[mojo-edk] Add explicit message object APIs

mojo/edk/system/message_for_transit.cc

+// Copyright 2016 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/edk/system/message_for_transit.h"
+
+#include <vector>
+
+#include "mojo/edk/embedder/platform_handle_vector.h"
+
+namespace mojo {
+namespace edk {
+
+namespace {
+
+static_assert(sizeof(MessageForTransit::MessageHeader) % 8 == 0,
+              "Invalid MessageHeader size.");
+static_assert(sizeof(MessageForTransit::DispatcherHeader) % 8 == 0,
+              "Invalid DispatcherHeader size.");
+
+}  // namespace
+
+MessageForTransit::~MessageForTransit() {}
+
+// static
+MojoResult MessageForTransit::Create(
+    std::unique_ptr<MessageForTransit>* message,
+    uint32_t num_bytes,
+    const Dispatcher::DispatcherInTransit* dispatchers,
+    uint32_t num_dispatchers) {
+  // A structure for retaining information about every Dispatcher that will be
+  // sent with this message.
+  struct DispatcherInfo {
+    uint32_t num_bytes;
+    uint32_t num_ports;
+    uint32_t num_handles;
+  };
+
+  // This is only the base header size. It will grow as we accumulate the
+  // size of serialized state for each dispatcher.
+  size_t header_size = sizeof(MessageHeader) +
+      num_dispatchers * sizeof(DispatcherHeader);
+  size_t num_ports = 0;
+  size_t num_handles = 0;
+
+  std::vector<DispatcherInfo> dispatcher_info(num_dispatchers);
+  for (size_t i = 0; i < num_dispatchers; ++i) {
+    Dispatcher* d = dispatchers[i].dispatcher.get();
+    d->StartSerialize(&dispatcher_info[i].num_bytes,
+                      &dispatcher_info[i].num_ports,
+                      &dispatcher_info[i].num_handles);
+    header_size += dispatcher_info[i].num_bytes;
+    num_ports += dispatcher_info[i].num_ports;
+    num_handles += dispatcher_info[i].num_handles;
+  }
+
+  // We now have enough information to fully allocate the message storage.
+  std::unique_ptr<PortsMessage> msg = PortsMessage::NewUserMessage(
+      header_size + num_bytes, num_ports, num_handles);
+  if (!msg)
+    return MOJO_RESULT_RESOURCE_EXHAUSTED;
+
+  // Populate the message header with information about serialized dispatchers.
+  //
+  // The front of the message is always a MessageHeader followed by a
+  // DispatcherHeader for each dispatcher to be sent.
+  MessageHeader* header =
+      static_cast<MessageHeader*>(msg->mutable_payload_bytes());
+  DispatcherHeader* dispatcher_headers =
+      reinterpret_cast<DispatcherHeader*>(header + 1);
+
+  // Serialized dispatcher state immediately follows the series of
+  // DispatcherHeaders.
+  char* dispatcher_data =
+      reinterpret_cast<char*>(dispatcher_headers + num_dispatchers);
+
+  header->num_dispatchers = num_dispatchers;
+
+  // |header_size| is the total number of bytes preceding the message payload,
+  // including all dispatcher headers and serialized dispatcher state.
+  DCHECK_LE(header_size, std::numeric_limits<uint32_t>::max());
+  header->header_size = static_cast<uint32_t>(header_size);
+
+  if (num_dispatchers > 0) {
+    ScopedPlatformHandleVectorPtr handles(
+        new PlatformHandleVector(num_handles));
+    size_t port_index = 0;
+    size_t handle_index = 0;
+    bool fail = false;
+    for (size_t i = 0; i < num_dispatchers; ++i) {
+      Dispatcher* d = dispatchers[i].dispatcher.get();
+      DispatcherHeader* dh = &dispatcher_headers[i];
+      const DispatcherInfo& info = dispatcher_info[i];
+
+      // Fill in the header for this dispatcher.
+      dh->type = static_cast<int32_t>(d->GetType());
+      dh->num_bytes = info.num_bytes;
+      dh->num_ports = info.num_ports;
+      dh->num_platform_handles = info.num_handles;
+
+      // Fill in serialized state, ports, and platform handles. We'll cancel
+      // the send if the dispatcher implementation rejects for some reason.
+      if (!d->EndSerialize(static_cast<void*>(dispatcher_data),
+                           msg->mutable_ports() + port_index,
+                           handles->data() + handle_index)) {
+        fail = true;
+        break;
+      }
+
+      dispatcher_data += info.num_bytes;
+      port_index += info.num_ports;
+      handle_index += info.num_handles;
+    }
+
+    if (fail) {
+      // Release any platform handles we've accumulated. Their dispatchers
+      // retain ownership when message creation fails, so these are not actually
+      // leaking.
+      handles->clear();
+      return MOJO_RESULT_INVALID_ARGUMENT;
+    }
+
+    // Take ownership of all the handles and move them into message storage.
+    msg->SetHandles(std::move(handles));
+  }
+
+  message->reset(new MessageForTransit(std::move(msg)));
+  return MOJO_RESULT_OK;
+}
+
+MessageForTransit::MessageForTransit(std::unique_ptr<PortsMessage> message)
+    : message_(std::move(message)) {
+}
+
+}  // namespace edk
+}  // namespace mojo