mojo: Support sync calls through ThreadSafeInterfacePtr

mojo/public/cpp/bindings/thread_safe_interface_ptr.h

Index: mojo/public/cpp/bindings/thread_safe_interface_ptr.h
diff --git a/mojo/public/cpp/bindings/thread_safe_interface_ptr.h b/mojo/public/cpp/bindings/thread_safe_interface_ptr.h
index 8b32b30b4fbefe1a5a07e1bb089b97e337d9be74..740687f37984f4864f9ddebf445e897bb346b8f1 100644
--- a/mojo/public/cpp/bindings/thread_safe_interface_ptr.h
+++ b/mojo/public/cpp/bindings/thread_safe_interface_ptr.h
@@ -10,12 +10,26 @@
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/stl_util.h"
+#include "base/synchronization/waitable_event.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_call_restrictions.h"
+#include "mojo/public/cpp/bindings/sync_event_watcher.h"
+
+// ThreadSafeInterfacePtr wraps a non-thread-safe InterfacePtr and proxies
+// messages to it. Async calls are posted to the thread that the InteracePtr is
+// bound to, and the responses are posted back. Sync calls are dispatched
+// directly if the call is made on the thread that the wrapped InterfacePtr is
+// bound to, or posted otherwise. It's important to be aware that sync calls
+// block both the calling thread and the InterfacePtr thread. That means that
+// you cannot make sync calls through a ThreadSafeInterfacePtr if the
+// underlying InterfacePtr is bound to a thread that cannot block, like the IO
+// thread.
 
 namespace mojo {
 
@@ -46,9 +60,15 @@ class ThreadSafeForwarder : public MessageReceiverWithResponder {
         task_runner_(task_runner),
         forward_(forward),
         forward_with_responder_(forward_with_responder),
-        associated_group_(associated_group) {}
+        associated_group_(associated_group),
+        sync_calls_(new InProgressSyncCalls()) {}
 
-  ~ThreadSafeForwarder() override {}
+  ~ThreadSafeForwarder() override {
+    // If there are ongoing sync calls signal their completion now.
+    base::AutoLock l(sync_calls_->lock);
+    for (const auto& pending_response : sync_calls_->pending_responses)
+      pending_response->event.Signal();
+  }
 
   ProxyType& proxy() { return proxy_; }
 
@@ -75,34 +95,128 @@ class ThreadSafeForwarder : public MessageReceiverWithResponder {
 
   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));
+
+    // Async messages are always posted (even if |task_runner_| runs tasks on
+    // this thread) to guarantee that two async calls can't be reordered.
+    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;
+    }
+
+    SyncCallRestrictions::AssertSyncCallAllowed();
+
+    // If the InterfacePtr is bound to this thread, dispatch it directly.
+    if (task_runner_->RunsTasksOnCurrentThread()) {
+      forward_with_responder_.Run(std::move(*message), std::move(responder));
+      return true;
+    }
+
+    // If the InterfacePtr is bound on another thread, post the call.
+    // TODO(yzshen, watk): We block both this thread and the InterfacePtr
+    // thread. Ideally only this thread would block.
+    auto response = make_scoped_refptr(new SyncResponseInfo());
+    auto response_signaler = base::MakeUnique<SyncResponseSignaler>(response);
     task_runner_->PostTask(
         FROM_HERE, base::Bind(forward_with_responder_, base::Passed(message),
-                              base::Passed(&responder)));
+                              base::Passed(&response_signaler)));
+
+    // Save the pending SyncResponseInfo so that if the sync call deletes
+    // |this|, we can signal the completion of the call to return from
+    // SyncWatch().
+    auto sync_calls = sync_calls_;
+    {
+      base::AutoLock l(sync_calls->lock);
+      sync_calls->pending_responses.push_back(response.get());
+    }
+
+    auto assign_true = [](bool* b) { *b = true; };
+    bool event_signaled = false;
+    SyncEventWatcher watcher(&response->event,
+                             base::Bind(assign_true, &event_signaled));
+    watcher.SyncWatch(&event_signaled);
+
+    {
+      base::AutoLock l(sync_calls->lock);
+      base::Erase(sync_calls->pending_responses, response.get());
+    }
+
+    if (response->received)
+      ignore_result(responder->Accept(&response->message));
+
     return true;
   }
 
+  // Data that we need to share between the threads involved in a sync call.
+  struct SyncResponseInfo
+      : public base::RefCountedThreadSafe<SyncResponseInfo> {
+    Message message;
+    bool received = false;
+    base::WaitableEvent event{base::WaitableEvent::ResetPolicy::MANUAL,
+                              base::WaitableEvent::InitialState::NOT_SIGNALED};
+
+   private:
+    friend class base::RefCountedThreadSafe<SyncResponseInfo>;
+  };
+
+  // A MessageReceiver that signals |response| when it either accepts the
+  // response message, or is destructed.
+  class SyncResponseSignaler : public MessageReceiver {
+   public:
+    explicit SyncResponseSignaler(scoped_refptr<SyncResponseInfo> response)
+        : response_(response) {}
+
+    ~SyncResponseSignaler() override {
+      // If Accept() was not called we must still notify the waiter that the
+      // sync call is finished.
+      if (response_)
+        response_->event.Signal();
+    }
+
+    bool Accept(Message* message) {
+      response_->message = std::move(*message);
+      response_->received = true;
+      response_->event.Signal();
+      response_ = nullptr;
+      return true;
+    }
+
+   private:
+    scoped_refptr<SyncResponseInfo> response_;
+  };
+
+  // A record of the pending sync responses for canceling pending sync calls
+  // when the owning ThreadSafeForwarder is destructed.
+  struct InProgressSyncCalls
+      : public base::RefCountedThreadSafe<InProgressSyncCalls> {
+    // |lock| protects access to |pending_responses|.
+    base::Lock lock;
+    std::vector<SyncResponseInfo*> pending_responses;
+  };
+
   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,
+      caller_task_runner_->PostTask(
+          FROM_HERE,
           base::Bind(&ForwardToCallingThread::CallAcceptAndDeleteResponder,
                      base::Passed(std::move(responder_)),
                      base::Passed(std::move(*message))));
@@ -124,6 +238,7 @@ class ThreadSafeForwarder : public MessageReceiverWithResponder {
   const ForwardMessageCallback forward_;
   const ForwardMessageWithResponderCallback forward_with_responder_;
   AssociatedGroup associated_group_;
+  scoped_refptr<InProgressSyncCalls> sync_calls_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadSafeForwarder);
 };