[BlobAsync] Disk support for blob storage

storage/browser/blob/blob_storage_context.cc

Index: storage/browser/blob/blob_storage_context.cc
diff --git a/storage/browser/blob/blob_storage_context.cc b/storage/browser/blob/blob_storage_context.cc
index 6ce45e53f90663cdda7d7b9d1293715ec92aabeb..4cd89a4247fdb3ee78cbbf3e0cc3167e1c6c3870 100644
--- a/storage/browser/blob/blob_storage_context.cc
+++ b/storage/browser/blob/blob_storage_context.cc
@@ -4,12 +4,10 @@
 
 #include "storage/browser/blob/blob_storage_context.h"
 
-#include <stddef.h>
-#include <stdint.h>
-
 #include <algorithm>
 #include <limits>
 #include <memory>
+#include <set>
 #include <utility>
 
 #include "base/bind.h"
@@ -19,20 +17,55 @@
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/histogram.h"
+#include "base/numerics/safe_conversions.h"
+#include "base/numerics/safe_math.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "storage/browser/blob/blob_data_builder.h"
 #include "storage/browser/blob/blob_data_handle.h"
 #include "storage/browser/blob/blob_data_item.h"
 #include "storage/browser/blob/blob_data_snapshot.h"
+#include "storage/browser/blob/blob_flattener.h"
+#include "storage/browser/blob/blob_slice.h"
 #include "storage/browser/blob/shareable_blob_data_item.h"
 #include "url/gurl.h"
 
 namespace storage {
+using ItemCopyEntry = BlobStorageRegistry::ItemCopyEntry;
+
+namespace {
+
+BlobStatus ConvertReferencedBlobErrorToConstructingError(
+    BlobStatus referenced_blob_error) {
+  switch (referenced_blob_error) {
+    // For most cases we propagate the error.
+    case BlobStatus::FILE_WRITE_FAILED:
+    case BlobStatus::SOURCE_DIED_IN_TRANSIT:
+    case BlobStatus::REFERENCED_BLOB_BROKEN:
+    case BlobStatus::OUT_OF_MEMORY:
+      return referenced_blob_error;
+    // Others we report that the referenced blob is broken, as we don't know
+    // why (the BLOB_DEREFERENCED_WHILE_BUILDING should never happen, as we hold
+    // onto the reference of the blobs we're using).
+    case BlobStatus::BLOB_DEREFERENCED_WHILE_BUILDING:
+      DCHECK(false) << "Referenced blob should never be dereferenced while we "
+                    << "are depending on it, as our system holds a handle.";
+    case BlobStatus::INVALID_CONSTRUCTION_ARGUMENTS:
+      return BlobStatus::REFERENCED_BLOB_BROKEN;
+    case BlobStatus::DONE:
+    case BlobStatus::PENDING:
+      NOTREACHED();
+  }
+  NOTREACHED();
+  return BlobStatus::REFERENCED_BLOB_BROKEN;
+}
+
+}  // namespace
+
 using BlobRegistryEntry = BlobStorageRegistry::Entry;
-using BlobState = BlobStorageRegistry::BlobState;
 
-BlobStorageContext::BlobStorageContext() : memory_usage_(0) {}
+BlobStorageContext::BlobStorageContext() {}
 
 BlobStorageContext::~BlobStorageContext() {
 }
@@ -43,9 +76,7 @@ std::unique_ptr<BlobDataHandle> BlobStorageContext::GetBlobDataFromUUID(
   if (!entry) {
     return nullptr;
   }
-  return base::WrapUnique(
-      new BlobDataHandle(uuid, entry->content_type, entry->content_disposition,
-                         this, base::ThreadTaskRunnerHandle::Get().get()));
+  return CreateHandle(uuid, entry);
 }
 
 std::unique_ptr<BlobDataHandle> BlobStorageContext::GetBlobDataFromPublicURL(
@@ -55,21 +86,14 @@ std::unique_ptr<BlobDataHandle> BlobStorageContext::GetBlobDataFromPublicURL(
   if (!entry) {
     return nullptr;
   }
-  return base::WrapUnique(
-      new BlobDataHandle(uuid, entry->content_type, entry->content_disposition,
-                         this, base::ThreadTaskRunnerHandle::Get().get()));
+  return CreateHandle(uuid, entry);
 }
 
 std::unique_ptr<BlobDataHandle> BlobStorageContext::AddFinishedBlob(
     const BlobDataBuilder& external_builder) {
   TRACE_EVENT0("Blob", "Context::AddFinishedBlob");
-  CreatePendingBlob(external_builder.uuid(), external_builder.content_type_,
-                    external_builder.content_disposition_);
-  CompletePendingBlob(external_builder);
-  std::unique_ptr<BlobDataHandle> handle =
-      GetBlobDataFromUUID(external_builder.uuid_);
-  DecrementBlobRefCount(external_builder.uuid_);
-  return handle;
+
+  return BuildBlob(external_builder, BlobStatusCallback());
 }
 
 std::unique_ptr<BlobDataHandle> BlobStorageContext::AddFinishedBlob(
@@ -95,86 +119,288 @@ void BlobStorageContext::RevokePublicBlobURL(const GURL& blob_url) {
   DecrementBlobRefCount(uuid);
 }
 
-void BlobStorageContext::CreatePendingBlob(
+void BlobStorageContext::EnableDisk(
+    const base::FilePath storage_directory,
+    scoped_refptr<base::SequencedWorkerPool> file_worker_pool) {
+  memory_controller_.EnableDisk(storage_directory, std::move(file_worker_pool));
+}
+
+std::unique_ptr<BlobDataHandle> BlobStorageContext::BuildBrokenBlob(
     const std::string& uuid,
     const std::string& content_type,
-    const std::string& content_disposition) {
-  DCHECK(!registry_.GetEntry(uuid) && !uuid.empty());
-  registry_.CreateEntry(uuid, content_type, content_disposition);
+    const std::string& content_disposition,
+    BlobStatus reason) {
+  DCHECK(!registry_.HasEntry(uuid));
+  DCHECK(BlobStatusIsError(reason));
+  BlobRegistryEntry* entry =
+      registry_.CreateEntry(uuid, content_type, content_disposition);
+  entry->status = reason;
+  FinishBuilding(entry);
+  return CreateHandle(uuid, entry);
 }
 
-void BlobStorageContext::CompletePendingBlob(
-    const BlobDataBuilder& external_builder) {
-  BlobRegistryEntry* entry = registry_.GetEntry(external_builder.uuid());
+std::unique_ptr<BlobDataHandle> BlobStorageContext::BuildBlob(
+    const BlobDataBuilder& content,
+    const BlobStatusCallback& can_populate_memory) {
+  DCHECK(!registry_.HasEntry(content.uuid_));
+
+  BlobRegistryEntry* entry = registry_.CreateEntry(
+      content.uuid_, content.content_type_, content.content_disposition_);
+
+  // This flattens all blob references in the transportion content out and
+  // stores the complete item representation in the internal data.
+  BlobFlattener flattener(content, &(entry->data), &registry_);
+
+  // If we contain broken references (or we reference ourself) we want to fail.
+  if (flattener.contains_broken_references) {
+    LOG(ERROR) << "references don't exist";
+    BreakAndFinishBlob(content.uuid_, BlobStatus::REFERENCED_BLOB_BROKEN);
+    return CreateHandle(content.uuid_, entry);
+  }
+
+  // We check to make sure that our memory calculations didn't overflow, which
+  // would mean that the user is trying to save more memory than is in the
+  // physical address space.
+  if (!flattener.memory_needed.IsValid() || !flattener.total_size.IsValid() ||
+      !memory_controller_.CanFitInSystem(
+          flattener.memory_needed.ValueOrDie())) {
+    LOG(ERROR) << "sizes aren't valid!";
+    BreakAndFinishBlob(content.uuid_, BlobStatus::OUT_OF_MEMORY);
+    return CreateHandle(content.uuid_, entry);
+  }
+  // We know we're < max_size_t now.
+  size_t new_memory_needed =
+      static_cast<size_t>(flattener.memory_needed.ValueOrDie());
+
+  // We store if we're waiting for the user to finish populating data in the
+  // |content| builder object.
+  if (flattener.contains_pending_content) {
+    entry->waiting_until_user_population = flattener.contains_pending_content;
+    entry->ready_for_user_population_callback = can_populate_memory;
+  }
+  entry->data.size_ = flattener.total_size.ValueOrDie();
+
+  std::unique_ptr<BlobDataHandle> handle = CreateHandle(content.uuid_, entry);
+
+  std::swap(entry->copies, flattener.copies);
+
+  // We hold a handle to all blobs we're using. This is important, as our memory
+  // accounting can be delayed until OnEnoughSizeForBlobData is called, and we
+  // only free memory on canceling when we've done this accounting. If a
+  // dependent blob is dereferenced, then we're the last blob holding onto that
+  // data item, and we need to account for that. So we prevent that case by
+  // holding onto all blobs.
+  entry->dependent_blobs_building = 0;
+  for (const std::pair<std::string, BlobRegistryEntry*>& pending_blob :
+       flattener.dependent_blobs) {
+    entry->dependent_blobs.push_back(
+        CreateHandle(pending_blob.first, pending_blob.second));
+    if (pending_blob.second->status == BlobStatus::PENDING) {
+      pending_blob.second->build_completion_callbacks.push_back(
+          base::Bind(&BlobStorageContext::OnDependentBlobFinished, AsWeakPtr(),
+                     content.uuid_));
+      entry->dependent_blobs_building++;
+    }
+  }
+
+  if (new_memory_needed == 0) {
+    entry->memory_accounted_for = true;
+    if (CanFinishBuilding(entry))
+      FinishBuilding(entry);
+    return handle;
+  }
+
+  // So we need to transport/copy memory.
+  base::Optional<BlobMemoryController::PendingConstructionEntry>
+      pending_construction_entry =
+          memory_controller_.NotifyWhenMemoryCanPopulated(
+              new_memory_needed,
+              base::Bind(&BlobStorageContext::OnEnoughSizeForBlobData,
+                         AsWeakPtr(), content.uuid_));
+
+  if (pending_construction_entry) {
+    // This means that we're waiting until the memory is available.
+    entry->can_fit = false;
+    entry->pending_copies_memory_entry = pending_construction_entry.value();
+  } else {
+    entry->memory_accounted_for = true;
+    if (!can_populate_memory.is_null())
+      can_populate_memory.Run(BlobStatus::PENDING);
+  }
+
+  if (CanFinishBuilding(entry))
+    FinishBuilding(entry);
+
+  return handle;
+}
+
+void BlobStorageContext::BreakAndFinishBlob(const std::string& uuid,
+                                            BlobStatus reason) {
+  BlobRegistryEntry* entry = registry_.GetEntry(uuid);
+  DCHECK(entry);
+  DCHECK(BlobStatusIsError(reason));
+  ClearAndFreeMemory(uuid, entry);
+  entry->copies.clear();
+  entry->dependent_blobs.clear();
+  if (!entry->ready_for_user_population_callback.is_null()) {
+    entry->ready_for_user_population_callback.Run(reason);
+    entry->ready_for_user_population_callback.Reset();
+  }
+  entry->data.items_.clear();
+  entry->data.offsets_.clear();
+  entry->data.size_ = 0;
+  entry->status = reason;
+  entry->waiting_until_user_population = false;
+  entry->can_fit = true;
+  entry->memory_accounted_for = true;
+  FinishBuilding(entry);
+}
+
+void BlobStorageContext::FinishedPopulatingBlob(const std::string& uuid) {
+  BlobRegistryEntry* entry = registry_.GetEntry(uuid);
+  entry->waiting_until_user_population = false;
+  if (CanFinishBuilding(entry)) {
+    FinishBuilding(entry);
+  }
+}
+
+std::unique_ptr<BlobDataHandle> BlobStorageContext::CreateHandle(
+    const std::string& uuid,
+    BlobRegistryEntry* entry) {
+  return base::WrapUnique(new BlobDataHandle(
+      uuid, entry->content_type, entry->content_disposition, entry->data.size_,
+      this, base::ThreadTaskRunnerHandle::Get().get()));
+}
+
+bool BlobStorageContext::CanFinishBuilding(BlobRegistryEntry* entry) {
+  return entry->status == BlobStatus::PENDING &&
+         entry->dependent_blobs_building == 0 && entry->can_fit &&
+         !entry->waiting_until_user_population;
+}
+
+void BlobStorageContext::FinishBuilding(BlobRegistryEntry* entry) {
   DCHECK(entry);
-  DCHECK(!entry->data.get()) << "Blob already constructed: "
-                             << external_builder.uuid();
-  // We want to handle storing our broken blob as well.
-  switch (entry->state) {
-    case BlobState::PENDING: {
-      entry->data_builder.reset(new InternalBlobData::Builder());
-      InternalBlobData::Builder* internal_data_builder =
-          entry->data_builder.get();
-
-      bool broken = false;
-      for (const auto& blob_item : external_builder.items_) {
-        IPCBlobCreationCancelCode error_code;
-        if (!AppendAllocatedBlobItem(external_builder.uuid_, blob_item,
-                                     internal_data_builder, &error_code)) {
-          broken = true;
-          memory_usage_ -= entry->data_builder->GetNonsharedMemoryUsage();
-          entry->state = BlobState::BROKEN;
-          entry->broken_reason = error_code;
-          entry->data_builder.reset(new InternalBlobData::Builder());
+
+  if (entry->status == BlobStatus::PENDING) {
+    for (const ItemCopyEntry& copy : entry->copies) {
+      DCHECK_EQ(copy.dest_item->item()->type(),
+                DataElement::TYPE_BYTES_DESCRIPTION);
+
+      // We check to see if our source item has been paged to disk.
+      size_t dest_size = static_cast<size_t>(copy.dest_item->item()->length());
+      switch (copy.source_item->item()->type()) {
+        case DataElement::TYPE_BYTES: {
+          const char* src_data =
+              copy.source_item->item()->bytes() + copy.source_item_offset;
+          copy.dest_item->item()->item_->SetToBytes(src_data, dest_size);
+        } break;
+        case DataElement::TYPE_FILE: {
+          // We've been paged to disk, so free the memory of our temporary item,
+          // and create a new shared item with appropriate offset and length.
+          const DataElement& source_element =
+              copy.source_item->item()->data_element();
+          std::unique_ptr<DataElement> new_element(new DataElement());
+          new_element->SetToFilePathRange(
+              source_element.path(),
+              source_element.offset() + copy.source_item_offset, dest_size,
+              source_element.expected_modification_time());
+          scoped_refptr<BlobDataItem> new_item(new BlobDataItem(
+              std::move(new_element), copy.source_item->item()->data_handle()));
+          copy.dest_item->item_.swap(new_item);
+          memory_controller_.FreeMemory(dest_size);
+        } break;
+        case DataElement::TYPE_UNKNOWN:
+        case DataElement::TYPE_BLOB:
+        case DataElement::TYPE_BYTES_DESCRIPTION:
+        case DataElement::TYPE_FILE_FILESYSTEM:
+        case DataElement::TYPE_DISK_CACHE_ENTRY:
+          NOTREACHED();
           break;
-        }
       }
-      entry->data = entry->data_builder->Build();
-      entry->data_builder.reset();
-      entry->state = broken ? BlobState::BROKEN : BlobState::COMPLETE;
-      break;
-    }
-    case BlobState::BROKEN: {
-      InternalBlobData::Builder builder;
-      entry->data = builder.Build();
-      break;
     }
-    case BlobState::COMPLETE:
-      DCHECK(false) << "Blob already constructed: " << external_builder.uuid();
-      return;
-  }
+    entry->copies.clear();
 
-  UMA_HISTOGRAM_COUNTS("Storage.Blob.ItemCount", entry->data->items().size());
-  UMA_HISTOGRAM_BOOLEAN("Storage.Blob.Broken",
-                        entry->state == BlobState::BROKEN);
-  if (entry->state == BlobState::BROKEN) {
-    UMA_HISTOGRAM_ENUMERATION(
-        "Storage.Blob.BrokenReason", static_cast<int>(entry->broken_reason),
-        (static_cast<int>(IPCBlobCreationCancelCode::LAST) + 1));
+    entry->status = BlobStatus::DONE;
   }
-  size_t total_memory = 0, nonshared_memory = 0;
-  entry->data->GetMemoryUsage(&total_memory, &nonshared_memory);
-  UMA_HISTOGRAM_COUNTS("Storage.Blob.TotalSize", total_memory / 1024);
-  UMA_HISTOGRAM_COUNTS("Storage.Blob.TotalUnsharedSize",
-                       nonshared_memory / 1024);
-  TRACE_COUNTER1("Blob", "MemoryStoreUsageBytes", memory_usage_);
+  entry->dependent_blobs.clear();
 
   auto runner = base::ThreadTaskRunnerHandle::Get();
   for (const auto& callback : entry->build_completion_callbacks) {
-    runner->PostTask(FROM_HERE,
-                     base::Bind(callback, entry->state == BlobState::COMPLETE,
-                                entry->broken_reason));
+    runner->PostTask(FROM_HERE, base::Bind(callback, entry->status));
   }
   entry->build_completion_callbacks.clear();
+
+  for (const auto& shareable_item : entry->data.items_) {
+    DCHECK_NE(DataElement::TYPE_BYTES_DESCRIPTION,
+              shareable_item->item()->type());
+    if (shareable_item->item()->type() != DataElement::TYPE_BYTES)
+      continue;
+    memory_controller_.UpdateBlobItemInRecents(shareable_item.get());
+  }
 }
 
-void BlobStorageContext::CancelPendingBlob(const std::string& uuid,
-                                           IPCBlobCreationCancelCode reason) {
+void BlobStorageContext::OnEnoughSizeForBlobData(const std::string& uuid,
+                                                 bool success) {
+  if (!success) {
+    BreakAndFinishBlob(uuid, BlobStatus::OUT_OF_MEMORY);
+    return;
+  }
   BlobRegistryEntry* entry = registry_.GetEntry(uuid);
-  DCHECK(entry && entry->state == BlobState::PENDING);
-  entry->state = BlobState::BROKEN;
-  entry->broken_reason = reason;
-  CompletePendingBlob(BlobDataBuilder(uuid));
+  if (entry == nullptr) {
+    return;
+  }
+  entry->can_fit = true;
+  entry->memory_accounted_for = true;
+  if (!entry->ready_for_user_population_callback.is_null()) {
+    entry->ready_for_user_population_callback.Run(BlobStatus::PENDING);
+    entry->ready_for_user_population_callback.Reset();
+  }
+  if (CanFinishBuilding(entry)) {
+    FinishBuilding(entry);
+  }
+}
+
+void BlobStorageContext::OnDependentBlobFinished(
+    const std::string& owning_blob_uuid,
+    BlobStatus status) {
+  BlobRegistryEntry* entry = registry_.GetEntry(owning_blob_uuid);
+  if (!entry) {
+    return;
+  }
+  if (BlobStatusIsError(status)) {
+    BreakAndFinishBlob(owning_blob_uuid,
+                       ConvertReferencedBlobErrorToConstructingError(status));
+    return;
+  }
+  DCHECK_GT(entry->dependent_blobs_building, 0u);
+  --entry->dependent_blobs_building;
+  if (CanFinishBuilding(entry)) {
+    FinishBuilding(entry);
+  }
+}
+
+void BlobStorageContext::ClearAndFreeMemory(const std::string& uuid,
+                                            BlobRegistryEntry* entry) {
+  if (entry->status == BlobStatus::PENDING) {
+    if (!entry->can_fit) {
+      memory_controller_.RemovePendingConstructionEntry(
+          entry->pending_copies_memory_entry);
+    }
+    entry->dependent_blobs.clear();
+  }
+  // If this is false, that means we can't fit yet and our memory size hasn't
+  // been recorded in the memory manager yet.
+  if (entry->memory_accounted_for) {
+    memory_controller_.FreeMemory(entry->data.GetUnsharedMemoryUsage());
+  }
+  entry->data.RemoveBlobFromShareableItems(uuid);
+  for (const auto& item_refptr : entry->data.items_) {
+    if (item_refptr->referencing_blobs().size() == 0) {
+      memory_controller_.RemoveBlobItemInRecents(*item_refptr);
+    }
+  }
+  entry->data.items_.clear();
+  entry->data.offsets_.clear();
 }
 
 void BlobStorageContext::IncrementBlobRefCount(const std::string& uuid) {
@@ -188,13 +414,7 @@ void BlobStorageContext::DecrementBlobRefCount(const std::string& uuid) {
   DCHECK(entry);
   DCHECK_GT(entry->refcount, 0u);
   if (--(entry->refcount) == 0) {
-    size_t memory_freeing = 0;
-    if (entry->state == BlobState::COMPLETE) {
-      memory_freeing = entry->data->GetUnsharedMemoryUsage();
-      entry->data->RemoveBlobFromShareableItems(uuid);
-    }
-    DCHECK_LE(memory_freeing, memory_usage_);
-    memory_usage_ -= memory_freeing;
+    ClearAndFreeMemory(uuid, entry);
     registry_.DeleteEntry(uuid);
   }
 }
@@ -203,254 +423,39 @@ std::unique_ptr<BlobDataSnapshot> BlobStorageContext::CreateSnapshot(
     const std::string& uuid) {
   std::unique_ptr<BlobDataSnapshot> result;
   BlobRegistryEntry* entry = registry_.GetEntry(uuid);
-  if (entry->state != BlobState::COMPLETE) {
+  if (entry->status != BlobStatus::DONE) {
     return result;
   }
 
-  const InternalBlobData& data = *entry->data;
+  const InternalBlobData& data = entry->data;
   std::unique_ptr<BlobDataSnapshot> snapshot(new BlobDataSnapshot(
       uuid, entry->content_type, entry->content_disposition));
   snapshot->items_.reserve(data.items().size());
   for (const auto& shareable_item : data.items()) {
     snapshot->items_.push_back(shareable_item->item());
+    memory_controller_.UpdateBlobItemInRecents(shareable_item.get());
   }
   return snapshot;
 }
 
-bool BlobStorageContext::IsBroken(const std::string& uuid) const {
+BlobStatus BlobStorageContext::GetBlobStatus(const std::string& uuid) const {
   const BlobRegistryEntry* entry = registry_.GetEntry(uuid);
   if (!entry) {
-    return true;
-  }
-  return entry->state == BlobState::BROKEN;
-}
-
-bool BlobStorageContext::IsBeingBuilt(const std::string& uuid) const {
-  const BlobRegistryEntry* entry = registry_.GetEntry(uuid);
-  if (!entry) {
-    return false;
+    return BlobStatus::INVALID_CONSTRUCTION_ARGUMENTS;
   }
-  return entry->state == BlobState::PENDING;
+  return entry->status;
 }
 
 void BlobStorageContext::RunOnConstructionComplete(
     const std::string& uuid,
-    const BlobConstructedCallback& done) {
+    const BlobStatusCallback& done) {
   BlobRegistryEntry* entry = registry_.GetEntry(uuid);
   DCHECK(entry);
-  switch (entry->state) {
-    case BlobState::COMPLETE:
-      done.Run(true, IPCBlobCreationCancelCode::UNKNOWN);
-      return;
-    case BlobState::BROKEN:
-      done.Run(false, entry->broken_reason);
-      return;
-    case BlobState::PENDING:
-      entry->build_completion_callbacks.push_back(done);
-      return;
-  }
-  NOTREACHED();
-}
-
-bool BlobStorageContext::AppendAllocatedBlobItem(
-    const std::string& target_blob_uuid,
-    scoped_refptr<BlobDataItem> blob_item,
-    InternalBlobData::Builder* target_blob_builder,
-    IPCBlobCreationCancelCode* error_code) {
-  DCHECK(error_code);
-  *error_code = IPCBlobCreationCancelCode::UNKNOWN;
-  bool error = false;
-
-  // The blob data is stored in the canonical way which only contains a
-  // list of Data, File, and FileSystem items. Aggregated TYPE_BLOB items
-  // are expanded into the primitive constituent types and reused if possible.
-  // 1) The Data item is denoted by the raw data and length.
-  // 2) The File item is denoted by the file path, the range and the expected
-  //    modification time.
-  // 3) The FileSystem File item is denoted by the FileSystem URL, the range
-  //    and the expected modification time.
-  // 4) The Blob item is denoted by the source blob and an offset and size.
-  //    Internal items that are fully used by the new blob (not cut by the
-  //    offset or size) are shared between the blobs.  Otherwise, the relevant
-  //    portion of the item is copied.
-
-  DCHECK(blob_item->data_element_ptr());
-  const DataElement& data_element = blob_item->data_element();
-  uint64_t length = data_element.length();
-  uint64_t offset = data_element.offset();
-  UMA_HISTOGRAM_COUNTS("Storage.Blob.StorageSizeBeforeAppend",
-                       memory_usage_ / 1024);
-  switch (data_element.type()) {
-    case DataElement::TYPE_BYTES:
-      UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.Bytes", length / 1024);
-      DCHECK(!offset);
-      if (memory_usage_ + length > kBlobStorageMaxMemoryUsage) {
-        error = true;
-        *error_code = IPCBlobCreationCancelCode::OUT_OF_MEMORY;
-        break;
-      }
-      memory_usage_ += length;
-      target_blob_builder->AppendSharedBlobItem(
-          new ShareableBlobDataItem(target_blob_uuid, blob_item));
-      break;
-    case DataElement::TYPE_FILE: {
-      bool full_file = (length == std::numeric_limits<uint64_t>::max());
-      UMA_HISTOGRAM_BOOLEAN("Storage.BlobItemSize.File.Unknown", full_file);
-      if (!full_file) {
-        UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.File",
-                             (length - offset) / 1024);
-      }
-      target_blob_builder->AppendSharedBlobItem(
-          new ShareableBlobDataItem(target_blob_uuid, blob_item));
-      break;
-    }
-    case DataElement::TYPE_FILE_FILESYSTEM: {
-      bool full_file = (length == std::numeric_limits<uint64_t>::max());
-      UMA_HISTOGRAM_BOOLEAN("Storage.BlobItemSize.FileSystem.Unknown",
-                            full_file);
-      if (!full_file) {
-        UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.FileSystem",
-                             (length - offset) / 1024);
-      }
-      target_blob_builder->AppendSharedBlobItem(
-          new ShareableBlobDataItem(target_blob_uuid, blob_item));
-      break;
-    }
-    case DataElement::TYPE_BLOB: {
-      UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.Blob",
-                           (length - offset) / 1024);
-      // We grab the handle to ensure it stays around while we copy it.
-      std::unique_ptr<BlobDataHandle> src =
-          GetBlobDataFromUUID(data_element.blob_uuid());
-      if (!src || src->IsBroken() || src->IsBeingBuilt()) {
-        error = true;
-        *error_code = IPCBlobCreationCancelCode::REFERENCED_BLOB_BROKEN;
-        break;
-      }
-      BlobRegistryEntry* other_entry =
-          registry_.GetEntry(data_element.blob_uuid());
-      DCHECK(other_entry->data);
-      if (!AppendBlob(target_blob_uuid, *other_entry->data, offset, length,
-                      target_blob_builder)) {
-        error = true;
-        *error_code = IPCBlobCreationCancelCode::OUT_OF_MEMORY;
-      }
-      break;
-    }
-    case DataElement::TYPE_DISK_CACHE_ENTRY: {
-      UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.CacheEntry",
-                           (length - offset) / 1024);
-      target_blob_builder->AppendSharedBlobItem(
-          new ShareableBlobDataItem(target_blob_uuid, blob_item));
-      break;
-    }
-    case DataElement::TYPE_BYTES_DESCRIPTION:
-    case DataElement::TYPE_UNKNOWN:
-      NOTREACHED();
-      break;
-  }
-  UMA_HISTOGRAM_COUNTS("Storage.Blob.StorageSizeAfterAppend",
-                       memory_usage_ / 1024);
-  return !error;
-}
-
-bool BlobStorageContext::AppendBlob(
-    const std::string& target_blob_uuid,
-    const InternalBlobData& blob,
-    uint64_t offset,
-    uint64_t length,
-    InternalBlobData::Builder* target_blob_builder) {
-  DCHECK_GT(length, 0ull);
-
-  const std::vector<scoped_refptr<ShareableBlobDataItem>>& items = blob.items();
-  auto iter = items.begin();
-  if (offset) {
-    for (; iter != items.end(); ++iter) {
-      const BlobDataItem& item = *(iter->get()->item());
-      if (offset >= item.length())
-        offset -= item.length();
-      else
-        break;
-    }
-  }
-
-  for (; iter != items.end() && length > 0; ++iter) {
-    scoped_refptr<ShareableBlobDataItem> shareable_item = iter->get();
-    const BlobDataItem& item = *(shareable_item->item());
-    uint64_t item_length = item.length();
-    DCHECK_GT(item_length, offset);
-    uint64_t current_length = item_length - offset;
-    uint64_t new_length = current_length > length ? length : current_length;
-
-    bool reusing_blob_item = offset == 0 && new_length == item.length();
-    UMA_HISTOGRAM_BOOLEAN("Storage.Blob.ReusedItem", reusing_blob_item);
-    if (reusing_blob_item) {
-      shareable_item->referencing_blobs().insert(target_blob_uuid);
-      target_blob_builder->AppendSharedBlobItem(shareable_item);
-      length -= new_length;
-      continue;
-    }
-
-    // We need to do copying of the items when we have a different offset or
-    // length
-    switch (item.type()) {
-      case DataElement::TYPE_BYTES: {
-        UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.BlobSlice.Bytes",
-                             new_length / 1024);
-        if (memory_usage_ + new_length > kBlobStorageMaxMemoryUsage) {
-          return false;
-        }
-        DCHECK(!item.offset());
-        std::unique_ptr<DataElement> element(new DataElement());
-        element->SetToBytes(item.bytes() + offset,
-                            static_cast<int64_t>(new_length));
-        memory_usage_ += new_length;
-        target_blob_builder->AppendSharedBlobItem(new ShareableBlobDataItem(
-            target_blob_uuid, new BlobDataItem(std::move(element))));
-      } break;
-      case DataElement::TYPE_FILE: {
-        DCHECK_NE(item.length(), std::numeric_limits<uint64_t>::max())
-            << "We cannot use a section of a file with an unknown length";
-        UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.BlobSlice.File",
-                             new_length / 1024);
-        std::unique_ptr<DataElement> element(new DataElement());
-        element->SetToFilePathRange(item.path(), item.offset() + offset,
-                                    new_length,
-                                    item.expected_modification_time());
-        target_blob_builder->AppendSharedBlobItem(new ShareableBlobDataItem(
-            target_blob_uuid,
-            new BlobDataItem(std::move(element), item.data_handle_)));
-      } break;
-      case DataElement::TYPE_FILE_FILESYSTEM: {
-        UMA_HISTOGRAM_COUNTS("Storage.BlobItemSize.BlobSlice.FileSystem",
-                             new_length / 1024);
-        std::unique_ptr<DataElement> element(new DataElement());
-        element->SetToFileSystemUrlRange(item.filesystem_url(),
-                                         item.offset() + offset, new_length,
-                                         item.expected_modification_time());
-        target_blob_builder->AppendSharedBlobItem(new ShareableBlobDataItem(
-            target_blob_uuid, new BlobDataItem(std::move(element))));
-      } break;
-      case DataElement::TYPE_DISK_CACHE_ENTRY: {
-        std::unique_ptr<DataElement> element(new DataElement());
-        element->SetToDiskCacheEntryRange(item.offset() + offset,
-                                          new_length);
-        target_blob_builder->AppendSharedBlobItem(new ShareableBlobDataItem(
-            target_blob_uuid,
-            new BlobDataItem(std::move(element), item.data_handle_,
-                             item.disk_cache_entry(),
-                             item.disk_cache_stream_index(),
-                             item.disk_cache_side_stream_index())));
-      } break;
-      case DataElement::TYPE_BYTES_DESCRIPTION:
-      case DataElement::TYPE_BLOB:
-      case DataElement::TYPE_UNKNOWN:
-        CHECK(false) << "Illegal blob item type: " << item.type();
-    }
-    length -= new_length;
-    offset = 0;
+  if (entry->status == BlobStatus::PENDING) {
+    entry->build_completion_callbacks.push_back(done);
+    return;
   }
-  return true;
+  done.Run(entry->status);
 }
 
 }  // namespace storage