Thread IO safety: file_util annotated, IO thread blocked from doing IO

net/url_request/url_request_file_job.cc

 // Copyright (c) 2006-2010 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.
 
 // For loading files, we make use of overlapped i/o to ensure that reading from
 // the filesystem (e.g., a network filesystem) does not block the calling
 // thread.  An alternative approach would be to use a background thread or pool
 // of threads, but it seems better to leverage the operating system's ability
 // to do background file reads for us.
 //
 // Since overlapped reads require a 'static' buffer for the duration of the
 // asynchronous read, the URLRequestFileJob keeps a buffer as a member var.  In
 // URLRequestFileJob::Read, data is simply copied from the object's buffer into
 // the given buffer.  If there is no data to copy, the URLRequestFileJob
 // attempts to read more from the file to fill its buffer.  If reading from the
 // file does not complete synchronously, then the URLRequestFileJob waits for a
 // signal from the OS that the overlapped read has completed.  It does so by
 // leveraging the MessageLoop::WatchObject API.
 
 #include "net/url_request/url_request_file_job.h"
 
 #include "base/compiler_specific.h"
 #include "base/message_loop.h"
 #include "base/platform_file.h"
 #include "base/string_util.h"
+#include "base/thread_restrictions.h"
 #include "build/build_config.h"
 #include "googleurl/src/gurl.h"
 #include "net/base/io_buffer.h"
 #include "net/base/load_flags.h"
 #include "net/base/mime_util.h"
 #include "net/base/net_errors.h"
 #include "net/base/net_util.h"
 #include "net/http/http_util.h"
 #include "net/url_request/url_request.h"
 #include "net/url_request/url_request_file_dir_job.h"
@@ skipping 85 matching lines @@
 #if defined(OS_WIN)
   // Resolve UNC paths on a background thread.
   if (!file_path_.value().compare(0, 2, L"\\\\")) {
     DCHECK(!async_resolver_);
     async_resolver_ = new AsyncResolver(this);
     WorkerPool::PostTask(FROM_HERE, NewRunnableMethod(
         async_resolver_.get(), &AsyncResolver::Resolve, file_path_), true);
     return;
   }
 #endif
+
+  // URL requests should not block on the disk!
+  //   http://code.google.com/p/chromium/issues/detail?id=59849
+  bool exists;
   base::PlatformFileInfo file_info;
-  bool exists = file_util::GetFileInfo(file_path_, &file_info);
+  {
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
+    exists = file_util::GetFileInfo(file_path_, &file_info);
+  }
 
   // Continue asynchronously.
   MessageLoop::current()->PostTask(FROM_HERE, NewRunnableMethod(
       this, &URLRequestFileJob::DidResolve, exists, file_info));
 }
 
 void URLRequestFileJob::Kill() {
   stream_.Close();
 
 #if defined(OS_WIN)
@@ skipping 180 matching lines @@
   if (!resolved)
     return false;
 
   *location = net::FilePathToFileURL(new_path);
   *http_status_code = 301;
   return true;
 #else
   return false;
 #endif
 }