[serial] Cancel read watcher if immediate read succeeded.

device/serial/serial_io_handler_posix.cc

Index: device/serial/serial_io_handler_posix.cc
diff --git a/device/serial/serial_io_handler_posix.cc b/device/serial/serial_io_handler_posix.cc
index 71b398c889b9eae7284bb699319f9114a50772a5..59582c5b84e7dea1ffe4b2d8306a103e6e0baebe 100644
--- a/device/serial/serial_io_handler_posix.cc
+++ b/device/serial/serial_io_handler_posix.cc
@@ -348,10 +348,16 @@ bool SerialIoHandlerPosix::AttemptRead(bool within_read) {
 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) {