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

According to profiling the first two read passes over SyncData DB
are from DirectoryBackingStore::DropDeletedEntries. This function
executes two SQL queries that drop entries pending deletion.
Since it is uncommon to have entries pending deletion it is far
more efficient to remove DirectoryBackingStore::DropDeletedEntries
and instead check for the entries matching this criteria inside
DirectoryBackingStore::LoadEntries, skip those entries from being
loaded and put them straight into Directory's metahandles_to_purge
collection which sets them up for deletion during a subsequent database
save.

See the bug for the preliminary performance results.

BUG=464073

Review URL: https://codereview.chromium.org/1008103002

Cr-Commit-Position: refs/heads/master@{#320861}

18 files changed, 188 insertions, 75 deletions

diff --git a/sync/syncable/deferred_on_disk_directory_backing_store.cc b/sync/syncable/deferred_on_disk_directory_backing_store.cc
index 0367e5b..fbb7845 100644
--- a/sync/syncable/deferred_on_disk_directory_backing_store.cc
+++ b/sync/syncable/deferred_on_disk_directory_backing_store.cc
@@ -52,6 +52,7 @@ bool DeferredOnDiskDirectoryBackingStore::SaveChanges(
 DirOpenResult DeferredOnDiskDirectoryBackingStore::Load(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   // Open an in-memory database at first to create initial sync data needed by
   // Directory.
@@ -61,7 +62,7 @@ DirOpenResult DeferredOnDiskDirectoryBackingStore::Load(
 
   if (!InitializeTables())
     return FAILED_OPEN_DATABASE;
-  if (!LoadEntries(handles_map))
+  if (!LoadEntries(handles_map, metahandles_to_purge))
     return FAILED_DATABASE_CORRUPT;
   if (!LoadInfo(kernel_load_info))
     return FAILED_DATABASE_CORRUPT;
diff --git a/sync/syncable/deferred_on_disk_directory_backing_store.h b/sync/syncable/deferred_on_disk_directory_backing_store.h
index c697120..b50da38 100644
--- a/sync/syncable/deferred_on_disk_directory_backing_store.h
+++ b/sync/syncable/deferred_on_disk_directory_backing_store.h
@@ -25,6 +25,7 @@ class SYNC_EXPORT_PRIVATE DeferredOnDiskDirectoryBackingStore
   ~DeferredOnDiskDirectoryBackingStore() override;
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
   bool SaveChanges(const Directory::SaveChangesSnapshot& snapshot) override;
 
diff --git a/sync/syncable/deferred_on_disk_directory_backing_store_unittest.cc b/sync/syncable/deferred_on_disk_directory_backing_store_unittest.cc
index fbba75e..020eeb4 100644
--- a/sync/syncable/deferred_on_disk_directory_backing_store_unittest.cc
+++ b/sync/syncable/deferred_on_disk_directory_backing_store_unittest.cc
@@ -31,6 +31,7 @@ class DeferredOnDiskDirectoryBackingStoreTest : public testing::Test {
   base::FilePath db_path_;
   Directory::MetahandlesMap handles_map_;
   JournalIndex delete_journals_;
+  MetahandleSet metahandles_to_purge_;
   Directory::KernelLoadInfo kernel_load_info_;
 };
 
@@ -38,8 +39,9 @@ class DeferredOnDiskDirectoryBackingStoreTest : public testing::Test {
 TEST_F(DeferredOnDiskDirectoryBackingStoreTest, Load) {
   DeferredOnDiskDirectoryBackingStore store("test", db_path_);
   EXPECT_EQ(OPENED, store.Load(&handles_map_, &delete_journals_,
-                               &kernel_load_info_));
+                               &metahandles_to_purge_, &kernel_load_info_));
   EXPECT_TRUE(delete_journals_.empty());
+  EXPECT_TRUE(metahandles_to_purge_.empty());
   ASSERT_EQ(1u, handles_map_.size());   // root node
   ASSERT_TRUE(handles_map_.count(1));
   EntryKernel* root = handles_map_[1];
@@ -55,7 +57,7 @@ TEST_F(DeferredOnDiskDirectoryBackingStoreTest,
     // Open and close.
     DeferredOnDiskDirectoryBackingStore store("test", db_path_);
     EXPECT_EQ(OPENED, store.Load(&handles_map_, &delete_journals_,
-                                 &kernel_load_info_));
+                                 &metahandles_to_purge_, &kernel_load_info_));
   }
 
   EXPECT_FALSE(base::PathExists(db_path_));
@@ -68,7 +70,7 @@ TEST_F(DeferredOnDiskDirectoryBackingStoreTest,
     // Open and close.
     DeferredOnDiskDirectoryBackingStore store("test", db_path_);
     EXPECT_EQ(OPENED, store.Load(&handles_map_, &delete_journals_,
-                                 &kernel_load_info_));
+                                 &metahandles_to_purge_, &kernel_load_info_));
 
     Directory::SaveChangesSnapshot snapshot;
     store.SaveChanges(snapshot);
@@ -83,7 +85,7 @@ TEST_F(DeferredOnDiskDirectoryBackingStoreTest, PersistWhenSavingValidChanges) {
     // Open and close.
     DeferredOnDiskDirectoryBackingStore store("test", db_path_);
     EXPECT_EQ(OPENED, store.Load(&handles_map_, &delete_journals_,
-                                 &kernel_load_info_));
+                                 &metahandles_to_purge_, &kernel_load_info_));
 
     Directory::SaveChangesSnapshot snapshot;
     EntryKernel* entry = new EntryKernel();
@@ -98,8 +100,9 @@ TEST_F(DeferredOnDiskDirectoryBackingStoreTest, PersistWhenSavingValidChanges) {
 
   ASSERT_TRUE(base::PathExists(db_path_));
   OnDiskDirectoryBackingStore read_store("test", db_path_);
-  EXPECT_EQ(OPENED, read_store.Load(&handles_map_, &delete_journals_,
-                                    &kernel_load_info_));
+  EXPECT_EQ(OPENED,
+            read_store.Load(&handles_map_, &delete_journals_,
+                            &metahandles_to_purge_, &kernel_load_info_));
   ASSERT_EQ(2u, handles_map_.size());
   ASSERT_TRUE(handles_map_.count(1));     // root node
   ASSERT_TRUE(handles_map_.count(2));
diff --git a/sync/syncable/directory.cc b/sync/syncable/directory.cc
index 1261695..9a689ac 100644
--- a/sync/syncable/directory.cc
+++ b/sync/syncable/directory.cc
@@ -77,7 +77,8 @@ Directory::SaveChangesSnapshot::~SaveChangesSnapshot() {
 
 Directory::Kernel::Kernel(
     const std::string& name,
-    const KernelLoadInfo& info, DirectoryChangeDelegate* delegate,
+    const KernelLoadInfo& info,
+    DirectoryChangeDelegate* delegate,
     const WeakHandle<TransactionObserver>& transaction_observer)
     : next_write_transaction_id(0),
       name(name),
@@ -178,12 +179,13 @@ DirOpenResult Directory::OpenImpl(
 
   // Avoids mem leaks on failure.  Harmlessly deletes the empty hash map after
   // the swap in the success case.
-  STLValueDeleter<Directory::MetahandlesMap> deleter(&tmp_handles_map);
+  STLValueDeleter<MetahandlesMap> deleter(&tmp_handles_map);
 
   JournalIndex delete_journals;
+  MetahandleSet metahandles_to_purge;
 
-  DirOpenResult result =
-      store_->Load(&tmp_handles_map, &delete_journals, &info);
+  DirOpenResult result = store_->Load(&tmp_handles_map, &delete_journals,
+                                      &metahandles_to_purge, &info);
   if (OPENED != result)
     return result;
 
@@ -195,6 +197,8 @@ DirOpenResult Directory::OpenImpl(
   // prevent local ID reuse in the case of an early crash.  See the comments in
   // TakeSnapshotForSaveChanges() or crbug.com/142987 for more information.
   kernel_->info_status = KERNEL_SHARE_INFO_DIRTY;
+
+  kernel_->metahandles_to_purge.swap(metahandles_to_purge);
   if (!SaveChanges())
     return FAILED_INITIAL_WRITE;
 
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index 98706b6..89eec17 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -599,11 +599,12 @@ class SYNC_EXPORT Directory {
   // processed |snapshot| failed, for example, due to no disk space.
   void HandleSaveChangesFailure(const SaveChangesSnapshot& snapshot);
 
-  // Used by CheckTreeInvariants
+  // Used by CheckTreeInvariants.
   void GetAllMetaHandles(BaseTransaction* trans, MetahandleSet* result);
+
+  // Used by VacuumAfterSaveChanges.
   bool SafeToPurgeFromMemory(WriteTransaction* trans,
                              const EntryKernel* const entry) const;
-
   // A helper used by GetTotalNodeCount.
   void GetChildSetForKernel(
       BaseTransaction*,
diff --git a/sync/syncable/directory_backing_store.cc b/sync/syncable/directory_backing_store.cc
index f87933b..9d18a2d 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);
diff --git a/sync/syncable/directory_backing_store.h b/sync/syncable/directory_backing_store.h
index 254fff4..36a2f0a 100644
--- a/sync/syncable/directory_backing_store.h
+++ b/sync/syncable/directory_backing_store.h
@@ -49,16 +49,18 @@ class SYNC_EXPORT_PRIVATE DirectoryBackingStore : public base::NonThreadSafe {
   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
@@ -90,16 +92,15 @@ class SYNC_EXPORT_PRIVATE DirectoryBackingStore : public base::NonThreadSafe {
   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,
diff --git a/sync/syncable/directory_backing_store_unittest.cc b/sync/syncable/directory_backing_store_unittest.cc
index 8f679f1..7394b0d 100644
--- a/sync/syncable/directory_backing_store_unittest.cc
+++ b/sync/syncable/directory_backing_store_unittest.cc
@@ -46,10 +46,11 @@ class MigrationTest : public testing::TestWithParam<int> {
   static bool LoadAndIgnoreReturnedData(DirectoryBackingStore *dbs) {
     Directory::MetahandlesMap tmp_handles_map;
     JournalIndex  delete_journals;
+    MetahandleSet metahandles_to_purge;
     STLValueDeleter<Directory::MetahandlesMap> deleter(&tmp_handles_map);
     Directory::KernelLoadInfo kernel_load_info;
-    return dbs->Load(&tmp_handles_map, &delete_journals, &kernel_load_info) ==
-        OPENED;
+    return dbs->Load(&tmp_handles_map, &delete_journals, &metahandles_to_purge,
+                     &kernel_load_info) == OPENED;
   }
 
   void SetUpVersion67Database(sql::Connection* connection);
@@ -3202,12 +3203,14 @@ TEST_F(DirectoryBackingStoreTest, MigrateVersion78To79) {
 
   // Ensure the next_id has been incremented.
   Directory::MetahandlesMap handles_map;
-  JournalIndex  delete_journals;;
+  JournalIndex delete_journals;
+  MetahandleSet metahandles_to_purge;
   STLValueDeleter<Directory::MetahandlesMap> deleter(&handles_map);
   Directory::KernelLoadInfo load_info;
 
   s.Clear();
-  ASSERT_TRUE(dbs->Load(&handles_map, &delete_journals, &load_info));
+  ASSERT_TRUE(dbs->Load(&handles_map, &delete_journals, &metahandles_to_purge,
+                        &load_info));
   EXPECT_LE(load_info.kernel_info.next_id, kInitialNextId - 65536);
 }
 
@@ -3225,11 +3228,13 @@ TEST_F(DirectoryBackingStoreTest, MigrateVersion79To80) {
 
   // Ensure the bag_of_chips has been set.
   Directory::MetahandlesMap handles_map;
-  JournalIndex  delete_journals;;
+  JournalIndex delete_journals;
+  MetahandleSet metahandles_to_purge;
   STLValueDeleter<Directory::MetahandlesMap> deleter(&handles_map);
   Directory::KernelLoadInfo load_info;
 
-  ASSERT_TRUE(dbs->Load(&handles_map, &delete_journals, &load_info));
+  ASSERT_TRUE(dbs->Load(&handles_map, &delete_journals, &metahandles_to_purge,
+                        &load_info));
   // Check that the initial value is the serialization of an empty ChipBag.
   sync_pb::ChipBag chip_bag;
   std::string serialized_chip_bag;
@@ -3438,11 +3443,13 @@ TEST_F(DirectoryBackingStoreTest, DetectInvalidPosition) {
 
   // Trying to unpack this entry should signal that the DB is corrupted.
   Directory::MetahandlesMap handles_map;
-  JournalIndex  delete_journals;;
+  JournalIndex delete_journals;
+  MetahandleSet metahandles_to_purge;
   STLValueDeleter<Directory::MetahandlesMap> deleter(&handles_map);
   Directory::KernelLoadInfo kernel_load_info;
   ASSERT_EQ(FAILED_DATABASE_CORRUPT,
-            dbs->Load(&handles_map, &delete_journals, &kernel_load_info));
+            dbs->Load(&handles_map, &delete_journals, &metahandles_to_purge,
+                      &kernel_load_info));
 }
 
 TEST_P(MigrationTest, ToCurrentVersion) {
@@ -3529,13 +3536,17 @@ TEST_P(MigrationTest, ToCurrentVersion) {
 
   syncable::Directory::KernelLoadInfo dir_info;
   Directory::MetahandlesMap handles_map;
-  JournalIndex  delete_journals;;
+  JournalIndex delete_journals;
+  MetahandleSet metahandles_to_purge;
   STLValueDeleter<Directory::MetahandlesMap> index_deleter(&handles_map);
 
   {
     scoped_ptr<TestDirectoryBackingStore> dbs(
         new TestDirectoryBackingStore(GetUsername(), &connection));
-    ASSERT_EQ(OPENED, dbs->Load(&handles_map, &delete_journals, &dir_info));
+    ASSERT_EQ(OPENED, dbs->Load(&handles_map, &delete_journals,
+                                &metahandles_to_purge, &dir_info));
+    if (!metahandles_to_purge.empty())
+      dbs->DeleteEntries(metahandles_to_purge);
     ASSERT_FALSE(dbs->needs_column_refresh_);
     ASSERT_EQ(kCurrentDBVersion, dbs->GetVersion());
   }
@@ -3903,20 +3914,22 @@ TEST_F(DirectoryBackingStoreTest, DeleteEntries) {
       new TestDirectoryBackingStore(GetUsername(), &connection));
   Directory::MetahandlesMap handles_map;
   JournalIndex  delete_journals;
+  MetahandleSet metahandles_to_purge;
   Directory::KernelLoadInfo kernel_load_info;
   STLValueDeleter<Directory::MetahandlesMap> index_deleter(&handles_map);
 
-  dbs->Load(&handles_map, &delete_journals, &kernel_load_info);
+  dbs->Load(&handles_map, &delete_journals, &metahandles_to_purge,
+            &kernel_load_info);
   size_t initial_size = handles_map.size();
   ASSERT_LT(0U, initial_size) << "Test requires handles_map to delete.";
   int64 first_to_die = handles_map.begin()->second->ref(META_HANDLE);
   MetahandleSet to_delete;
   to_delete.insert(first_to_die);
-  EXPECT_TRUE(dbs->DeleteEntries(TestDirectoryBackingStore::METAS_TABLE,
-                                 to_delete));
+  EXPECT_TRUE(dbs->DeleteEntries(to_delete));
 
   STLDeleteValues(&handles_map);
-  dbs->LoadEntries(&handles_map);
+  metahandles_to_purge.clear();
+  dbs->LoadEntries(&handles_map, &metahandles_to_purge);
 
   EXPECT_EQ(initial_size - 1, handles_map.size());
   bool delete_failed = false;
@@ -3935,11 +3948,11 @@ TEST_F(DirectoryBackingStoreTest, DeleteEntries) {
     to_delete.insert(it->first);
   }
 
-  EXPECT_TRUE(dbs->DeleteEntries(TestDirectoryBackingStore::METAS_TABLE,
-                                 to_delete));
+  EXPECT_TRUE(dbs->DeleteEntries(to_delete));
 
   STLDeleteValues(&handles_map);
-  dbs->LoadEntries(&handles_map);
+  metahandles_to_purge.clear();
+  dbs->LoadEntries(&handles_map, &metahandles_to_purge);
   EXPECT_EQ(0U, handles_map.size());
 }
 
diff --git a/sync/syncable/directory_unittest.cc b/sync/syncable/directory_unittest.cc
index 5f8cbed..aa974d5 100644
--- a/sync/syncable/directory_unittest.cc
+++ b/sync/syncable/directory_unittest.cc
@@ -552,6 +552,87 @@ TEST_F(SyncableDirectoryTest, ManageDeleteJournals) {
   }
 }
 
+TEST_F(SyncableDirectoryTest, TestPurgeDeletedEntriesOnReload) {
+  sync_pb::EntitySpecifics specifics;
+  AddDefaultFieldValue(PREFERENCES, &specifics);
+
+  const int kClientCount = 2;
+  const int kServerCount = 5;
+  const int kTestCount = kClientCount + kServerCount;
+  int64 handles[kTestCount];
+
+  // The idea is to recreate various combinations of IDs, IS_DEL,
+  // IS_UNSYNCED, and IS_UNAPPLIED_UPDATE flags to test all combinations
+  // for DirectoryBackingStore::SafeToPurgeOnLoading.
+  // 0: client ID, IS_DEL, IS_UNSYNCED
+  // 1: client ID, IS_UNSYNCED
+  // 2: server ID, IS_DEL, IS_UNSYNCED, IS_UNAPPLIED_UPDATE
+  // 3: server ID, IS_DEL, IS_UNSYNCED
+  // 4: server ID, IS_DEL, IS_UNAPPLIED_UPDATE
+  // 5: server ID, IS_DEL
+  // 6: server ID
+  {
+    WriteTransaction trans(FROM_HERE, UNITTEST, dir().get());
+
+    for (int i = 0; i < kTestCount; i++) {
+      std::string name = base::StringPrintf("item%d", i);
+      MutableEntry item(&trans, CREATE, PREFERENCES, trans.root_id(), name);
+      ASSERT_TRUE(item.good());
+
+      handles[i] = item.GetMetahandle();
+
+      if (i < kClientCount) {
+        item.PutId(TestIdFactory::FromNumber(i - kClientCount));
+      } else {
+        item.PutId(TestIdFactory::FromNumber(i));
+      }
+
+      item.PutUniqueClientTag(name);
+      item.PutIsUnsynced(true);
+      item.PutSpecifics(specifics);
+      item.PutServerSpecifics(specifics);
+
+      if (i >= kClientCount) {
+        item.PutBaseVersion(10);
+        item.PutServerVersion(10);
+      }
+
+      // Set flags
+      if (i != 1 && i != 6)
+        item.PutIsDel(true);
+
+      if (i >= 4)
+        item.PutIsUnsynced(false);
+
+      if (i == 2 || i == 4)
+        item.PutIsUnappliedUpdate(true);
+    }
+  }
+  ASSERT_EQ(OPENED, SimulateSaveAndReloadDir());
+
+  // Expect items 0 and 5 to be purged according to
+  // DirectoryBackingStore::SafeToPurgeOnLoading:
+  // - Item 0 is an item with IS_DEL flag and client ID.
+  // - Item 5 is an item with IS_DEL flag which has both
+  //   IS_UNSYNCED and IS_UNAPPLIED_UPDATE unset.
+  std::vector<int64> expected_purged;
+  expected_purged.push_back(0);
+  expected_purged.push_back(5);
+
+  std::vector<int64> actually_purged;
+  {
+    ReadTransaction trans(FROM_HERE, dir().get());
+    for (int i = 0; i < kTestCount; i++) {
+      Entry item(&trans, GET_BY_HANDLE, handles[i]);
+      if (!item.good()) {
+        actually_purged.push_back(i);
+      }
+    }
+  }
+
+  EXPECT_EQ(expected_purged, actually_purged);
+}
+
 TEST_F(SyncableDirectoryTest, TestBasicLookupNonExistantID) {
   ReadTransaction rtrans(FROM_HERE, dir().get());
   Entry e(&rtrans, GET_BY_ID, TestIdFactory::FromNumber(-99));
diff --git a/sync/syncable/in_memory_directory_backing_store.cc b/sync/syncable/in_memory_directory_backing_store.cc
index 57995f8..8f84baa 100644
--- a/sync/syncable/in_memory_directory_backing_store.cc
+++ b/sync/syncable/in_memory_directory_backing_store.cc
@@ -16,6 +16,7 @@ InMemoryDirectoryBackingStore::InMemoryDirectoryBackingStore(
 DirOpenResult InMemoryDirectoryBackingStore::Load(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   if (!db_->is_open()) {
     if (!db_->OpenInMemory())
@@ -32,9 +33,7 @@ DirOpenResult InMemoryDirectoryBackingStore::Load(
     }
   }
 
-  if (!DropDeletedEntries())
-    return FAILED_DATABASE_CORRUPT;
-  if (!LoadEntries(handles_map))
+  if (!LoadEntries(handles_map, metahandles_to_purge))
     return FAILED_DATABASE_CORRUPT;
   if (!LoadDeleteJournals(delete_journals))
     return FAILED_DATABASE_CORRUPT;
diff --git a/sync/syncable/in_memory_directory_backing_store.h b/sync/syncable/in_memory_directory_backing_store.h
index 0a0f9ba..d22e65f 100644
--- a/sync/syncable/in_memory_directory_backing_store.h
+++ b/sync/syncable/in_memory_directory_backing_store.h
@@ -26,6 +26,7 @@ class SYNC_EXPORT_PRIVATE InMemoryDirectoryBackingStore
   explicit InMemoryDirectoryBackingStore(const std::string& dir_name);
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
 
   void request_consistent_cache_guid() {
diff --git a/sync/syncable/invalid_directory_backing_store.cc b/sync/syncable/invalid_directory_backing_store.cc
index 6345521..1844d84 100644
--- a/sync/syncable/invalid_directory_backing_store.cc
+++ b/sync/syncable/invalid_directory_backing_store.cc
@@ -17,6 +17,7 @@ InvalidDirectoryBackingStore::~InvalidDirectoryBackingStore() {
 DirOpenResult InvalidDirectoryBackingStore::Load(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   return FAILED_OPEN_DATABASE;
 }
diff --git a/sync/syncable/invalid_directory_backing_store.h b/sync/syncable/invalid_directory_backing_store.h
index fd99653..2a6d065 100644
--- a/sync/syncable/invalid_directory_backing_store.h
+++ b/sync/syncable/invalid_directory_backing_store.h
@@ -19,6 +19,7 @@ class SYNC_EXPORT_PRIVATE InvalidDirectoryBackingStore
   ~InvalidDirectoryBackingStore() override;
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
 
  private:
diff --git a/sync/syncable/mutable_entry.cc b/sync/syncable/mutable_entry.cc
index 89c8b05..d627cf4 100644
--- a/sync/syncable/mutable_entry.cc
+++ b/sync/syncable/mutable_entry.cc
@@ -184,9 +184,9 @@ void MutableEntry::PutIsDel(bool value) {
     // - Ensure that the item is never committed to the server.
     // - Allow any items with the same UNIQUE_CLIENT_TAG created on other
     //   clients to override this entry.
-    // - Let us delete this entry permanently through
-    //   DirectoryBackingStore::DropDeletedEntries() when we next restart sync.
-    //   This will save memory and avoid crbug.com/125381.
+    // - Let us delete this entry permanently when we next restart sync - see
+    //   DirectoryBackingStore::SafeToPurgeOnLoading.
+    //   This will avoid crbug.com/125381.
     // Note: do not unset IsUnsynced if the syncer is in the middle of
     // attempting to commit this entity.
     if (!GetId().ServerKnows() && !GetSyncing()) {
diff --git a/sync/syncable/on_disk_directory_backing_store.cc b/sync/syncable/on_disk_directory_backing_store.cc
index 1bebf9a..dcbaedd 100644
--- a/sync/syncable/on_disk_directory_backing_store.cc
+++ b/sync/syncable/on_disk_directory_backing_store.cc
@@ -37,6 +37,7 @@ OnDiskDirectoryBackingStore::~OnDiskDirectoryBackingStore() { }
 DirOpenResult OnDiskDirectoryBackingStore::TryLoad(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   DCHECK(CalledOnValidThread());
   if (!db_->is_open()) {
@@ -47,9 +48,7 @@ DirOpenResult OnDiskDirectoryBackingStore::TryLoad(
   if (!InitializeTables())
     return FAILED_OPEN_DATABASE;
 
-  if (!DropDeletedEntries())
-    return FAILED_DATABASE_CORRUPT;
-  if (!LoadEntries(handles_map))
+  if (!LoadEntries(handles_map, metahandles_to_purge))
     return FAILED_DATABASE_CORRUPT;
   if (!LoadDeleteJournals(delete_journals))
     return FAILED_DATABASE_CORRUPT;
@@ -65,9 +64,10 @@ DirOpenResult OnDiskDirectoryBackingStore::TryLoad(
 DirOpenResult OnDiskDirectoryBackingStore::Load(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   DirOpenResult result = TryLoad(handles_map, delete_journals,
-                                 kernel_load_info);
+                                 metahandles_to_purge, kernel_load_info);
   if (result == OPENED) {
     UMA_HISTOGRAM_ENUMERATION(
         "Sync.DirectoryOpenResult", FIRST_TRY_SUCCESS, RESULT_COUNT);
@@ -89,7 +89,8 @@ DirOpenResult OnDiskDirectoryBackingStore::Load(
   db_->set_histogram_tag("SyncDirectory");
   base::DeleteFile(backing_filepath_, false);
 
-  result = TryLoad(handles_map, delete_journals, kernel_load_info);
+  result = TryLoad(handles_map, delete_journals, metahandles_to_purge,
+                   kernel_load_info);
   if (result == OPENED) {
     UMA_HISTOGRAM_ENUMERATION(
         "Sync.DirectoryOpenResult", SECOND_TRY_SUCCESS, RESULT_COUNT);
diff --git a/sync/syncable/on_disk_directory_backing_store.h b/sync/syncable/on_disk_directory_backing_store.h
index fdeda4d..43bc74d 100644
--- a/sync/syncable/on_disk_directory_backing_store.h
+++ b/sync/syncable/on_disk_directory_backing_store.h
@@ -22,14 +22,15 @@ class SYNC_EXPORT_PRIVATE OnDiskDirectoryBackingStore
   ~OnDiskDirectoryBackingStore() override;
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
 
   // A helper function that will make one attempt to load the directory.
   // Unlike Load(), it does not attempt to recover from failure.
-  DirOpenResult TryLoad(
-      Directory::MetahandlesMap* handles_map,
-      JournalIndex* delete_journals,
-      Directory::KernelLoadInfo* kernel_load_info);
+  DirOpenResult TryLoad(Directory::MetahandlesMap* handles_map,
+                        JournalIndex* delete_journals,
+                        MetahandleSet* metahandles_to_purge,
+                        Directory::KernelLoadInfo* kernel_load_info);
 
  protected:
   // Subclasses may override this to avoid a possible DCHECK.
diff --git a/sync/test/test_directory_backing_store.cc b/sync/test/test_directory_backing_store.cc
index 6a16a06..f382cc5 100644
--- a/sync/test/test_directory_backing_store.cc
+++ b/sync/test/test_directory_backing_store.cc
@@ -24,15 +24,14 @@ TestDirectoryBackingStore::~TestDirectoryBackingStore() {
 DirOpenResult TestDirectoryBackingStore::Load(
     Directory::MetahandlesMap* handles_map,
     JournalIndex* delete_journals,
+    MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   DCHECK(db_->is_open());
 
   if (!InitializeTables())
     return FAILED_OPEN_DATABASE;
 
-  if (!DropDeletedEntries())
-    return FAILED_DATABASE_CORRUPT;
-  if (!LoadEntries(handles_map))
+  if (!LoadEntries(handles_map, metahandles_to_purge))
     return FAILED_DATABASE_CORRUPT;
   if (!LoadDeleteJournals(delete_journals))
     return FAILED_DATABASE_CORRUPT;
@@ -44,5 +43,10 @@ DirOpenResult TestDirectoryBackingStore::Load(
   return OPENED;
 }
 
+bool TestDirectoryBackingStore::DeleteEntries(const MetahandleSet& handles) {
+  return DirectoryBackingStore::DeleteEntries(
+      DirectoryBackingStore::METAS_TABLE, handles);
+}
+
 }  // namespace syncable
 }  // namespace syncer
diff --git a/sync/test/test_directory_backing_store.h b/sync/test/test_directory_backing_store.h
index 541528e..1e71816 100644
--- a/sync/test/test_directory_backing_store.h
+++ b/sync/test/test_directory_backing_store.h
@@ -27,8 +27,9 @@ class TestDirectoryBackingStore : public DirectoryBackingStore {
   ~TestDirectoryBackingStore() override;
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
-
+  bool DeleteEntries(const MetahandleSet& handles);
   FRIEND_TEST_ALL_PREFIXES(DirectoryBackingStoreTest, MigrateVersion67To68);
   FRIEND_TEST_ALL_PREFIXES(DirectoryBackingStoreTest, MigrateVersion68To69);
   FRIEND_TEST_ALL_PREFIXES(DirectoryBackingStoreTest, MigrateVersion69To70);