[mojo] Add pickling support for native-only structs

mojo/public/cpp/bindings/lib/native_serialization.h

+// 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.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_SERIALIZATION_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_SERIALIZATION_H_
+
+#include <stdint.h>
+
+#include <limits>
+
+#include "base/logging.h"
+#include "base/pickle.h"
+#include "ipc/ipc_param_traits.h"
+#include "mojo/public/cpp/bindings/lib/array_internal.h"
+#include "mojo/public/cpp/bindings/lib/bindings_internal.h"
+#include "mojo/public/cpp/bindings/lib/bindings_serialization.h"
+#include "mojo/public/cpp/bindings/lib/pickle_buffer.h"
+
+namespace mojo {
+namespace internal {
+
+template <typename T>
+size_t GetSerializedSizeNative_(const T& value) {
+  return IPC::ParamTraits<T>::GetSize(value);
+}
+
+template <typename T>
+void SerializeNative_(const T& value,
+                      Buffer* buffer,
+                      Array_Data<uint8_t>** out) {
+  PickleBuffer* pickler = buffer->AsPickleBuffer();
+  DCHECK(pickler) << "Native types can only be used with PickleBuffers.";
+
+  ArrayHeader* header =
+      reinterpret_cast<ArrayHeader*>(buffer->Allocate(sizeof(ArrayHeader)));
+  header->num_bytes = sizeof(ArrayHeader);

[yzshen1, 2015/12/15 21:20:26]

header->num_bytes should be the total size of the array, including the array
header and the payload.

Making it a standard array has the benefits:
- avoid confusion
- use the standard Array_Data<uint_8>::Validate() (Please see my comment in
validation_util)
- other language bindings can handle the object as an opaque array if they want
to.

[Ken Rockot(use gerrit already), 2015/12/15 23:31:07]

Oops, I was confused by this and at some point convinced myself it was supposed
to be the header size. Fixed.

+
+  // Remember where the Pickle started before writing.
+  const char* data_start = pickler->end_of_payload();
+
+  IPC::ParamTraits<T>::Write(pickler, value);
+
+  // Fix up the ArrayHeader so that num_elements contains the length of the
+  // pickled data.
+  size_t pickled_size = pickler->end_of_payload() - data_start;
+  DCHECK_LT(pickled_size, std::numeric_limits<uint32_t>::max());
+  header->num_elements = static_cast<uint32_t>(pickled_size);
+
+  *out = reinterpret_cast<Array_Data<uint8_t>*>(header);
+}
+
+template <typename T>
+bool DeserializeNative_(Array_Data<uint8_t>* data,

[yzshen1, 2015/12/15 21:20:26]

Please handle the case |data| is nullptr, in that case, |out| should be set to
default.

With the current code, we won't encountered a null |data| because the
serialization code never generate null. But it is legal to declare a native
struct field to be nullable. And theoretically other bindings (if they deal with
opaque arrays) could send us null.

[Ken Rockot(use gerrit already), 2015/12/15 23:31:07]

Done, sort of. I'd rather not force native types to be copyable, and since the
bindings already provide the address of a default-constructed output, I simply
return true in the null case and assume |out| is already a default.

[yzshen1, 2015/12/15 23:55:29]

Okay. The other deserialization routines don't make that assumption (which may
be useful when we expose serialization/deserialization as a public API), but I
don't think we need to worry too much now.

+                        T* out,
+                        SerializationContext* context) {
+  // Construct a temporary base::Pickle view over the array data. Note that
+  // the Array_Data is laid out like this:
+  //
+  //   [header_size (4 bytes)] [element_count (4 bytes)] [elements...]
+  //
+  // and base::Pickle expects to view data like this:
+  //
+  //   [payload_size (4 bytes)] [header bytes ...] [payload...]
+  //
+  // Conveniently we can get Pickle to do what we want by hacking up the first
+  // 4 bytes so the element count is ignored as part of the header, and the
+  // Pickle payload consists of only the ParamTraits-serialized bytes.
+  ArrayHeader* header = reinterpret_cast<ArrayHeader*>(data);
+  DCHECK_EQ(header->num_bytes, 8u);

[yzshen1, 2015/12/15 21:20:26]

Please see my comment on line 37.

[Ken Rockot(use gerrit already), 2015/12/15 23:31:07]

Fixed

+
+  // Write the number of elements to the Pickle header location. It will treat
+  // this as the bytes size.
+  header->num_bytes = header->num_elements;
+
+  {
+    // Construct a view over the full Array_Data, including our hacked up
+    // header. Pickle will infer from this that the header is 8 bytes long,
+    // and the payload will contain all of the pickled bytes.
+    base::Pickle pickle_view(reinterpret_cast<const char*>(header),
+                             header->num_elements + 8);

[yzshen1, 2015/12/15 21:20:26]

Please use sizeof(ArrayHeader) instead of 8.

[Ken Rockot(use gerrit already), 2015/12/15 23:31:07]

Done

+    base::PickleIterator iter(pickle_view);
+    if (!IPC::ParamTraits<T>::Read(&pickle_view, &iter, out)) {
+      return false;
+    }
+  }
+
+  // Just to be nice, we return the message buffer to its original state.
+  header->num_bytes = sizeof(ArrayHeader);
+  return true;
+}
+
+}  // namespace internal
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_SERIALIZATION_H_