Make base:ReplaceFile return an informative error.

base/file_util_posix.cc

 // Copyright (c) 2012 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 "base/file_util.h"
 
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <fnmatch.h>
@@ skipping 214 matching lines @@
   if (rename(from_path.value().c_str(), to_path.value().c_str()) == 0)
     return true;
 
   if (!CopyDirectory(from_path, to_path, true))
     return false;
 
   Delete(from_path, true);
   return true;
 }
 
-bool ReplaceFile(const FilePath& from_path, const FilePath& to_path) {
+bool ReplaceFile(const FilePath& from_path, const FilePath& to_path,
+                 base::PlatformFileError* error) {
   base::ThreadRestrictions::AssertIOAllowed();
-  return (rename(from_path.value().c_str(), to_path.value().c_str()) == 0);
+  if (rename(from_path.value().c_str(), to_path.value().c_str()) == 0)
+    return true;
+  if (error)
+    *error = base::ErrnoToPlatformFileError(errno);
+  return false;
 }
 
 bool CopyDirectory(const FilePath& from_path,
                    const FilePath& to_path,
                    bool recursive) {
   base::ThreadRestrictions::AssertIOAllowed();
   // Some old callers of CopyDirectory want it to support wildcards.
   // After some discussion, we decided to fix those callers.
   // Break loudly here if anyone tries to do this.
   // TODO(evanm): remove this once we're sure it's ok.
@@ skipping 813 matching lines @@
       kFileSystemRoot, path, kRootUid, allowed_group_ids);
 }
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
 int GetMaximumPathComponentLength(const FilePath& path) {
   base::ThreadRestrictions::AssertIOAllowed();
   return pathconf(path.value().c_str(), _PC_NAME_MAX);
 }
 
 }  // namespace file_util