[serial] Cancel read watcher if immediate read succeeded.

device/serial/serial_io_handler_posix.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "device/serial/serial_io_handler_posix.h"
 
 #include <sys/ioctl.h>
 #include <termios.h>
 
 #include "base/posix/eintr_wrapper.h"
@@ skipping 330 matching lines @@
     is_watching_reads_ = false;
     file_read_watcher_.StopWatchingFileDescriptor();
   }
 
   return true;
 }
 
 void SerialIoHandlerPosix::RunReadCompleted(bool within_read,
                                             int bytes_read,
                                             serial::ReceiveError error) {
-  if (within_read)
+  if (within_read) {
+    // Stop watching the fd to avoid more reads until the queued ReadCompleted()
+    // completes and releases the pending_read_buffer.
+    is_watching_reads_ = false;
+    file_read_watcher_.StopWatchingFileDescriptor();
+
     QueueReadCompleted(bytes_read, error);
-  else
+  } else {
     ReadCompleted(bytes_read, error);
+  }
 }
 
 void SerialIoHandlerPosix::OnFileCanWriteWithoutBlocking(int fd) {
   DCHECK(CalledOnValidThread());
   DCHECK_EQ(fd, file().GetPlatformFile());
 
   if (pending_write_buffer()) {
     int bytes_written = HANDLE_EINTR(write(file().GetPlatformFile(),
                                            pending_write_buffer(),
                                            pending_write_buffer_len()));
@@ skipping 272 matching lines @@
   memcpy(buffer, chars_stashed_, std::min(new_bytes_read, 2));
   memcpy(chars_stashed_, tmp, num_chars_stashed_);
   return new_bytes_read;
 }
 
 std::string SerialIoHandler::MaybeFixUpPortName(const std::string& port_name) {
   return port_name;
 }
 
 }  // namespace device