this bug.

Fixes #33357.

Change-Id: Iea8f24b20672c8d4b03f55ef298d128431dc3fac
Reviewed-on: https://go-review.googlesource.com/c/go/+/561937
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/internal/syscall/windows/reparse_windows.go

@@ -9,10 +9,13 @@ import (
 	"unsafe"
 )
 
+// Reparse tag values are taken from
+// https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/c8e77b37-3909-4fe6-a4ea-2b9d423b1ee4
 const (
 	FSCTL_SET_REPARSE_POINT    = 0x000900A4
 	IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
 	IO_REPARSE_TAG_DEDUP       = 0x80000013
+	IO_REPARSE_TAG_AF_UNIX     = 0x80000023
 
 	SYMLINK_FLAG_RELATIVE = 1
 )

src/net/unixsock_windows_test.go

@@ -95,3 +95,23 @@ func TestUnixConnLocalWindows(t *testing.T) {
 		}
 	}
 }
+
+func TestModeSocket(t *testing.T) {
+	addr := testUnixAddr(t)
+
+	l, err := Listen("unix", addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+
+	stat, err := os.Stat(addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mode := stat.Mode()
+	if mode&os.ModeSocket == 0 {
+		t.Fatalf("%v should have ModeSocket", mode)
+	}
+}

src/os/types_windows.go

@@ -184,23 +184,28 @@ func (fs *fileStat) Mode() (m FileMode) {
 	case syscall.FILE_TYPE_CHAR:
 		m |= ModeDevice | ModeCharDevice
 	}
-	if fs.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT != 0 && m&ModeType == 0 {
-		if fs.ReparseTag == windows.IO_REPARSE_TAG_DEDUP {
-			// If the Data Deduplication service is enabled on Windows Server, its
-			// Optimization job may convert regular files to IO_REPARSE_TAG_DEDUP
-			// whenever that job runs.
-			//
-			// However, DEDUP reparse points remain similar in most respects to
-			// regular files: they continue to support random-access reads and writes
-			// of persistent data, and they shouldn't add unexpected latency or
-			// unavailability in the way that a network filesystem might.
-			//
-			// Go programs may use ModeIrregular to filter out unusual files (such as
-			// raw device files on Linux, POSIX FIFO special files, and so on), so
-			// to avoid files changing unpredictably from regular to irregular we will
-			// consider DEDUP files to be close enough to regular to treat as such.
-		} else {
-			m |= ModeIrregular
+	if fs.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+		if fs.ReparseTag == windows.IO_REPARSE_TAG_AF_UNIX {
+			m |= ModeSocket
+		}
+		if m&ModeType == 0 {
+			if fs.ReparseTag == windows.IO_REPARSE_TAG_DEDUP {
+				// If the Data Deduplication service is enabled on Windows Server, its
+				// Optimization job may convert regular files to IO_REPARSE_TAG_DEDUP
+				// whenever that job runs.
+				//
+				// However, DEDUP reparse points remain similar in most respects to
+				// regular files: they continue to support random-access reads and writes
+				// of persistent data, and they shouldn't add unexpected latency or
+				// unavailability in the way that a network filesystem might.
+				//
+				// Go programs may use ModeIrregular to filter out unusual files (such as
+				// raw device files on Linux, POSIX FIFO special files, and so on), so
+				// to avoid files changing unpredictably from regular to irregular we will
+				// consider DEDUP files to be close enough to regular to treat as such.
+			} else {
+				m |= ModeIrregular
+			}
 		}
 	}
 	return m