mojo: Support sync calls through ThreadSafeInterfacePtr

mojo/public/cpp/bindings/thread_safe_interface_ptr.h

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_THREAD_SAFE_INTERFACE_PTR_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_THREAD_SAFE_INTERFACE_PTR_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "mojo/public/cpp/bindings/associated_group.h"
 #include "mojo/public/cpp/bindings/associated_interface_ptr.h"
 #include "mojo/public/cpp/bindings/interface_ptr.h"
 #include "mojo/public/cpp/bindings/message.h"
+#include "mojo/public/cpp/bindings/sync_event_watcher.h"
 
 namespace mojo {
+namespace {
+void AssignTrue(bool* b) {
+  *b = true;
+}
+}  // namespace
 
 // Instances of this class may be used from any thread to serialize |Interface|
 // messages and forward them elsewhere. In general you should use one of the
 // ThreadSafeInterfacePtrBase helper aliases defined below, but this type may be
 // useful if you need/want to manually manage the lifetime of the underlying
 // proxy object which will be used to ultimately send messages.
 template <typename Interface>
 class ThreadSafeForwarder : public MessageReceiverWithResponder {
  public:
   using ProxyType = typename Interface::Proxy_;
   using ForwardMessageCallback = base::Callback<void(Message)>;
   using ForwardMessageWithResponderCallback =
       base::Callback<void(Message, std::unique_ptr<MessageReceiver>)>;
 
   // Constructs a ThreadSafeForwarder through which Messages are forwarded to
   // |forward| or |forward_with_responder| by posting to |task_runner|.
   //
   // Any message sent through this forwarding interface will dispatch its reply,
   // if any, back to the thread which called the corresponding interface method.
   ThreadSafeForwarder(
       const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
       const ForwardMessageCallback& forward,
       const ForwardMessageWithResponderCallback& forward_with_responder,
       const AssociatedGroup& associated_group)
       : proxy_(this),
         task_runner_(task_runner),
         forward_(forward),
         forward_with_responder_(forward_with_responder),
-        associated_group_(associated_group) {}
+        associated_group_(associated_group),
+        sync_message_event_(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED),
+        weak_factory_(this) {}
 
   ~ThreadSafeForwarder() override {}
 
   ProxyType& proxy() { return proxy_; }
 
  private:
   // MessageReceiverWithResponder implementation:
   bool Accept(Message* message) override {
     if (!message->associated_endpoint_handles()->empty()) {
       // If this DCHECK fails, it is likely because:
       // - This is a non-associated interface pointer setup using
       //     PtrWrapper::BindOnTaskRunner(
       //         InterfacePtrInfo<InterfaceType> ptr_info);
       //   Please see the TODO in that method.
       // - This is an associated interface which hasn't been associated with a
       //   message pipe. In other words, the corresponding
       //   AssociatedInterfaceRequest hasn't been sent.
       DCHECK(associated_group_.GetController());
       message->SerializeAssociatedEndpointHandles(
           associated_group_.GetController());
     }
     task_runner_->PostTask(FROM_HERE,
                            base::Bind(forward_, base::Passed(message)));
     return true;
   }
 
   bool AcceptWithResponder(
       Message* message,
-      std::unique_ptr<MessageReceiver> response_receiver) override {
+      std::unique_ptr<MessageReceiver> responder) override {
     if (!message->associated_endpoint_handles()->empty()) {
       // Please see comment for the DCHECK in the previous method.
       DCHECK(associated_group_.GetController());
       message->SerializeAssociatedEndpointHandles(
           associated_group_.GetController());
     }
-    auto responder =
-        base::MakeUnique<ForwardToCallingThread>(std::move(response_receiver));
+
+    if (!message->has_flag(Message::kFlagIsSync)) {
+      auto reply_forwarder =
+          base::MakeUnique<ForwardToCallingThread>(std::move(responder));
+      task_runner_->PostTask(
+          FROM_HERE, base::Bind(forward_with_responder_, base::Passed(message),
+                                base::Passed(&reply_forwarder)));
+      return true;
+    }
+
+    // Sync call on this thread.
+    if (task_runner_->RunsTasksOnCurrentThread()) {
+      forward_with_responder_.Run(std::move(*message), std::move(responder));
+      return true;
+    }
+
+    // Sync call on a different thread.
+    // TODO: allow the call to be async on the other thread.
+    Message response;
+    auto reply_signaler =
+        base::MakeUnique<SyncReplySignaler>(&response, &sync_message_event_);
     task_runner_->PostTask(
         FROM_HERE, base::Bind(forward_with_responder_, base::Passed(message),
-                              base::Passed(&responder)));
+                              base::Passed(&reply_signaler)));
+    bool response_received = false;
+    // XXX: What does it mean to have reentrant sync calls that are all waiting
+    // on the same sync_message_event_? Correct because only reentrant sync
+    // calls are allowed?
+    SyncEventWatcher watcher(&sync_message_event_,
+                             base::Bind(&AssignTrue, &response_received));
+    watcher.AllowWokenUpBySyncWatchOnSameThread();
+    // XXX: Check that this handles the case that sync_message_event_ is already
+    // signaled.
+    auto weak_self = weak_factory_.GetWeakPtr();
+    watcher.SyncWatch(&response_received);
+    if (weak_self) {
+      sync_message_event_.Reset();
+      if (response_received)
+        ignore_result(responder->Accept(&response));
+    }
+
     return true;
   }
 
+  class SyncReplySignaler : public MessageReceiver {
+   public:
+    SyncReplySignaler(Message* sync_response_destination,
+                      base::WaitableEvent* sync_response_event)
+        : sync_response_destination_(sync_response_destination),
+          sync_response_event_(sync_response_event) {}
+
+   private:
+    bool Accept(Message* message) {
+      *sync_response_destination_ = std::move(*message);
+      sync_response_event_->Signal();
+      return true;
+    }
+
+    // XXX: What guarantees do we have about the lifetime of these?
+    Message* sync_response_destination_;
+    base::WaitableEvent* sync_response_event_;
+  };
+
   class ForwardToCallingThread : public MessageReceiver {
    public:
     explicit ForwardToCallingThread(std::unique_ptr<MessageReceiver> responder)
         : responder_(std::move(responder)),
-          caller_task_runner_(base::ThreadTaskRunnerHandle::Get()) {
-    }
+          caller_task_runner_(base::ThreadTaskRunnerHandle::Get()) {}
 
    private:
     bool Accept(Message* message) {
       // The current instance will be deleted when this method returns, so we
       // have to relinquish the responder's ownership so it does not get
       // deleted.
       caller_task_runner_->PostTask(FROM_HERE,
           base::Bind(&ForwardToCallingThread::CallAcceptAndDeleteResponder,
                      base::Passed(std::move(responder_)),
                      base::Passed(std::move(*message))));
       return true;
     }
 
     static void CallAcceptAndDeleteResponder(
         std::unique_ptr<MessageReceiver> responder,
         Message message) {
       ignore_result(responder->Accept(&message));
     }
 
     std::unique_ptr<MessageReceiver> responder_;
     scoped_refptr<base::SingleThreadTaskRunner> caller_task_runner_;
   };
 
   ProxyType proxy_;
   const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   const ForwardMessageCallback forward_;
   const ForwardMessageWithResponderCallback forward_with_responder_;
   AssociatedGroup associated_group_;
 
+  // An event used to signal that sync messages are available.
+  base::WaitableEvent sync_message_event_;
+  base::WeakPtrFactory<ThreadSafeForwarder> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(ThreadSafeForwarder);
 };
 
 template <typename InterfacePtrType>
 class ThreadSafeInterfacePtrBase
     : public base::RefCountedThreadSafe<
           ThreadSafeInterfacePtrBase<InterfacePtrType>> {
  public:
   using InterfaceType = typename InterfacePtrType::InterfaceType;
   using PtrInfoType = typename InterfacePtrType::PtrInfoType;
@@ skipping 132 matching lines @@
 using ThreadSafeAssociatedInterfacePtr =
     ThreadSafeInterfacePtrBase<AssociatedInterfacePtr<Interface>>;
 
 template <typename Interface>
 using ThreadSafeInterfacePtr =
     ThreadSafeInterfacePtrBase<InterfacePtr<Interface>>;
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_THREAD_SAFE_INTERFACE_PTR_H_