Add sequencing comment to PpdCacheImpl.

chromeos/printing/ppd_cache.cc

 // Copyright 2016 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 "chromeos/printing/ppd_cache.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 98 matching lines @@
             static_cast<int>(checksum.size())) {
       LOG(ERROR) << "Failed to create ppd cache file";
       file.Close();
       if (!base::DeleteFile(path, false)) {
         LOG(ERROR) << "Failed to cleanup failed creation.";
       }
     }
   }
 }
 
+// Implementation of the PpdCache that uses two separate task runners for Store
+// and Fetch since the two operations have different priorities. Note that the
+// two operations are not sequenced so there should be no expectation that a
+// call to Find will return a file that was previously Stored until the Store
+// callback is run.
 class PpdCacheImpl : public PpdCache {
  public:
   explicit PpdCacheImpl(const base::FilePath& cache_base_dir)
       : cache_base_dir_(cache_base_dir),
         fetch_task_runner_(base::CreateSequencedTaskRunnerWithTraits(
             {base::TaskPriority::USER_VISIBLE, base::MayBlock(),
              base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN})),
         store_task_runner_(base::CreateSequencedTaskRunnerWithTraits(
             {base::TaskPriority::BACKGROUND, base::MayBlock(),
              base::TaskShutdownBehavior::BLOCK_SHUTDOWN})) {}
@@ skipping 26 matching lines @@
 
 }  // namespace
 
 // static
 scoped_refptr<PpdCache> PpdCache::Create(const base::FilePath& cache_base_dir) {
   return scoped_refptr<PpdCache>(new PpdCacheImpl(cache_base_dir));
 }
 
 }  // namespace printing
 }  // namespace chromeos