Fixes warn_unused_result of fwrite.

mojo/common/data_pipe_utils.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 "mojo/common/data_pipe_utils.h"
 
 #include <stdio.h>
 
 #include "base/file_util.h"
 #include "base/files/file_path.h"
@@ skipping 10 matching lines @@
   base::ScopedFILE fp(base::OpenFile(destination, "wb"));
   if (!fp)
     return false;
 
   for (;;) {
     const void* buffer;
     uint32_t num_bytes;
     MojoResult result = BeginReadDataRaw(source.get(), &buffer, &num_bytes,
                                          MOJO_READ_DATA_FLAG_NONE);
     if (result == MOJO_RESULT_OK) {
-      fwrite(buffer, 1, num_bytes, fp.get());
+      size_t bytes_written = fwrite(buffer, 1, num_bytes, fp.get());
       result = EndReadDataRaw(source.get(), num_bytes);
-      if (result != MOJO_RESULT_OK)
+      if (bytes_written < num_bytes || result != MOJO_RESULT_OK)
         return false;
     } else if (result == MOJO_RESULT_SHOULD_WAIT) {
       result = Wait(source.get(),
                     MOJO_WAIT_FLAG_READABLE,
                     MOJO_DEADLINE_INDEFINITE);
       if (result != MOJO_RESULT_OK) {
         // If the producer handle was closed, then treat as EOF.
         return result == MOJO_RESULT_FAILED_PRECONDITION;
       }
     } else if (result == MOJO_RESULT_FAILED_PRECONDITION) {
@@ skipping 19 matching lines @@
                 const base::Callback<void(bool)>& callback) {
   base::PostTaskAndReplyWithResult(
       task_runner,
       FROM_HERE,
       base::Bind(&BlockingCopyToFile, base::Passed(&source), destination),
       callback);
 }
 
 }  // namespace common
 }  // namespace mojo