internal/poll: set the correct file offset in FD.Seek for Windows overlapped handles

Windows doesn't keep the file pointer for overlapped file handles.
To work around this, we keep track of the current offset ourselves
and use it on every Read/Write operation.

When the user calls File.Seek with whence == io.SeekCurrent, it expects
that the offset we keep track of is also accounted for, else the
the seek'ed value won't match the file pointer seen by the user.

Updates #74951.
Fixes #75081.

Change-Id: Ieca7c3779e5349292883ffc293a8474088a4dec7
Reviewed-on: https://go-review.googlesource.com/c/go/+/697275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/internal/poll/fd_windows.go

@@ -1107,6 +1107,12 @@ func (fd *FD) Seek(offset int64, whence int) (int64, error) {
 	fd.l.Lock()
 	defer fd.l.Unlock()
 
+	if !fd.isBlocking && whence == io.SeekCurrent {
+		// Windows doesn't keep the file pointer for overlapped file handles.
+		// We do it ourselves in case to account for any read or write
+		// operations that may have occurred.
+		offset += fd.offset
+	}
 	n, err := syscall.Seek(fd.Sysfd, offset, whence)
 	fd.setOffset(n)
 	return n, err

src/os/os_windows_test.go

@@ -1845,6 +1845,44 @@ func TestFile(t *testing.T) {
 	}
 }
 
+func TestFileOverlappedSeek(t *testing.T) {
+	t.Parallel()
+	name := filepath.Join(t.TempDir(), "foo")
+	f := newFileOverlapped(t, name, true)
+	content := []byte("foo")
+	if _, err := f.Write(content); err != nil {
+		t.Fatal(err)
+	}
+	// Check that the file pointer is at the expected offset.
+	n, err := f.Seek(0, io.SeekCurrent)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != int64(len(content)) {
+		t.Errorf("expected file pointer to be at offset %d, got %d", len(content), n)
+	}
+	// Set the file pointer to the start of the file.
+	if _, err := f.Seek(0, io.SeekStart); err != nil {
+		t.Fatal(err)
+	}
+	// Read the first byte.
+	var buf [1]byte
+	if _, err := f.Read(buf[:]); err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(buf[:], content[:len(buf)]) {
+		t.Errorf("expected %q, got %q", content[:len(buf)], buf[:])
+	}
+	// Check that the file pointer is at the expected offset.
+	n, err = f.Seek(0, io.SeekCurrent)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != int64(len(buf)) {
+		t.Errorf("expected file pointer to be at offset %d, got %d", len(buf), n)
+	}
+}
+
 func TestPipe(t *testing.T) {
 	t.Parallel()
 	r, w, err := os.Pipe()