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

sync/syncable/directory_backing_store.cc

Index: sync/syncable/directory_backing_store.cc
diff --git a/sync/syncable/directory_backing_store.cc b/sync/syncable/directory_backing_store.cc
index f87933bea6323bfa9612ed6f2c031141985f2e66..9d18a2dbb3bcc5eb27bc430a0dde410c13187099 100644
--- a/sync/syncable/directory_backing_store.cc
+++ b/sync/syncable/directory_backing_store.cc
@@ -538,8 +538,8 @@ bool DirectoryBackingStore::RefreshColumns() {
   return true;
 }
 
-bool DirectoryBackingStore::LoadEntries(
-    Directory::MetahandlesMap* handles_map) {
+bool DirectoryBackingStore::LoadEntries(Directory::MetahandlesMap* handles_map,
+                                        MetahandleSet* metahandles_to_purge) {
   string select;
   select.reserve(kUpdateStatementBufferSize);
   select.append("SELECT ");
@@ -555,11 +555,25 @@ bool DirectoryBackingStore::LoadEntries(
       return false;
 
     int64 handle = kernel->ref(META_HANDLE);
-    (*handles_map)[handle] = kernel.release();
+    if (SafeToPurgeOnLoading(*kernel))
+      metahandles_to_purge->insert(handle);
+    else
+      (*handles_map)[handle] = kernel.release();
   }
   return s.Succeeded();
 }
 
+bool DirectoryBackingStore::SafeToPurgeOnLoading(
+    const EntryKernel& entry) const {
+  if (entry.ref(IS_DEL)) {
+    if (!entry.ref(IS_UNSYNCED) && !entry.ref(IS_UNAPPLIED_UPDATE))
+      return true;
+    else if (!entry.ref(ID).ServerKnows())
+      return true;
+  }
+  return false;
+}
+
 bool DirectoryBackingStore::LoadDeleteJournals(
     JournalIndex* delete_journals) {
   string select;
@@ -643,21 +657,6 @@ bool DirectoryBackingStore::SaveEntryToDB(sql::Statement* save_statement,
   return save_statement->Run();
 }
 
-bool DirectoryBackingStore::DropDeletedEntries() {
-  if (!db_->Execute("DELETE FROM metas "
-                    "WHERE is_del > 0 "
-                    "AND is_unsynced < 1 "
-                    "AND is_unapplied_update < 1")) {
-    return false;
-  }
-  if (!db_->Execute("DELETE FROM metas "
-                    "WHERE is_del > 0 "
-                    "AND id LIKE 'c%'")) {
-    return false;
-  }
-  return true;
-}
-
 bool DirectoryBackingStore::SafeDropTable(const char* table_name) {
   string query = "DROP TABLE IF EXISTS ";
   query.append(table_name);