use variadic macros/templates in IPC message implementation

ipc/ipc_message_templates.h

Index: ipc/ipc_message_templates.h
diff --git a/ipc/ipc_message_templates.h b/ipc/ipc_message_templates.h
new file mode 100644
index 0000000000000000000000000000000000000000..17dfc1b9ff221169167c2f3103089a219e14fa5e
--- /dev/null
+++ b/ipc/ipc_message_templates.h
@@ -0,0 +1,210 @@
+// 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 IPC_IPC_MESSAGE_TEMPLATES_H_
+#define IPC_IPC_MESSAGE_TEMPLATES_H_
+
+#include <stdint.h>
+
+#include <type_traits>
+
+#include "base/logging.h"
+#include "base/tuple.h"
+#include "build/build_config.h"
+#include "ipc/ipc_message.h"
+#include "ipc/ipc_message_utils.h"
+
+namespace IPC {
+
+// This function is for all the async IPCs that don't pass an extra parameter
+// using IPC_BEGIN_MESSAGE_MAP_WITH_PARAM.
+template <typename ObjT, typename Method, typename P, typename Tuple>
+void DispatchToMethod(ObjT* obj, Method method, P*, const Tuple& tuple) {
+  base::DispatchToMethod(obj, method, tuple);
+}
+
+template <typename ObjT,
+          typename Method,
+          typename P,
+          typename Tuple,
+          size_t... Ns>
+void DispatchToMethodImpl(ObjT* obj,
+                          Method method,
+                          P* parameter,
+                          const Tuple& tuple,
+                          base::IndexSequence<Ns...>) {
+  // TODO(mdempsky): Apply UnwrapTraits like base::DispatchToMethod?

[dcheng, 2016/02/05 21:31:41]

Do we need that here though? I don't think we'd need to forward WeakPtrs or any
sort of fun stuff like that.

[mdempsky, 2016/02/05 22:36:06]

I don't know, hence the question mark. :)

IPC is essentially the only user of base::DispatchToMethod, which does apply
UnwrapTraits, so it just seemed odd/inconsistent to me that the only time we
skip applying UnwrapTraits is when dispatching a message to a method that takes
an extra WITH_PARAM parameter.

+  (obj->*method)(parameter, base::get<Ns>(tuple)...);
+}
+
+// The following function is for async IPCs which have a dispatcher with an
+// extra parameter specified using IPC_BEGIN_MESSAGE_MAP_WITH_PARAM.
+template <typename ObjT, typename P, typename... Args, typename... Ts>
+typename std::enable_if<sizeof...(Args) == sizeof...(Ts)>::type
+DispatchToMethod(ObjT* obj,
+                 void (ObjT::*method)(P*, Args...),
+                 P* parameter,
+                 const base::Tuple<Ts...>& tuple) {
+  DispatchToMethodImpl(obj, method, parameter, tuple,
+                       base::MakeIndexSequence<sizeof...(Ts)>());
+}
+
+enum class MessageKind {
+  CONTROL,
+  ROUTED,
+};
+
+// Routing is a helper struct so MessageT's private common constructor has a
+// different type signature than the public "int32_t routing_id" one.
+struct Routing {
+  explicit Routing(int32_t id) : id(id) {}
+  int32_t id;
+};
+
+// We want to restrict MessageT's constructors so that a routing_id is always
+// provided for ROUTED messages and never provided for CONTROL messages, so
+// use the SFINAE technique from N4387's "Implementation Hint" section.
+#if defined(COMPILER_MSVC)
+// MSVC 2013 doesn't support default arguments for template member functions
+// of templated classes, so there we have to rely on the DCHECKs instead.
+// TODO(mdempsky): Reevaluate once MSVC 2015.
+#define IPC_MESSAGET_SFINAE(x)
+#else
+#define IPC_MESSAGET_SFINAE(x) \
+  template <bool X = (x), typename std::enable_if<X, bool>::type = false>
+#endif
+
+// MessageT is the common template used for all user-defined message types.
+// It's intended to be used via the macros defined in ipc_message_macros.h.
+template <typename Meta,
+          typename InTuple = typename Meta::InTuple,
+          typename OutTuple = typename Meta::OutTuple>
+class MessageT;
+
+// Asynchronous message partial specialization.
+template <typename Meta, typename... Ins>
+class MessageT<Meta, base::Tuple<Ins...>, void> : public Message {
+ public:
+  using Param = base::Tuple<Ins...>;
+  enum { ID = Meta::ID };
+
+  // TODO(mdempsky): Remove.  Uses of MyMessage::Schema::Param can be replaced
+  // with just MyMessage::Param.
+  using Schema = MessageT;
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::CONTROL)
+  MessageT(const Ins&... ins) : MessageT(Routing(MSG_ROUTING_CONTROL), ins...) {
+    DCHECK(Meta::kKind == MessageKind::CONTROL) << Meta::kName;
+  }
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::ROUTED)
+  MessageT(int32_t routing_id, const Ins&... ins)
+      : MessageT(Routing(routing_id), ins...) {
+    DCHECK(Meta::kKind == MessageKind::ROUTED) << Meta::kName;
+  }
+
+  static bool Read(const Message* msg, Param* p);
+  static void Log(std::string* name, const Message* msg, std::string* l);
+
+  template <class T, class S, class P, class Method>
+  static bool Dispatch(const Message* msg,
+                       T* obj,
+                       S* sender,
+                       P* parameter,
+                       Method func) {
+    Param p;
+    if (Read(msg, &p)) {
+      DispatchToMethod(obj, func, parameter, p);
+      return true;
+    }
+    return false;
+  }
+
+ private:
+  MessageT(Routing routing, const Ins&... ins);
+};
+
+// Synchronous message partial specialization.
+template <typename Meta, typename... Ins, typename... Outs>
+class MessageT<Meta, base::Tuple<Ins...>, base::Tuple<Outs...>>
+    : public SyncMessage {
+ public:
+  using SendParam = base::Tuple<Ins...>;
+  using ReplyParam = base::Tuple<Outs...>;
+  enum { ID = Meta::ID };
+
+  // TODO(mdempsky): Remove.  Uses of MyMessage::Schema::{Send,Reply}Param can
+  // be replaced with just MyMessage::{Send,Reply}Param.
+  using Schema = MessageT;
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::CONTROL)
+  MessageT(const Ins&... ins, Outs*... outs)
+      : MessageT(Routing(MSG_ROUTING_CONTROL), ins..., outs...) {
+    DCHECK(Meta::kKind == MessageKind::CONTROL) << Meta::kName;
+  }
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::ROUTED)
+  MessageT(int32_t routing_id, const Ins&... ins, Outs*... outs)
+      : MessageT(Routing(routing_id), ins..., outs...) {
+    DCHECK(Meta::kKind == MessageKind::ROUTED) << Meta::kName;
+  }
+
+  static bool ReadSendParam(const Message* msg, SendParam* p);
+  static bool ReadReplyParam(const Message* msg, ReplyParam* p);
+  static void WriteReplyParams(Message* reply, const Outs&... outs);
+  static void Log(std::string* name, const Message* msg, std::string* l);
+
+  template <class T, class S, class P, class Method>
+  static bool Dispatch(const Message* msg,
+                       T* obj,
+                       S* sender,
+                       P* parameter,
+                       Method func) {
+    SendParam send_params;
+    bool ok = ReadSendParam(msg, &send_params);
+    Message* reply = SyncMessage::GenerateReply(msg);
+    if (ok) {
+      ReplyParam reply_params;
+      base::DispatchToMethod(obj, func, send_params, &reply_params);
+      WriteParam(reply, reply_params);
+      LogReplyParamsToMessage(reply_params, msg);
+    } else {
+      NOTREACHED() << "Error deserializing message " << msg->type();
+      reply->set_reply_error();
+    }
+    sender->Send(reply);
+    return ok;
+  }
+
+  template <class T, class P, class Method>
+  static bool DispatchDelayReply(const Message* msg,
+                                 T* obj,
+                                 P* parameter,
+                                 Method func) {
+    SendParam send_params;
+    bool ok = ReadSendParam(msg, &send_params);
+    Message* reply = SyncMessage::GenerateReply(msg);
+    if (ok) {
+      base::Tuple<Message&> t = base::MakeRefTuple(*reply);
+      ConnectMessageAndReply(msg, reply);
+      base::DispatchToMethod(obj, func, send_params, &t);
+    } else {
+      NOTREACHED() << "Error deserializing message " << msg->type();
+      reply->set_reply_error();
+      obj->Send(reply);
+    }
+    return ok;
+  }
+
+ private:
+  MessageT(Routing routing, const Ins&... ins, Outs*... outs);
+};
+
+}  // namespace IPC
+
+#if defined(IPC_MESSAGE_IMPL)
+#include "ipc/ipc_message_templates_impl.h"
+#endif
+
+#endif  // IPC_IPC_MESSAGE_TEMPLATES_H_