[Sync] Start implementation of migration for USS.

components/sync/engine_impl/uss_migrator.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 "components/sync/engine_impl/uss_migrator.h"
+
+#include <algorithm>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "base/memory/ptr_util.h"
+#include "components/sync/base/time.h"
+#include "components/sync/engine_impl/model_type_worker.h"
+#include "components/sync/protocol/sync.pb.h"
+#include "components/sync/syncable/directory.h"
+#include "components/sync/syncable/entry.h"
+#include "components/sync/syncable/read_node.h"
+#include "components/sync/syncable/read_transaction.h"
+#include "components/sync/syncable/user_share.h"
+
+namespace syncer {
+
+namespace {
+
+bool ExtractSyncEntity(ReadTransaction* trans,
+                       int64_t id,
+                       sync_pb::SyncEntity* entity) {
+  ReadNode read_node(trans);
+  if (read_node.InitByIdLookup(id) != BaseNode::INIT_OK)
+    return false;
+
+  const syncable::Entry& entry = *read_node.GetEntry();
+
+  // Copy the fields USS cares about from the server side of the directory so
+  // that we don't miss things that haven't been applied yet. See
+  // ModelTypeWorker::ProcessGetUpdatesResponse for which fields are used.
+  entity->set_id_string(entry.GetId().GetServerId());
+  entity->set_version(entry.GetServerVersion());
+  entity->set_mtime(TimeToProtoTime(entry.GetServerMtime()));
+  entity->set_ctime(TimeToProtoTime(entry.GetServerCtime()));
+  entity->set_name(entry.GetServerNonUniqueName());
+  entity->set_deleted(entry.GetServerIsDel());
+  entity->set_client_defined_unique_tag(entry.GetUniqueClientTag());
+
+  // It looks like there are fancy other ways to get e.g. passwords specifics
+  // out of Entry. Do we need to special-case them when we ship those types?
+  entity->mutable_specifics()->CopyFrom(entry.GetServerSpecifics());

[pavely, 2016/10/27 23:20:19]

If you were offline for a while then your local specifics would be newer than
server specifics.
Taking server specifics for Migration would cause data loss in this case. I
would take local specifics.

[maxbogue, 2016/10/28 16:17:41]

Wouldn't any local changes be in the model type's local storage and need to be
migrated separately? The purpose of this migration is to prevent the need for an
initial GetUpdates from the server, so I figured having the data align with the
progress marker was most important.

[pavely, 2016/10/28 17:26:58]

You are right. I agree. 

+  return true;
+}
+
+}  // namespace
+
+bool MigrateDirectoryData(ModelType type,
+                          UserShare* user_share,
+                          ModelTypeWorker* worker) {
+  return MigrateDirectoryDataWithBatchSize(type, user_share, worker, 64);
+}
+
+bool MigrateDirectoryDataWithBatchSize(ModelType type,
+                                       UserShare* user_share,
+                                       ModelTypeWorker* worker,
+                                       int batch_size) {
+  DCHECK_NE(BOOKMARKS, type);
+  DCHECK_NE(PASSWORDS, type);
+  ReadTransaction trans(FROM_HERE, user_share);
+
+  ReadNode root(&trans);
+  if (root.InitTypeRoot(type) != BaseNode::INIT_OK) {
+    LOG(ERROR) << "Missing root node for " << ModelTypeToString(type);
+    // Inform the worker so it can trigger a fallback initial GetUpdates.
+    worker->AbortMigration();
+    return false;
+  }
+
+  // Get the progress marker and context from the directory.
+  sync_pb::DataTypeProgressMarker progress;
+  sync_pb::DataTypeContext context;
+  user_share->directory->GetDownloadProgress(type, &progress);
+  user_share->directory->GetDataTypeContext(trans.GetWrappedTrans(), type,
+                                            &context);
+
+  std::vector<int64_t> child_ids;
+  root.GetChildIds(&child_ids);
+
+  // Process |batch_size| entities at a time to reduce memory usage.
+  size_t i = 0;
+  while (i < child_ids.size()) {
+    // Vector to own the temporary entities.
+    std::vector<std::unique_ptr<sync_pb::SyncEntity>> entities;
+    // Vector of raw pointers for passing to ProcessGetUpdatesResponse().
+    SyncEntityList entity_ptrs;
+
+    const size_t batch_limit = std::min(i + batch_size, child_ids.size());
+    for (; i < batch_limit; i++) {
+      auto entity = base::MakeUnique<sync_pb::SyncEntity>();
+      if (!ExtractSyncEntity(&trans, child_ids[i], entity.get())) {
+        LOG(ERROR) << "Failed to fetch child node for "
+                   << ModelTypeToString(type);
+        // Inform the worker so it can clear any partial data and trigger a
+        // fallback initial GetUpdates.
+        worker->AbortMigration();
+        return false;
+      }
+      entity_ptrs.push_back(entity.get());
+      entities.push_back(std::move(entity));
+    }
+
+    worker->ProcessGetUpdatesResponse(progress, context, entity_ptrs, nullptr);
+  }
+
+  worker->PassiveApplyUpdates(nullptr);
+  return true;
+}
+
+}  // namespace syncer