[mojo] Use base::Pickle for Message storage.

mojo/public/cpp/bindings/lib/pickle_buffer.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/cpp/bindings/lib/pickle_buffer.h"
+
+#include <stdlib.h>
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/pickle.h"
+#include "mojo/public/cpp/bindings/lib/bindings_serialization.h"
+
+namespace mojo {
+namespace internal {
+
+class PickleBuffer::Storage : public base::Pickle {
+ public:
+  explicit Storage(size_t num_bytes);
+  ~Storage() override {}
+
+  size_t available_capacity() const {
+    return capacity_after_header() - payload_size();
+  }
+
+  void* GetData() { return static_cast<void*>(mutable_payload()); }
+  void* Claim(size_t num_bytes) { return ClaimBytes(num_bytes); }
+
+ private:
+  // TODO(rockot): Stop wasting 8 bytes per buffer.
+  //
+  // We don't use Pickle's header at all, but its base header type consumes 4
+  // bytes. We waste another 4 bytes to keep our actual buffer aligned to an
+  // 8-byte boundary.
+  //
+  // The reason we don't use base::Pickle's header is that it stores payload
+  // length in the first 4 bytes. Mojo Messages are packed like mojom structs,
+  // where the first 4 bytes are header size rather than payload size.
+  struct PaddedHeader : public base::Pickle::Header {
+    uint32_t padding;
+  };
+
+  static_assert(sizeof(PaddedHeader) % 8 == 0,
+      "PickleBuffer requires a Pickle header size with 8-byte alignment.");
+
+  DISALLOW_COPY_AND_ASSIGN(Storage);
+};
+
+PickleBuffer::Storage::Storage(size_t num_bytes)
+    : base::Pickle(sizeof(PaddedHeader)) {
+  headerT<PaddedHeader>()->padding = 0;
+  Resize(num_bytes);
+}
+
+PickleBuffer::PickleBuffer(size_t num_bytes, bool zero_initialized)
+    : storage_(new Storage(num_bytes)) {
+  if (zero_initialized)
+    memset(storage_->GetData(), 0, num_bytes);
+}
+
+PickleBuffer::~PickleBuffer() {
+}
+
+const void* PickleBuffer::data() const { return storage_->GetData(); }
+
+size_t PickleBuffer::data_num_bytes() const { return storage_->payload_size(); }
+
+base::Pickle* PickleBuffer::pickle() const {
+  return storage_.get();
+}
+
+void* PickleBuffer::Allocate(size_t num_bytes) {
+  DCHECK(storage_);
+
+  // The last allocation may terminate in between 8-byte boundaries. Pad the
+  // front of this allocation if that's the case.
+  size_t padded_capacity = Align(storage_->payload_size());
+  DCHECK_GE(padded_capacity, storage_->payload_size());
+
+  size_t padding_bytes = padded_capacity - storage_->payload_size();
+  size_t allocation_size = padding_bytes + num_bytes;
+  const void* previous_data_location = storage_->GetData();
+  if (storage_->available_capacity() < allocation_size) {
+    NOTREACHED() <<
+        "Message buffers must be fully allocated before serialization.";
+    return nullptr;
+  }
+  char* p = static_cast<char*>(storage_->Claim(allocation_size));
+  DCHECK_EQ(storage_->GetData(), previous_data_location);
+  return p + padding_bytes;
+}
+
+PickleBuffer* PickleBuffer::AsPickleBuffer() { return this; }
+
+}  // namespace internal
+}  // namespace mojo