[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..3043546a7b793ed0f56c6447788abcb95c28d6ae 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 +17,54 @@
 #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/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 +75,7 @@ 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 +85,14 @@ 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 +118,280 @@ void BlobStorageContext::RevokePublicBlobURL(const GURL& blob_url) {
   DecrementBlobRefCount(uuid);
 }
 
-void BlobStorageContext::CreatePendingBlob(
+void BlobStorageContext::EnableDisk(
+    const base::FilePath storage_directory,
+    scoped_refptr<base::SingleThreadTaskRunner> file_runner) {
+  memory_controller_.EnableDisk(storage_directory, std::move(file_runner));
+}
+
+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);
+  DecrementBlobRefCount(content.uuid_);
+
+  std::swap(entry->copies, flattener.copies);
+
+  entry->dependent_blobs_building = flattener.pending_dependent_blobs.size();
+  for (const std::pair<std::string, BlobRegistryEntry*>& pending_blob :
+       flattener.pending_dependent_blobs) {
+    entry->dependent_blobs.emplace_back(

[Marijn Kruisselbrink, 2016/07/08 23:37:37]

nit: Couldn't this just be push_back? Either way you're moving the
std::unique_ptr CreateHandle returns into the method, and the method then has to
move it in place (in the case of push_back), or call the constructor that
matches the arguments you passed to emplace_back to construct it in place (which
in this case would be the move constructor). Emplace is useful to construct
something in place, but if that constructing is just moving an already
constructed item it really is more or less equivalent to push_back.

[dmurph, 2016/07/11 23:33:28]

Ah, yes, forgot to not use that.

+        CreateHandle(pending_blob.first, pending_blob.second));
+    pending_blob.second->build_completion_callbacks.push_back(
+        base::Bind(&BlobStorageContext::OnDependentBlobFinished, AsWeakPtr(),
+                   content.uuid_));
+  }
+
+  if (new_memory_needed == 0) {
+    if (CanFinishBuilding(entry)) {
+      FinishBuilding(entry);
+    }
+    return handle;
+  }
+  // So we need to transport/copy memory.
+  base::Optional<BlobMemoryController::PendingContructionEntry>
+      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 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(!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());
+  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;
+  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);
+  }
+}
+
+static uint64_t sNextItemId = 0;

[Marijn Kruisselbrink, 2016/07/08 23:37:37]

nit: why not move this variable into GetAndIncrementItemId?

And I wonder if it would maybe make sense to have ShareableBlobDataItem create
its own ID rather then everybody having to pass the result of
GetAndIncrementItemId to its constructor?

[dmurph, 2016/07/11 23:33:29]

Sure, changed.

+
+/* static */
+uint64_t BlobStorageContext::GetAndIncrementItemId() {
+  return sNextItemId++;
+}
+
+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);
+
+  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, static_cast<size_t>(dest_size));

[Marijn Kruisselbrink, 2016/07/08 23:37:37]

why the cast? dest_size is already a size_t

[dmurph, 2016/07/11 23:33:28]

Done.

+        } 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) {

[Marijn Kruisselbrink, 2016/07/08 23:37:38]

nit: if (!entry)?

[dmurph, 2016/07/11 23:33:28]

Done.

+    return;
+  }
+  entry->can_fit = 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 == nullptr) {
+    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();
+  }
+  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->item_id());
+    }
+  }
+  entry->data.items_.clear();
+  entry->data.offsets_.clear();
 }
 
 void BlobStorageContext::IncrementBlobRefCount(const std::string& uuid) {
@@ -188,13 +405,7 @@ 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 +414,39 @@ 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 {
-  const BlobRegistryEntry* entry = registry_.GetEntry(uuid);
-  if (!entry) {
-    return true;
-  }
-  return entry->state == BlobState::BROKEN;
-}
-
-bool BlobStorageContext::IsBeingBuilt(const std::string& uuid) const {
+BlobStatus BlobStorageContext::GetBlobStatus(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