Wrap large IndexedDB values into Blobs before writing to LevelDB.

third_party/WebKit/Source/modules/indexeddb/IDBValueWrapper.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IDBValueWrapper_h
+#define IDBValueWrapper_h
+
+#include "bindings/core/v8/ExceptionState.h"
+#include "platform/SharedBuffer.h"
+#include "platform/wtf/PassRefPtr.h"
+#include "platform/wtf/RefPtr.h"
+#include "platform/wtf/Vector.h"
+#include "public/platform/WebBlobInfo.h"
+#include "v8/include/v8.h"
+
+namespace blink {
+
+class Blob;
+class ExceptionState;
+class ScriptState;
+class ScriptValue;
+class SerializedScriptValue;
+class SharedBuffer;
+
+// Logic for wrapping large IndexedDB values in Blobs.
+class IDBValueWrapper {
+ public:
+  // Wrapper for an IndexedDB value.
+  //
+  // The serialization process can throw an exception. The caller is responsible
+  // for checking exception_state.
+  IDBValueWrapper(v8::Isolate*,
+                  v8::Local<v8::Value>,
+                  bool write_wasm_to_stream,
+                  ExceptionState&);
+
+  // Creates a clone of the serialized value.
+  //
+  // This method is necessary because the IndexedDB specification demands that a
+  // value's key and index keys are extracted from a structured clone of the
+  // value.
+  //
+  // This method cannot be called after WrapIfBiggerThan().
+  void Clone(ScriptState*, ScriptValue* clone);
+
+  // Wraps the serialized value into a Blob if it exceeds the given size.
+  //
+  // In production, the threshold is currently always kWrapThreshold.
+  //
+  // This method must be called before ExtractWireBytes() and cannot be called
+  // after ExtractWireBytes().
+  bool WrapIfBiggerThan(unsigned max_bytes);
+
+  PassRefPtr<SharedBuffer> ExtractWireBytes();
+
+  inline Vector<WebBlobInfo>& WrappedBlobInfo() {
+    DCHECK(!had_exception_)
+        << "WrapBlobInfo() called on wrapper with serialization exception";
+    return blob_info_;
+  }
+
+  // Default threshold for WrapIfBiggerThan().
+  //
+  // This should be tuned to achieve a compromise between short-term IndexedDB
+  // throughput and long-term I/O load and memory usage. LevelDB, the underlying
+  // storage for IndexedDB, was not designed with large values in mind. At the
+  // very least, large values will slow down compaction, causing occasional I/O
+  // spikes.
+  static constexpr unsigned kWrapThreshold = 64 * 1024 * 1024;
+
+  // MIME type used for Blobs that wrap IDBValues.
+  static constexpr const char* kWrapMimeType =
+      "application/x-blink-idb-value-wrapper";
+
+ private:
+  // Used to serialize the wrapped value.
+  static void WriteVarint(unsigned value, Vector<char>& output);
+  static void WriteAsciiString(const String& value, Vector<char>& output);
+
+  RefPtr<SerializedScriptValue> serialized_value_;
+  Vector<WebBlobInfo> blob_info_;
+  Vector<char> wire_bytes_;
+#if DCHECK_IS_ON()
+  bool had_exception_ = false;
+  bool wrap_called_ = false;
+#endif  // DCHECK_IS_ON()
+};
+
+}  // namespace blink
+
+#endif  // IDBValueWrapper_h