internal/poll: don't call Seek for overlapped Windows handles

Overlapped handles don't have the file pointer updated when performing
I/O operations, so there is no need to call FD.Seek to reset the file
pointer.

Also, some overlapped file handles don't support seeking. See golang#74951.

Fixes golang#74951.

Change-Id: I0edd53beed7d3862730f3b2ed5fe9ba490e66c06
Reviewed-on: https://go-review.googlesource.com/c/go/+/697295
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: qmuntal

src/internal/poll/fd_windows.go

@@ -622,12 +622,22 @@ func (fd *FD) Pread(b []byte, off int64) (int, error) {
 
 	fd.l.Lock()
 	defer fd.l.Unlock()
-	curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
-	if err != nil {
-		return 0, err
+	if fd.isBlocking {
+		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+		if err != nil {
+			return 0, err
+		}
+		defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+		defer fd.setOffset(curoffset)
+	} else {
+		// Overlapped handles don't have the file pointer updated
+		// when performing I/O operations, so there is no need to
+		// call Seek to reset the file pointer.
+		// Also, some overlapped file handles don't support seeking.
+		// See https://go.dev/issues/74951.
+		curoffset := fd.offset
+		defer fd.setOffset(curoffset)
 	}
-	defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
-	defer fd.setOffset(curoffset)
 	o := &fd.rop
 	o.InitBuf(b)
 	fd.setOffset(off)
@@ -847,12 +857,22 @@ func (fd *FD) Pwrite(buf []byte, off int64) (int, error) {
 
 	fd.l.Lock()
 	defer fd.l.Unlock()
-	curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
-	if err != nil {
-		return 0, err
+	if fd.isBlocking {
+		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+		if err != nil {
+			return 0, err
+		}
+		defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+		defer fd.setOffset(curoffset)
+	} else {
+		// Overlapped handles don't have the file pointer updated
+		// when performing I/O operations, so there is no need to
+		// call Seek to reset the file pointer.
+		// Also, some overlapped file handles don't support seeking.
+		// See https://go.dev/issues/74951.
+		curoffset := fd.offset
+		defer fd.setOffset(curoffset)
 	}
-	defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
-	defer fd.setOffset(curoffset)
 
 	var ntotal int
 	for {

src/os/os_windows_test.go

@@ -1883,6 +1883,34 @@ func TestFileOverlappedSeek(t *testing.T) {
 	}
 }
 
+func TestFileOverlappedReadAtVolume(t *testing.T) {
+	// Test that we can use File.ReadAt with an overlapped volume handle.
+	// See https://go.dev/issues/74951.
+	t.Parallel()
+	name := `\\.\` + filepath.VolumeName(t.TempDir())
+	namep, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	h, err := syscall.CreateFile(namep,
+		syscall.GENERIC_READ|syscall.GENERIC_WRITE,
+		syscall.FILE_SHARE_WRITE|syscall.FILE_SHARE_READ,
+		nil, syscall.OPEN_ALWAYS, syscall.FILE_FLAG_OVERLAPPED, 0)
+	if err != nil {
+		if errors.Is(err, syscall.ERROR_ACCESS_DENIED) {
+			t.Skip("skipping test: access denied")
+		}
+		t.Fatal(err)
+	}
+	f := os.NewFile(uintptr(h), name)
+	defer f.Close()
+
+	var buf [0]byte
+	if _, err := f.ReadAt(buf[:], 0); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestPipe(t *testing.T) {
 	t.Parallel()
 	r, w, err := os.Pipe()