NTPSnippetsDatabase: support multiple concurrent loads

components/ntp_snippets/ntp_snippets_database.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_DATABASE_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_DATABASE_H_
 
 #include <memory>
+#include <string>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner.h"
 #include "components/leveldb_proto/proto_database.h"
 #include "components/ntp_snippets/ntp_snippet.h"
 
 namespace base {
 class FilePath;
 }
 
 namespace ntp_snippets {
 
 class SnippetProto;
 
 class NTPSnippetsDatabase {
  public:
   using SnippetsLoadedCallback = base::Callback<void(NTPSnippet::PtrVector)>;
 
   NTPSnippetsDatabase(
       const base::FilePath& database_dir,
       scoped_refptr<base::SequencedTaskRunner> file_task_runner);
   ~NTPSnippetsDatabase();
 
-  // Loads all snippets from storage and passes them to |callback|. Only one
-  // concurrent fetch is supported; it is only legal to call this again after
-  // the first load has completed.
+  // Loads all snippets from storage and passes them to |callback|.
   void Load(const SnippetsLoadedCallback& callback);
 
   // Adds or updates the given snippet.
   void Save(const NTPSnippet& snippet);
   // Adds or updates all the given snippets.
   void Save(const NTPSnippet::PtrVector& snippets);
 
   // Deletes the snippet with the given ID.
   void Delete(const std::string& snippet_id);
   // Deletes all the given snippets (identified by their IDs).
   void Delete(const NTPSnippet::PtrVector& snippets);
 
  private:
   friend class NTPSnippetsDatabaseTest;
 
   using KeyEntryVector =
       leveldb_proto::ProtoDatabase<SnippetProto>::KeyEntryVector;
 
   // Callbacks for ProtoDatabase operations.
   void OnDatabaseInited(bool success);
-  void OnDatabaseLoaded(bool success,
+  void OnDatabaseLoaded(const SnippetsLoadedCallback& callback,
+                        bool success,
                         std::unique_ptr<std::vector<SnippetProto>> entries);
   void OnDatabaseSaved(bool success);
 
-  void LoadImpl();
+  void LoadImpl(const SnippetsLoadedCallback& callback);
 
   void SaveImpl(std::unique_ptr<KeyEntryVector> entries_to_save);
 
   void DeleteImpl(std::unique_ptr<std::vector<std::string>> keys_to_remove);
 
   std::unique_ptr<leveldb_proto::ProtoDatabase<SnippetProto>> database_;
   bool database_initialized_;
 
-  SnippetsLoadedCallback callback_;
+  std::vector<SnippetsLoadedCallback> pending_load_callbacks_;
 
   base::WeakPtrFactory<NTPSnippetsDatabase> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsDatabase);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_DATABASE_H_