gvisor/fs: return a proper error from FileWriter.Write in case of a short-write

The io.Writer contract requires that Write writes all available
bytes and does not return short writes. This causes errors with
io.Copy, since our own Write interface does not have this same
contract.

PiperOrigin-RevId: 251368730

pkg/sentry/fs/file.go

@@ -545,12 +545,28 @@ type lockedWriter struct {
 
 // Write implements io.Writer.Write.
 func (w *lockedWriter) Write(buf []byte) (int, error) {
-	n, err := w.File.FileOperations.Write(w.Ctx, w.File, usermem.BytesIOSequence(buf), w.File.offset)
-	return int(n), err
+	return w.WriteAt(buf, w.File.offset)
 }
 
 // WriteAt implements io.Writer.WriteAt.
 func (w *lockedWriter) WriteAt(buf []byte, offset int64) (int, error) {
-	n, err := w.File.FileOperations.Write(w.Ctx, w.File, usermem.BytesIOSequence(buf), offset)
-	return int(n), err
+	var (
+		written int
+		err     error
+	)
+	// The io.Writer contract requires that Write writes all available
+	// bytes and does not return short writes. This causes errors with
+	// io.Copy, since our own Write interface does not have this same
+	// contract. Enforce that here.
+	for written < len(buf) {
+		var n int64
+		n, err = w.File.FileOperations.Write(w.Ctx, w.File, usermem.BytesIOSequence(buf[written:]), offset+int64(written))
+		if n > 0 {
+			written += int(n)
+		}
+		if err != nil {
+			break
+		}
+	}
+	return written, err
 }