[net] Subtract timestamps to determine if uploading file changed.

net/base/upload_file_element_reader.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 "net/base/upload_file_element_reader.h"
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/profiler/scoped_tracker.h"
@@ skipping 156 matching lines @@
     return;
   }
 
   int64 length = file_info->size;
   if (range_offset_ < static_cast<uint64>(length)) {
     // Compensate for the offset.
     length = std::min(length - range_offset_, range_length_);
   }
 
   // If the underlying file has been changed and the expected file modification
-  // time is set, treat it as error. Note that the expected modification time
-  // from WebKit is based on time_t precision. So we have to convert both to
-  // time_t to compare. This check is used for sliced files.
+  // time is set, treat it as error. Note that |expected_modification_time_| may
+  // have gone through multiple conversion steps involving loss of precision
+  // (including conversion to time_t). Therefore the check below only verifies
+  // that the timestamps are within one second of each other. This check is used
+  // for sliced files.
   if (!expected_modification_time_.is_null() &&
-      expected_modification_time_.ToTimeT() !=
-      file_info->last_modified.ToTimeT()) {
+      (expected_modification_time_ - file_info->last_modified)
+              .magnitude()
+              .InSeconds() != 0) {
     callback.Run(ERR_UPLOAD_FILE_CHANGED);
     return;
   }
 
   content_length_ = length;
   bytes_remaining_ = GetContentLength();
   callback.Run(OK);
 }
 
 int UploadFileElementReader::OnReadCompleted(
@@ skipping 16 matching lines @@
 ScopedOverridingContentLengthForTests(uint64 value) {
   overriding_content_length = value;
 }
 
 UploadFileElementReader::ScopedOverridingContentLengthForTests::
 ~ScopedOverridingContentLengthForTests() {
   overriding_content_length = 0;
 }
 
 }  // namespace net