Sync: Avoid 3 passes over SyncDarta DB when loading the directory from the disk

sync/syncable/directory_backing_store.h

 // Copyright 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.
 
 #ifndef SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_
 #define SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
@@ skipping 31 matching lines @@
 //
 // This class is abstract so that we can extend it in interesting ways for use
 // in tests.  The concrete class used in non-test scenarios is
 // OnDiskDirectoryBackingStore.
 class SYNC_EXPORT_PRIVATE DirectoryBackingStore : public base::NonThreadSafe {
  public:
   explicit DirectoryBackingStore(const std::string& dir_name);
   virtual ~DirectoryBackingStore();
 
   // Loads and drops all currently persisted meta entries into |handles_map|
-  // and loads appropriate persisted kernel info into |info_bucket|.
+  // and loads appropriate persisted kernel info into |kernel_load_info|.
+  // The function determines which entries can be safely dropped and inserts
+  // their keys into |metahandles_to_purge|. It is up to the caller to
+  // perform the actual cleanup.
   //
-  // This function can perform some cleanup tasks behind the scenes.  It will
-  // clean up unused entries from the database and migrate to the latest
-  // database version.  The caller can safely ignore these details.
+  // This function will migrate to the latest database version.
   //
   // NOTE: On success (return value of OPENED), the buckets are populated with
   // newly allocated items, meaning ownership is bestowed upon the caller.
   virtual DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                              JournalIndex* delete_journals,
+                             MetahandleSet* metahandles_to_purge,
                              Directory::KernelLoadInfo* kernel_load_info) = 0;
 
   // Updates the on-disk store with the input |snapshot| as a database
   // transaction.  Does NOT open any syncable transactions as this would cause
   // opening transactions elsewhere to block on synchronous I/O.
   // DO NOT CALL THIS FROM MORE THAN ONE THREAD EVER.  Also, whichever thread
   // calls SaveChanges *must* be the thread that owns/destroys |this|.
   virtual bool SaveChanges(const Directory::SaveChangesSnapshot& snapshot);
 
  protected:
@@ skipping 11 matching lines @@
 
   bool CreateShareInfoTableVersion71(bool is_temporary);
   // Create 'metas' or 'temp_metas' depending on value of is_temporary. Also
   // create a 'deleted_metas' table using same schema.
   bool CreateMetasTable(bool is_temporary);
   bool CreateModelsTable();
   bool CreateV71ModelsTable();
   bool CreateV75ModelsTable();
   bool CreateV81ModelsTable();
 
-  // We don't need to load any synced and applied deleted entries, we can
-  // in fact just purge them forever on startup.
-  bool DropDeletedEntries();
   // Drops a table if it exists, harmless if the table did not already exist.
   bool SafeDropTable(const char* table_name);
 
   // Load helpers for entries and attributes.
-  bool LoadEntries(Directory::MetahandlesMap* handles_map);
+  bool LoadEntries(Directory::MetahandlesMap* handles_map,
+                   MetahandleSet* metahandles_to_purge);
   bool LoadDeleteJournals(JournalIndex* delete_journals);
   bool LoadInfo(Directory::KernelLoadInfo* info);
+  bool SafeToPurgeOnLoading(const EntryKernel& entry) const;
 
   // Save/update helpers for entries.  Return false if sqlite commit fails.
   static bool SaveEntryToDB(sql::Statement* save_statement,
                             const EntryKernel& entry);
   bool SaveNewEntryToDB(const EntryKernel& entry);
   bool UpdateEntryToDB(const EntryKernel& entry);
 
   // Close save_dbhandle_.  Broken out for testing.
   void EndSave();
 
@@ skipping 76 matching lines @@
   void PrepareSaveEntryStatement(EntryTable table,
                                  sql::Statement* save_statement);
 
   DISALLOW_COPY_AND_ASSIGN(DirectoryBackingStore);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_