Replace AttachmentStore's StoreAttachments with UploadAttachments.

This change is lays groundwork for making attachment upload operations
persistent.

Make GenericChangeProcessor responsible for writing attachments to the
AttachmentStore and calling UploadAttachments.  In a future CL, datatype
code will be responsible for writing attachments to the store.

Queue up attachments for upload inside of AttachmentService.  In a
future CL we'll add rate limiting, retry, and back-off logic.

Expose AttachmentService's AttachmentStore via GetStore method.

BUG=

Review URL: https://codereview.chromium.org/512413003

Cr-Commit-Position: refs/heads/master@{#293536}

10 files changed, 354 insertions, 236 deletions

diff --git a/components/sync_driver/generic_change_processor.cc b/components/sync_driver/generic_change_processor.cc
index c44ce72..ddf6b67 100644
--- a/components/sync_driver/generic_change_processor.cc
+++ b/components/sync_driver/generic_change_processor.cc
@@ -85,6 +85,11 @@ syncer::SyncData BuildRemoteSyncData(
                                             attachment_service_proxy);
 }
 
+const syncer::AttachmentId& AttachmentToAttachmentId(
+    const syncer::Attachment& attachment) {
+  return attachment.GetId();
+}
+
 }  // namespace
 
 GenericChangeProcessor::GenericChangeProcessor(
@@ -102,7 +107,8 @@ GenericChangeProcessor::GenericChangeProcessor(
       attachment_service_weak_ptr_factory_(attachment_service_.get()),
       attachment_service_proxy_(
           base::MessageLoopProxy::current(),
-          attachment_service_weak_ptr_factory_.GetWeakPtr()) {
+          attachment_service_weak_ptr_factory_.GetWeakPtr()),
+      weak_ptr_factory_(this) {
   DCHECK(CalledOnValidThread());
   DCHECK(attachment_service_);
 }
@@ -387,25 +393,6 @@ syncer::SyncError AttemptDelete(const syncer::SyncChange& change,
   return syncer::SyncError();
 }
 
-// A callback invoked on completion of AttachmentService::StoreAttachment.
-void IgnoreStoreResult(const syncer::AttachmentService::StoreResult&) {
-  // TODO(maniscalco): Here is where we're going to update the in-directory
-  // entry to indicate that the attachments have been successfully stored on
-  // disk.  Why do we care?  Because we might crash after persisting the
-  // directory to disk, but before we have persisted its attachments, leaving us
-  // with danging attachment ids.  Having a flag that indicates we've stored the
-  // entry will allow us to detect and filter entries with dangling attachment
-  // ids (bug 368353).
-}
-
-void StoreAttachments(syncer::AttachmentService* attachment_service,
-                      const syncer::AttachmentList& attachments) {
-  DCHECK(attachment_service);
-  syncer::AttachmentService::StoreCallback ignore_store_result =
-      base::Bind(&IgnoreStoreResult);
-  attachment_service->StoreAttachments(attachments, ignore_store_result);
-}
-
 }  // namespace
 
 syncer::SyncError GenericChangeProcessor::ProcessSyncChanges(
@@ -465,7 +452,7 @@ syncer::SyncError GenericChangeProcessor::ProcessSyncChanges(
   }
 
   if (!new_attachments.empty()) {
-    StoreAttachments(attachment_service_.get(), new_attachments);
+    StoreAndUploadAttachments(new_attachments);
   }
 
   return syncer::SyncError();
@@ -711,4 +698,40 @@ syncer::UserShare* GenericChangeProcessor::share_handle() const {
   return share_handle_;
 }
 
+void GenericChangeProcessor::StoreAndUploadAttachments(
+    const syncer::AttachmentList& attachments) {
+  DCHECK(CalledOnValidThread());
+  attachment_service_->GetStore()->Write(
+      attachments,
+      base::Bind(&GenericChangeProcessor::WriteAttachmentsDone,
+                 weak_ptr_factory_.GetWeakPtr(),
+                 attachments));
+}
+
+void GenericChangeProcessor::WriteAttachmentsDone(
+    const syncer::AttachmentList& attachments,
+    const syncer::AttachmentStore::Result& result) {
+  DCHECK(CalledOnValidThread());
+  if (result != syncer::AttachmentStore::SUCCESS) {
+    // TODO(maniscalco): Deal with case where an error occurred (bug 361251).
+    return;
+  }
+
+  // TODO(maniscalco): Here is where we're going to update the in-directory
+  // entry to indicate that the attachments have been successfully stored on
+  // disk.  Why do we care?  Because we might crash after persisting the
+  // directory to disk, but before we have persisted its attachments, leaving us
+  // with danging attachment ids.  Having a flag that indicates we've stored the
+  // entry will allow us to detect and filter entries with dangling attachment
+  // ids (bug 368353).
+
+  // Begin uploading the attachments now that they are safe on disk.
+  syncer::AttachmentIdSet attachment_ids;
+  std::transform(attachments.begin(),
+                 attachments.end(),
+                 std::inserter(attachment_ids, attachment_ids.end()),
+                 AttachmentToAttachmentId);
+  attachment_service_->UploadAttachments(attachment_ids);
+}
+
 }  // namespace sync_driver
diff --git a/components/sync_driver/generic_change_processor.h b/components/sync_driver/generic_change_processor.h
index 1941600..a3139d8 100644
--- a/components/sync_driver/generic_change_processor.h
+++ b/components/sync_driver/generic_change_processor.h
@@ -13,6 +13,7 @@
 #include "components/sync_driver/change_processor.h"
 #include "components/sync_driver/data_type_controller.h"
 #include "components/sync_driver/data_type_error_handler.h"
+#include "sync/api/attachments/attachment_store.h"
 #include "sync/api/sync_change_processor.h"
 #include "sync/api/sync_merge_result.h"
 #include "sync/internal_api/public/attachments/attachment_service.h"
@@ -126,6 +127,18 @@ class GenericChangeProcessor : public ChangeProcessor,
                                        syncer::WriteNode* sync_node,
                                        syncer::AttachmentList* new_attachments);
 
+  // Store |attachments| locally then upload them to the sync server.
+  //
+  // Store and uploading are asynchronous operations.  |WriteAttachmentsDone|
+  // will be invoked once the attachments have been stored on the local device.
+  void StoreAndUploadAttachments(const syncer::AttachmentList& attachments);
+
+  // Invoked once attachments have been stored locally.
+  //
+  // See also AttachmentStore::WriteCallback.
+  void WriteAttachmentsDone(const syncer::AttachmentList& attachments,
+                            const syncer::AttachmentStore::Result& result);
+
   // The SyncableService this change processor will forward changes on to.
   const base::WeakPtr<syncer::SyncableService> local_service_;
 
@@ -155,6 +168,8 @@ class GenericChangeProcessor : public ChangeProcessor,
       attachment_service_weak_ptr_factory_;
   syncer::AttachmentServiceProxy attachment_service_proxy_;
 
+  base::WeakPtrFactory<GenericChangeProcessor> weak_ptr_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(GenericChangeProcessor);
 };
 
diff --git a/components/sync_driver/generic_change_processor_unittest.cc b/components/sync_driver/generic_change_processor_unittest.cc
index bdd7266..f1b35e6 100644
--- a/components/sync_driver/generic_change_processor_unittest.cc
+++ b/components/sync_driver/generic_change_processor_unittest.cc
@@ -7,6 +7,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
+#include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "components/sync_driver/data_type_error_handler_mock.h"
 #include "components/sync_driver/sync_api_component_factory.h"
@@ -25,6 +26,7 @@
 #include "sync/internal_api/public/user_share.h"
 #include "sync/internal_api/public/write_node.h"
 #include "sync/internal_api/public/write_transaction.h"
+#include "testing/gmock/include/gmock/gmock-matchers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace sync_driver {
@@ -33,17 +35,17 @@ namespace {
 
 const char kTestData[] = "some data";
 
-// A mock that keeps track of attachments passed to StoreAttachments.
+// A mock that keeps track of attachments passed to UploadAttachments.
 class MockAttachmentService : public syncer::AttachmentServiceImpl {
  public:
   MockAttachmentService();
   virtual ~MockAttachmentService();
-  virtual void StoreAttachments(const syncer::AttachmentList& attachments,
-                                const StoreCallback& callback) OVERRIDE;
-  std::vector<syncer::AttachmentList>* attachment_lists();
+  virtual void UploadAttachments(
+      const syncer::AttachmentIdSet& attachment_ids) OVERRIDE;
+  std::vector<syncer::AttachmentIdSet>* attachment_id_sets();
 
  private:
-  std::vector<syncer::AttachmentList> attachment_lists_;
+  std::vector<syncer::AttachmentIdSet> attachment_id_sets_;
 };
 
 MockAttachmentService::MockAttachmentService()
@@ -60,15 +62,15 @@ MockAttachmentService::MockAttachmentService()
 MockAttachmentService::~MockAttachmentService() {
 }
 
-void MockAttachmentService::StoreAttachments(
-    const syncer::AttachmentList& attachments,
-    const StoreCallback& callback) {
-  attachment_lists_.push_back(attachments);
-  AttachmentServiceImpl::StoreAttachments(attachments, callback);
+void MockAttachmentService::UploadAttachments(
+    const syncer::AttachmentIdSet& attachment_ids) {
+  attachment_id_sets_.push_back(attachment_ids);
+  AttachmentServiceImpl::UploadAttachments(attachment_ids);
 }
 
-std::vector<syncer::AttachmentList>* MockAttachmentService::attachment_lists() {
-  return &attachment_lists_;
+std::vector<syncer::AttachmentIdSet>*
+MockAttachmentService::attachment_id_sets() {
+  return &attachment_id_sets_;
 }
 
 // MockSyncApiComponentFactory needed to initialize GenericChangeProcessor and
@@ -161,6 +163,11 @@ class SyncGenericChangeProcessorTest : public testing::Test {
     return mock_attachment_service_;
   }
 
+  void RunLoop() {
+    base::RunLoop run_loop;
+    run_loop.RunUntilIdle();
+  }
+
  private:
   base::MessageLoopForUI loop_;
 
@@ -349,19 +356,20 @@ TEST_F(SyncGenericChangeProcessorTest,
                              tag, title, specifics, attachments)));
   ASSERT_FALSE(
       change_processor()->ProcessSyncChanges(FROM_HERE, change_list).IsSet());
+  RunLoop();
 
   // Check that the AttachmentService received the new attachments.
-  ASSERT_EQ(mock_attachment_service()->attachment_lists()->size(), 1U);
-  const syncer::AttachmentList& attachments_added =
-      mock_attachment_service()->attachment_lists()->front();
-  ASSERT_EQ(attachments_added.size(), 2U);
-  ASSERT_EQ(attachments_added[0].GetId(), attachments[0].GetId());
-  ASSERT_EQ(attachments_added[1].GetId(), attachments[1].GetId());
+  ASSERT_EQ(mock_attachment_service()->attachment_id_sets()->size(), 1U);
+  const syncer::AttachmentIdSet& attachments_added =
+      mock_attachment_service()->attachment_id_sets()->front();
+  ASSERT_THAT(attachments_added,
+              testing::UnorderedElementsAre(attachments[0].GetId(),
+                                            attachments[1].GetId()));
 
   // Update the SyncData, replacing its two attachments with one new attachment.
   syncer::AttachmentList new_attachments;
   new_attachments.push_back(syncer::Attachment::Create(attachment_data));
-  mock_attachment_service()->attachment_lists()->clear();
+  mock_attachment_service()->attachment_id_sets()->clear();
   change_list.clear();
   change_list.push_back(
       syncer::SyncChange(FROM_HERE,
@@ -370,13 +378,14 @@ TEST_F(SyncGenericChangeProcessorTest,
                              tag, title, specifics, new_attachments)));
   ASSERT_FALSE(
       change_processor()->ProcessSyncChanges(FROM_HERE, change_list).IsSet());
+  RunLoop();
 
   // Check that the AttachmentService received it.
-  ASSERT_EQ(mock_attachment_service()->attachment_lists()->size(), 1U);
-  const syncer::AttachmentList& new_attachments_added =
-      mock_attachment_service()->attachment_lists()->front();
-  ASSERT_EQ(new_attachments_added.size(), 1U);
-  ASSERT_EQ(new_attachments_added[0].GetId(), new_attachments[0].GetId());
+  ASSERT_EQ(mock_attachment_service()->attachment_id_sets()->size(), 1U);
+  const syncer::AttachmentIdSet& new_attachments_added =
+      mock_attachment_service()->attachment_id_sets()->front();
+  ASSERT_THAT(new_attachments_added,
+              testing::UnorderedElementsAre(new_attachments[0].GetId()));
 }
 
 // Verify that after attachment is uploaded GenericChangeProcessor updates
diff --git a/sync/internal_api/attachments/attachment_service_impl.cc b/sync/internal_api/attachments/attachment_service_impl.cc
index 61fb1c6..e9d3a82 100644
--- a/sync/internal_api/attachments/attachment_service_impl.cc
+++ b/sync/internal_api/attachments/attachment_service_impl.cc
@@ -4,6 +4,8 @@
 
 #include "sync/internal_api/public/attachments/attachment_service_impl.h"
 
+#include <iterator>
+
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/thread_task_runner_handle.h"
@@ -141,6 +143,10 @@ scoped_ptr<syncer::AttachmentService> AttachmentServiceImpl::CreateForTest() {
   return attachment_service.Pass();
 }
 
+AttachmentStore* AttachmentServiceImpl::GetStore() {
+  return attachment_store_.get();
+}
+
 void AttachmentServiceImpl::GetOrDownloadAttachments(
     const AttachmentIdList& attachment_ids,
     const GetOrDownloadCallback& callback) {
@@ -163,25 +169,6 @@ void AttachmentServiceImpl::DropAttachments(
                                      callback));
 }
 
-void AttachmentServiceImpl::StoreAttachments(const AttachmentList& attachments,
-                                             const StoreCallback& callback) {
-  DCHECK(CalledOnValidThread());
-  attachment_store_->Write(attachments,
-                           base::Bind(&AttachmentServiceImpl::WriteDone,
-                                      weak_ptr_factory_.GetWeakPtr(),
-                                      callback));
-  if (attachment_uploader_.get()) {
-    for (AttachmentList::const_iterator iter = attachments.begin();
-         iter != attachments.end();
-         ++iter) {
-      attachment_uploader_->UploadAttachment(
-          *iter,
-          base::Bind(&AttachmentServiceImpl::UploadDone,
-                     weak_ptr_factory_.GetWeakPtr()));
-    }
-  }
-}
-
 void AttachmentServiceImpl::ReadDone(
     const scoped_refptr<GetOrDownloadState>& state,
     const AttachmentStore::Result& result,
@@ -226,21 +213,10 @@ void AttachmentServiceImpl::DropDone(const DropCallback& callback,
                                          base::Bind(callback, drop_result));
 }
 
-void AttachmentServiceImpl::WriteDone(const StoreCallback& callback,
-                                      const AttachmentStore::Result& result) {
-  AttachmentService::StoreResult store_result =
-      AttachmentService::STORE_UNSPECIFIED_ERROR;
-  if (result == AttachmentStore::SUCCESS) {
-    store_result = AttachmentService::STORE_SUCCESS;
-  }
-  // TODO(maniscalco): Deal with case where an error occurred (bug 361251).
-  base::MessageLoop::current()->PostTask(FROM_HERE,
-                                         base::Bind(callback, store_result));
-}
-
 void AttachmentServiceImpl::UploadDone(
     const AttachmentUploader::UploadResult& result,
     const AttachmentId& attachment_id) {
+  ids_in_queue_.erase(attachment_id);
   // TODO(pavely): crbug/372622: Deal with UploadAttachment failures.
   if (result != AttachmentUploader::UPLOAD_SUCCESS)
     return;
@@ -261,4 +237,60 @@ void AttachmentServiceImpl::DownloadDone(
   }
 }
 
+void AttachmentServiceImpl::UploadAttachments(
+    const AttachmentIdSet& attachment_ids) {
+  DCHECK(CalledOnValidThread());
+  if (!attachment_uploader_.get()) {
+    return;
+  }
+
+  // Enqueue the attachment ids that aren't already in the queue.
+  AttachmentIdSet::const_iterator iter = attachment_ids.begin();
+  AttachmentIdSet::const_iterator end = attachment_ids.end();
+  for (; iter != end; ++iter) {
+    if (ids_in_queue_.find(*iter) == ids_in_queue_.end()) {
+      queue_.push_back(*iter);
+      ids_in_queue_.insert(*iter);
+    }
+  }
+
+  ProcessQueuedUploads();
+}
+
+void AttachmentServiceImpl::ProcessQueuedUploads() {
+  DCHECK(CalledOnValidThread());
+  // TODO(maniscalco): Don't dequeue them all.  Instead, limit the number of
+  // concurrent uploads and apply backoff on failure.
+  while (!queue_.empty()) {
+    const AttachmentId id = queue_.front();
+    queue_.pop_front();
+    AttachmentIdList attachment_ids;
+    attachment_ids.push_back(id);
+    attachment_store_->Read(
+        attachment_ids,
+        base::Bind(&AttachmentServiceImpl::ReadDoneNowUpload,
+                   weak_ptr_factory_.GetWeakPtr()));
+  }
+}
+
+void AttachmentServiceImpl::ReadDoneNowUpload(
+    const AttachmentStore::Result& result,
+    scoped_ptr<AttachmentMap> attachments,
+    scoped_ptr<AttachmentIdList> unavailable_attachment_ids) {
+  DCHECK(CalledOnValidThread());
+  if (!unavailable_attachment_ids->empty()) {
+    // TODO(maniscalco): We failed to read some attachments. What should we do
+    // now?
+  }
+
+  AttachmentMap::const_iterator iter = attachments->begin();
+  AttachmentMap::const_iterator end = attachments->end();
+  for (; iter != end; ++iter) {
+    attachment_uploader_->UploadAttachment(
+        iter->second,
+        base::Bind(&AttachmentServiceImpl::UploadDone,
+                   weak_ptr_factory_.GetWeakPtr()));
+  }
+}
+
 }  // namespace syncer
diff --git a/sync/internal_api/attachments/attachment_service_impl_unittest.cc b/sync/internal_api/attachments/attachment_service_impl_unittest.cc
index d8bf6b2..1b642e6 100644
--- a/sync/internal_api/attachments/attachment_service_impl_unittest.cc
+++ b/sync/internal_api/attachments/attachment_service_impl_unittest.cc
@@ -10,6 +10,7 @@
 #include "base/run_loop.h"
 #include "sync/internal_api/public/attachments/fake_attachment_downloader.h"
 #include "sync/internal_api/public/attachments/fake_attachment_uploader.h"
+#include "testing/gmock/include/gmock/gmock-matchers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
@@ -196,21 +197,12 @@ class AttachmentServiceImplTest : public testing::Test,
                       base::Unretained(this));
   }
 
-  AttachmentService::StoreCallback store_callback() {
-    return base::Bind(&AttachmentServiceImplTest::StoreDone,
-                      base::Unretained(this));
-  }
-
   void DownloadDone(const AttachmentService::GetOrDownloadResult& result,
                     scoped_ptr<AttachmentMap> attachments) {
     download_results_.push_back(result);
     last_download_attachments_ = attachments.Pass();
   }
 
-  void StoreDone(const AttachmentService::StoreResult& result) {
-    store_results_.push_back(result);
-  }
-
   void RunLoop() {
     base::RunLoop run_loop;
     run_loop.RunUntilIdle();
@@ -225,10 +217,6 @@ class AttachmentServiceImplTest : public testing::Test,
     return *last_download_attachments_.get();
   }
 
-  const std::vector<AttachmentService::StoreResult>& store_results() const {
-    return store_results_;
-  }
-
   MockAttachmentStore* store() { return attachment_store_.get(); }
 
   MockAttachmentDownloader* downloader() {
@@ -253,10 +241,12 @@ class AttachmentServiceImplTest : public testing::Test,
   std::vector<AttachmentService::GetOrDownloadResult> download_results_;
   scoped_ptr<AttachmentMap> last_download_attachments_;
   std::vector<AttachmentId> on_attachment_uploaded_list_;
-
-  std::vector<AttachmentService::StoreResult> store_results_;
 };
 
+TEST_F(AttachmentServiceImplTest, GetStore) {
+  EXPECT_EQ(store(), attachment_service()->GetStore());
+}
+
 TEST_F(AttachmentServiceImplTest, GetOrDownload_EmptyAttachmentList) {
   AttachmentIdList attachment_ids;
   attachment_service()->GetOrDownloadAttachments(attachment_ids,
@@ -348,105 +338,151 @@ TEST_F(AttachmentServiceImplTest, GetOrDownload_NoDownloader) {
   EXPECT_TRUE(last_download_attachments().empty());
 }
 
-TEST_F(AttachmentServiceImplTest, StoreAttachments_Success) {
-  scoped_refptr<base::RefCountedString> data = new base::RefCountedString();
-  Attachment attachment(Attachment::Create(data));
-  AttachmentList attachments;
-  attachments.push_back(attachment);
-  attachment_service()->StoreAttachments(attachments, store_callback());
-  EXPECT_EQ(1U, store()->write_attachments.size());
-  EXPECT_EQ(1U, uploader()->upload_requests.size());
+TEST_F(AttachmentServiceImplTest, UploadAttachments_Success) {
+  AttachmentIdSet attachment_ids;
+  const size_t num_attachments = 3;
+  for (unsigned i = 0; i < num_attachments; ++i) {
+    attachment_ids.insert(AttachmentId::Create());
+  }
+  attachment_service()->UploadAttachments(attachment_ids);
+  RunLoop();
+  // See that the service has issued reads for the attachments, but not yet
+  // uploaded anything.
+  EXPECT_EQ(num_attachments, store()->read_ids.size());
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
+  for (unsigned i = 0; i < num_attachments; ++i) {
+    store()->RespondToRead(attachment_ids);
+  }
 
-  store()->RespondToWrite(AttachmentStore::SUCCESS);
-  uploader()->RespondToUpload(attachment.GetId(),
-                              AttachmentUploader::UPLOAD_SUCCESS);
   RunLoop();
-  ASSERT_EQ(1U, store_results().size());
-  EXPECT_EQ(AttachmentService::STORE_SUCCESS, store_results()[0]);
-  ASSERT_EQ(1U, on_attachment_uploaded_list().size());
-  EXPECT_EQ(attachment.GetId(), on_attachment_uploaded_list()[0]);
+  EXPECT_EQ(0U, store()->read_ids.size());
+  EXPECT_EQ(num_attachments, uploader()->upload_requests.size());
+  AttachmentIdSet::const_iterator iter = attachment_ids.begin();
+  const AttachmentIdSet::const_iterator end = attachment_ids.end();
+  for (; iter != end; ++iter) {
+    uploader()->RespondToUpload(*iter, AttachmentUploader::UPLOAD_SUCCESS);
+  }
+  RunLoop();
+
+  // See that all the attachments were uploaded.
+  ASSERT_EQ(attachment_ids.size(), on_attachment_uploaded_list().size());
+  for (iter = attachment_ids.begin(); iter != end; ++iter) {
+    EXPECT_THAT(on_attachment_uploaded_list(), testing::Contains(*iter));
+  }
 }
 
-TEST_F(AttachmentServiceImplTest,
-       StoreAttachments_StoreFailsWithUnspecifiedError) {
-  scoped_refptr<base::RefCountedString> data = new base::RefCountedString();
-  Attachment attachment(Attachment::Create(data));
-  AttachmentList attachments;
-  attachments.push_back(attachment);
-  attachment_service()->StoreAttachments(attachments, store_callback());
-  EXPECT_EQ(1U, store()->write_attachments.size());
-  EXPECT_EQ(1U, uploader()->upload_requests.size());
+TEST_F(AttachmentServiceImplTest, UploadAttachments_Success_NoDelegate) {
+  InitializeAttachmentService(make_scoped_ptr(new MockAttachmentUploader()),
+                              make_scoped_ptr(new MockAttachmentDownloader()),
+                              NULL);  // No delegate.
 
-  store()->RespondToWrite(AttachmentStore::UNSPECIFIED_ERROR);
-  uploader()->RespondToUpload(attachment.GetId(),
+  AttachmentIdSet attachment_ids;
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_service()->UploadAttachments(attachment_ids);
+  RunLoop();
+  EXPECT_EQ(1U, store()->read_ids.size());
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
+  store()->RespondToRead(attachment_ids);
+  RunLoop();
+  EXPECT_EQ(0U, store()->read_ids.size());
+  EXPECT_EQ(1U, uploader()->upload_requests.size());
+  uploader()->RespondToUpload(*attachment_ids.begin(),
                               AttachmentUploader::UPLOAD_SUCCESS);
   RunLoop();
-  ASSERT_EQ(1U, store_results().size());
-  EXPECT_EQ(AttachmentService::STORE_UNSPECIFIED_ERROR, store_results()[0]);
-  ASSERT_EQ(1U, on_attachment_uploaded_list().size());
-  EXPECT_EQ(attachment.GetId(), on_attachment_uploaded_list()[0]);
+  ASSERT_TRUE(on_attachment_uploaded_list().empty());
 }
 
-TEST_F(AttachmentServiceImplTest,
-       StoreAttachments_UploadFailsWithUnspecifiedError) {
-  scoped_refptr<base::RefCountedString> data = new base::RefCountedString();
-  Attachment attachment(Attachment::Create(data));
-  AttachmentList attachments;
-  attachments.push_back(attachment);
-  attachment_service()->StoreAttachments(attachments, store_callback());
-  EXPECT_EQ(1U, store()->write_attachments.size());
-  EXPECT_EQ(1U, uploader()->upload_requests.size());
+TEST_F(AttachmentServiceImplTest, UploadAttachments_SomeMissingFromStore) {
+  AttachmentIdSet attachment_ids;
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_ids.insert(AttachmentId::Create());
 
-  store()->RespondToWrite(AttachmentStore::SUCCESS);
-  uploader()->RespondToUpload(attachment.GetId(),
-                              AttachmentUploader::UPLOAD_UNSPECIFIED_ERROR);
+  attachment_service()->UploadAttachments(attachment_ids);
   RunLoop();
-  ASSERT_EQ(1U, store_results().size());
-  // Even though the upload failed, the Store operation is successful.
-  EXPECT_EQ(AttachmentService::STORE_SUCCESS, store_results()[0]);
-  EXPECT_TRUE(on_attachment_uploaded_list().empty());
+  EXPECT_EQ(2U, store()->read_ids.size());
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
+  store()->RespondToRead(attachment_ids);
+  EXPECT_EQ(1U, store()->read_ids.size());
+  // Not found!
+  store()->RespondToRead(AttachmentIdSet());
+  EXPECT_EQ(0U, store()->read_ids.size());
+  RunLoop();
+
+  // One attachment went missing so we should see only one upload request.
+  EXPECT_EQ(1U, uploader()->upload_requests.size());
+  uploader()->RespondToUpload(uploader()->upload_requests.begin()->first,
+                              AttachmentUploader::UPLOAD_SUCCESS);
+  RunLoop();
+
+  // See that the delegate was called for only one.
+  ASSERT_EQ(1U, on_attachment_uploaded_list().size());
 }
 
-TEST_F(AttachmentServiceImplTest, StoreAttachments_NoDelegate) {
-  InitializeAttachmentService(make_scoped_ptr(new MockAttachmentUploader()),
-                              make_scoped_ptr(new MockAttachmentDownloader()),
-                              NULL);  // No delegate.
+TEST_F(AttachmentServiceImplTest, UploadAttachments_AllMissingFromStore) {
+  AttachmentIdSet attachment_ids;
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_ids.insert(AttachmentId::Create());
 
-  scoped_refptr<base::RefCountedString> data = new base::RefCountedString();
-  Attachment attachment(Attachment::Create(data));
-  AttachmentList attachments;
-  attachments.push_back(attachment);
-  attachment_service()->StoreAttachments(attachments, store_callback());
-  EXPECT_EQ(1U, store()->write_attachments.size());
-  EXPECT_EQ(1U, uploader()->upload_requests.size());
+  attachment_service()->UploadAttachments(attachment_ids);
+  RunLoop();
+  EXPECT_EQ(2U, store()->read_ids.size());
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
+  // None found!
+  store()->RespondToRead(AttachmentIdSet());
+  store()->RespondToRead(AttachmentIdSet());
+  EXPECT_EQ(0U, store()->read_ids.size());
+  RunLoop();
 
-  store()->RespondToWrite(AttachmentStore::SUCCESS);
-  uploader()->RespondToUpload(attachment.GetId(),
-                              AttachmentUploader::UPLOAD_SUCCESS);
+  // Nothing uploaded.
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
   RunLoop();
-  ASSERT_EQ(1U, store_results().size());
-  EXPECT_EQ(AttachmentService::STORE_SUCCESS, store_results()[0]);
-  EXPECT_TRUE(on_attachment_uploaded_list().empty());
+
+  // See that the delegate was never called.
+  ASSERT_EQ(0U, on_attachment_uploaded_list().size());
 }
 
-TEST_F(AttachmentServiceImplTest, StoreAttachments_NoUploader) {
-  // No uploader.
+TEST_F(AttachmentServiceImplTest, UploadAttachments_NoUploader) {
   InitializeAttachmentService(make_scoped_ptr<MockAttachmentUploader>(NULL),
                               make_scoped_ptr(new MockAttachmentDownloader()),
                               this);
 
-  scoped_refptr<base::RefCountedString> data = new base::RefCountedString();
-  Attachment attachment(Attachment::Create(data));
-  AttachmentList attachments;
-  attachments.push_back(attachment);
-  attachment_service()->StoreAttachments(attachments, store_callback());
-  EXPECT_EQ(1U, store()->write_attachments.size());
+  AttachmentIdSet attachment_ids;
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_service()->UploadAttachments(attachment_ids);
+  RunLoop();
+  EXPECT_EQ(0U, store()->read_ids.size());
+  ASSERT_EQ(0U, on_attachment_uploaded_list().size());
+}
+
+// Upload three attachments.  For one of them, server responds with error.
+TEST_F(AttachmentServiceImplTest, UploadAttachments_OneUploadFails) {
+  AttachmentIdSet attachment_ids;
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_ids.insert(AttachmentId::Create());
+  attachment_ids.insert(AttachmentId::Create());
+
+  attachment_service()->UploadAttachments(attachment_ids);
+  RunLoop();
+  EXPECT_EQ(3U, store()->read_ids.size());
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
 
-  store()->RespondToWrite(AttachmentStore::SUCCESS);
+  // All attachments found.
+  store()->RespondToRead(attachment_ids);
+  store()->RespondToRead(attachment_ids);
+  store()->RespondToRead(attachment_ids);
   RunLoop();
-  ASSERT_EQ(1U, store_results().size());
-  EXPECT_EQ(AttachmentService::STORE_SUCCESS, store_results()[0]);
-  EXPECT_TRUE(on_attachment_uploaded_list().empty());
+
+  EXPECT_EQ(3U, uploader()->upload_requests.size());
+  uploader()->RespondToUpload(uploader()->upload_requests.begin()->first,
+                              AttachmentUploader::UPLOAD_SUCCESS);
+  uploader()->RespondToUpload(uploader()->upload_requests.begin()->first,
+                              AttachmentUploader::UPLOAD_UNSPECIFIED_ERROR);
+  uploader()->RespondToUpload(uploader()->upload_requests.begin()->first,
+                              AttachmentUploader::UPLOAD_SUCCESS);
+  EXPECT_EQ(0U, uploader()->upload_requests.size());
+  RunLoop();
+
+  EXPECT_EQ(2U, on_attachment_uploaded_list().size());
 }
 
 }  // namespace syncer
diff --git a/sync/internal_api/attachments/attachment_service_proxy.cc b/sync/internal_api/attachments/attachment_service_proxy.cc
index 1d41878..ef858dd 100644
--- a/sync/internal_api/attachments/attachment_service_proxy.cc
+++ b/sync/internal_api/attachments/attachment_service_proxy.cc
@@ -35,15 +35,6 @@ void ProxyDropCallback(
   task_runner->PostTask(FROM_HERE, base::Bind(callback, result));
 }
 
-// Invokes |callback| with |result| and |attachments| in the |task_runner|
-// thread.
-void ProxyStoreCallback(
-    const scoped_refptr<base::SequencedTaskRunner>& task_runner,
-    const AttachmentService::StoreCallback& callback,
-    const AttachmentService::StoreResult& result) {
-  task_runner->PostTask(FROM_HERE, base::Bind(callback, result));
-}
-
 }  // namespace
 
 AttachmentServiceProxy::AttachmentServiceProxy() {
@@ -67,6 +58,10 @@ AttachmentServiceProxy::AttachmentServiceProxy(
 AttachmentServiceProxy::~AttachmentServiceProxy() {
 }
 
+AttachmentStore* AttachmentServiceProxy::GetStore() {
+  return NULL;
+}
+
 void AttachmentServiceProxy::GetOrDownloadAttachments(
     const AttachmentIdList& attachment_ids,
     const GetOrDownloadCallback& callback) {
@@ -96,17 +91,12 @@ void AttachmentServiceProxy::DropAttachments(
                                             proxy_callback));
 }
 
-void AttachmentServiceProxy::StoreAttachments(const AttachmentList& attachments,
-                                              const StoreCallback& callback) {
-  DCHECK(wrapped_task_runner_.get());
-  StoreCallback proxy_callback = base::Bind(
-      &ProxyStoreCallback, base::ThreadTaskRunnerHandle::Get(), callback);
+void AttachmentServiceProxy::UploadAttachments(
+    const AttachmentIdSet& attachment_ids) {
+  DCHECK(wrapped_task_runner_);
   wrapped_task_runner_->PostTask(
       FROM_HERE,
-      base::Bind(&AttachmentService::StoreAttachments,
-                 core_,
-                 attachments,
-                 proxy_callback));
+      base::Bind(&AttachmentService::UploadAttachments, core_, attachment_ids));
 }
 
 AttachmentServiceProxy::Core::Core(
@@ -117,6 +107,10 @@ AttachmentServiceProxy::Core::Core(
 AttachmentServiceProxy::Core::~Core() {
 }
 
+AttachmentStore* AttachmentServiceProxy::Core::GetStore() {
+  return NULL;
+}
+
 void AttachmentServiceProxy::Core::GetOrDownloadAttachments(
     const AttachmentIdList& attachment_ids,
     const GetOrDownloadCallback& callback) {
@@ -135,13 +129,12 @@ void AttachmentServiceProxy::Core::DropAttachments(
   wrapped_->DropAttachments(attachment_ids, callback);
 }
 
-void AttachmentServiceProxy::Core::StoreAttachments(
-    const AttachmentList& attachments,
-    const StoreCallback& callback) {
+void AttachmentServiceProxy::Core::UploadAttachments(
+    const AttachmentIdSet& attachment_ids) {
   if (!wrapped_) {
     return;
   }
-  wrapped_->StoreAttachments(attachments, callback);
+  wrapped_->UploadAttachments(attachment_ids);
 }
 
 }  // namespace syncer
diff --git a/sync/internal_api/attachments/attachment_service_proxy_unittest.cc b/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
index 545998d..a946382 100644
--- a/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
+++ b/sync/internal_api/attachments/attachment_service_proxy_unittest.cc
@@ -34,6 +34,8 @@ class StubAttachmentService : public AttachmentService,
 
   virtual ~StubAttachmentService() {}
 
+  virtual AttachmentStore* GetStore() OVERRIDE { return NULL; }
+
   virtual void GetOrDownloadAttachments(const AttachmentIdList& attachment_ids,
                                         const GetOrDownloadCallback& callback)
       OVERRIDE {
@@ -55,12 +57,10 @@ class StubAttachmentService : public AttachmentService,
         FROM_HERE, base::Bind(callback, AttachmentService::DROP_SUCCESS));
   }
 
-  virtual void StoreAttachments(const AttachmentList& attachments,
-                                const StoreCallback& callback) OVERRIDE {
+  virtual void UploadAttachments(
+      const AttachmentIdSet& attachments_ids) OVERRIDE {
     CalledOnValidThread();
     Increment();
-    base::MessageLoop::current()->PostTask(
-        FROM_HERE, base::Bind(callback, AttachmentService::STORE_SUCCESS));
   }
 
   virtual base::WeakPtr<AttachmentService> AsWeakPtr() {
@@ -105,11 +105,8 @@ class AttachmentServiceProxyTest : public testing::Test,
                    base::Unretained(this));
     callback_drop = base::Bind(&AttachmentServiceProxyTest::IncrementDrop,
                                base::Unretained(this));
-    callback_store = base::Bind(&AttachmentServiceProxyTest::IncrementStore,
-                                base::Unretained(this));
     count_callback_get_or_download = 0;
     count_callback_drop = 0;
-    count_callback_store = 0;
   }
 
   virtual void TearDown()
@@ -136,12 +133,6 @@ class AttachmentServiceProxyTest : public testing::Test,
     ++count_callback_drop;
   }
 
-  // a StoreCallback
-  void IncrementStore(const AttachmentService::StoreResult&) {
-    CalledOnValidThread();
-    ++count_callback_store;
-  }
-
   void WaitForStubThread() {
     base::WaitableEvent done(false, false);
     stub_thread->message_loop()->PostTask(
@@ -157,23 +148,24 @@ class AttachmentServiceProxyTest : public testing::Test,
 
   AttachmentService::GetOrDownloadCallback callback_get_or_download;
   AttachmentService::DropCallback callback_drop;
-  AttachmentService::StoreCallback callback_store;
 
   // number of times callback_get_or_download was invoked
   int count_callback_get_or_download;
   // number of times callback_drop was invoked
   int count_callback_drop;
-  // number of times callback_store was invoked
-  int count_callback_store;
 };
 
-// Verify that each of AttachmentServiceProxy's callback methods (those that
-// take callbacks) are invoked on the stub and that the passed callbacks are
-// invoked in this thread.
-TEST_F(AttachmentServiceProxyTest, MethodsWithCallbacksAreProxied) {
+TEST_F(AttachmentServiceProxyTest, GetStore) {
+  EXPECT_EQ(NULL, proxy->GetStore());
+}
+
+// Verify that each of AttachmentServiceProxy's methods are invoked on the stub.
+// Verify that the methods that take callbacks invoke passed callbacks on this
+// thread.
+TEST_F(AttachmentServiceProxyTest, MethodsAreProxied) {
   proxy->GetOrDownloadAttachments(AttachmentIdList(), callback_get_or_download);
   proxy->DropAttachments(AttachmentIdList(), callback_drop);
-  proxy->StoreAttachments(AttachmentList(), callback_store);
+  proxy->UploadAttachments(AttachmentIdSet());
   // Wait for the posted calls to execute in the stub thread.
   WaitForStubThread();
   EXPECT_EQ(3, stub->GetCallCount());
@@ -185,7 +177,6 @@ TEST_F(AttachmentServiceProxyTest, MethodsWithCallbacksAreProxied) {
   loop.RunUntilIdle();
   EXPECT_EQ(1, count_callback_get_or_download);
   EXPECT_EQ(1, count_callback_drop);
-  EXPECT_EQ(1, count_callback_store);
 }
 
 // Verify that it's safe to use an AttachmentServiceProxy even after its wrapped
diff --git a/sync/internal_api/public/attachments/attachment_service.h b/sync/internal_api/public/attachments/attachment_service.h
index 6ac8cc6..758f960 100644
--- a/sync/internal_api/public/attachments/attachment_service.h
+++ b/sync/internal_api/public/attachments/attachment_service.h
@@ -14,6 +14,7 @@
 
 namespace syncer {
 
+class AttachmentStore;
 class SyncData;
 
 // AttachmentService is responsible for managing a model type's attachments.
@@ -44,16 +45,6 @@ class SYNC_EXPORT AttachmentService {
 
   typedef base::Callback<void(const DropResult&)> DropCallback;
 
-  // The result of a StoreAttachments operation.
-  enum StoreResult {
-    STORE_SUCCESS,            // No error, all attachments stored (at least
-                              // locally).
-    STORE_UNSPECIFIED_ERROR,  // An unspecified error occurred. Some or all
-                              // attachments may not have been stored.
-  };
-
-  typedef base::Callback<void(const StoreResult&)> StoreCallback;
-
   // An interface that embedder code implements to be notified about different
   // events that originate from AttachmentService.
   // This interface will be called from the same thread AttachmentService was
@@ -70,6 +61,11 @@ class SYNC_EXPORT AttachmentService {
   AttachmentService();
   virtual ~AttachmentService();
 
+  // Return a pointer to the AttachmentStore owned by this object.
+  //
+  // May return NULL.
+  virtual AttachmentStore* GetStore() = 0;
+
   // See SyncData::GetOrDownloadAttachments.
   virtual void GetOrDownloadAttachments(
       const AttachmentIdList& attachment_ids,
@@ -79,12 +75,18 @@ class SYNC_EXPORT AttachmentService {
   virtual void DropAttachments(const AttachmentIdList& attachment_ids,
                                const DropCallback& callback) = 0;
 
-  // Store |attachments| on device and (eventually) upload them to the server.
+  // Schedules the attachments identified by |attachment_ids| to be uploaded to
+  // the server.
+  //
+  // Assumes the attachments are already in the attachment store.
+  //
+  // A request to upload attachments is persistent in that uploads will be
+  // automatically retried if transient errors occur.
+  //
+  // A request to upload attachments does not persist across restarts of Chrome.
   //
-  // Invokes |callback| once the attachments have been written to device
-  // storage.
-  virtual void StoreAttachments(const AttachmentList& attachments,
-                                const StoreCallback& callback) = 0;
+  // Invokes OnAttachmentUploaded on the Delegate (if provided).
+  virtual void UploadAttachments(const AttachmentIdSet& attachment_ids) = 0;
 };
 
 }  // namespace syncer
diff --git a/sync/internal_api/public/attachments/attachment_service_impl.h b/sync/internal_api/public/attachments/attachment_service_impl.h
index 53f6889..1fa0c54 100644
--- a/sync/internal_api/public/attachments/attachment_service_impl.h
+++ b/sync/internal_api/public/attachments/attachment_service_impl.h
@@ -5,6 +5,8 @@
 #ifndef SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_SERVICE_IMPL_H_
 #define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_SERVICE_IMPL_H_
 
+#include <deque>
+
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/non_thread_safe.h"
@@ -43,13 +45,14 @@ class SYNC_EXPORT AttachmentServiceImpl : public AttachmentService,
   static scoped_ptr<syncer::AttachmentService> CreateForTest();
 
   // AttachmentService implementation.
-  virtual void GetOrDownloadAttachments(const AttachmentIdList& attachment_ids,
-                                        const GetOrDownloadCallback& callback)
-      OVERRIDE;
+  virtual AttachmentStore* GetStore() OVERRIDE;
+  virtual void GetOrDownloadAttachments(
+      const AttachmentIdList& attachment_ids,
+      const GetOrDownloadCallback& callback) OVERRIDE;
   virtual void DropAttachments(const AttachmentIdList& attachment_ids,
                                const DropCallback& callback) OVERRIDE;
-  virtual void StoreAttachments(const AttachmentList& attachments,
-                                const StoreCallback& callback) OVERRIDE;
+  virtual void UploadAttachments(
+      const AttachmentIdSet& attachment_ids) OVERRIDE;
 
  private:
   class GetOrDownloadState;
@@ -60,14 +63,17 @@ class SYNC_EXPORT AttachmentServiceImpl : public AttachmentService,
                 scoped_ptr<AttachmentIdList> unavailable_attachment_ids);
   void DropDone(const DropCallback& callback,
                 const AttachmentStore::Result& result);
-  void WriteDone(const StoreCallback& callback,
-                 const AttachmentStore::Result& result);
   void UploadDone(const AttachmentUploader::UploadResult& result,
                   const AttachmentId& attachment_id);
   void DownloadDone(const scoped_refptr<GetOrDownloadState>& state,
                     const AttachmentId& attachment_id,
                     const AttachmentDownloader::DownloadResult& result,
                     scoped_ptr<Attachment> attachment);
+  void ProcessQueuedUploads();
+  void ReadDoneNowUpload(
+      const AttachmentStore::Result& result,
+      scoped_ptr<AttachmentMap> attachments,
+      scoped_ptr<AttachmentIdList> unavailable_attachment_ids);
 
   const scoped_ptr<AttachmentStore> attachment_store_;
 
@@ -80,6 +86,13 @@ class SYNC_EXPORT AttachmentServiceImpl : public AttachmentService,
   // May be null.
   Delegate* delegate_;
 
+  // Queue of attachment ids to be uploaded.  Every entry in this queue should
+  // also exist in ids_in_queue_.
+  std::deque<AttachmentId> queue_;
+
+  // Ids of attachments currently being uploaded or queued for upload.
+  AttachmentIdSet ids_in_queue_;
+
   // Must be last data member.
   base::WeakPtrFactory<AttachmentServiceImpl> weak_ptr_factory_;
 
diff --git a/sync/internal_api/public/attachments/attachment_service_proxy.h b/sync/internal_api/public/attachments/attachment_service_proxy.h
index fb7c08c..fa51457 100644
--- a/sync/internal_api/public/attachments/attachment_service_proxy.h
+++ b/sync/internal_api/public/attachments/attachment_service_proxy.h
@@ -51,13 +51,16 @@ class SYNC_EXPORT AttachmentServiceProxy : public AttachmentService {
   virtual ~AttachmentServiceProxy();
 
   // AttachmentService implementation.
+  //
+  // GetStore always returns NULL.
+  virtual AttachmentStore* GetStore() OVERRIDE;
   virtual void GetOrDownloadAttachments(
       const AttachmentIdList& attachment_ids,
       const GetOrDownloadCallback& callback) OVERRIDE;
   virtual void DropAttachments(const AttachmentIdList& attachment_ids,
                                const DropCallback& callback) OVERRIDE;
-  virtual void StoreAttachments(const AttachmentList& attachment,
-                                const StoreCallback& callback) OVERRIDE;
+  virtual void UploadAttachments(
+      const AttachmentIdSet& attachment_ids) OVERRIDE;
 
  protected:
   // Core does the work of proxying calls to AttachmentService methods from one
@@ -80,13 +83,14 @@ class SYNC_EXPORT AttachmentServiceProxy : public AttachmentService {
     Core(const base::WeakPtr<syncer::AttachmentService>& wrapped);
 
     // AttachmentService implementation.
+    virtual AttachmentStore* GetStore() OVERRIDE;
     virtual void GetOrDownloadAttachments(
         const AttachmentIdList& attachment_ids,
         const GetOrDownloadCallback& callback) OVERRIDE;
     virtual void DropAttachments(const AttachmentIdList& attachment_ids,
                                  const DropCallback& callback) OVERRIDE;
-    virtual void StoreAttachments(const AttachmentList& attachment,
-                                  const StoreCallback& callback) OVERRIDE;
+    virtual void UploadAttachments(
+        const AttachmentIdSet& attachment_ids) OVERRIDE;
 
    protected:
     friend class base::RefCountedThreadSafe<Core>;