Mojo C++ bindings: support sync methods - part 2

mojo/public/cpp/bindings/lib/sync_handle_watcher.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_LIB_SYNC_HANDLE_WATCHER_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_LIB_SYNC_HANDLE_WATCHER_H_
+
+#include <unordered_map>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/message_loop/message_loop.h"
+#include "base/threading/thread_checker.h"
+#include "mojo/public/cpp/system/core.h"
+
+namespace mojo {
+namespace internal {
+
+// SyncHandleWatcher is used to support sync methods. While a sync call is
+// waiting for response, we would like incoming sync method requests on the same
+// thread to be able to reenter. We also would like master endpoints to continue
+// dispatching messages for associated endpoints on different threads.
+// Therefore, SyncHandleWatcher is used as thread-local storage to register all
+// handles that need to be watched while waiting for sync call responses.
+//
+// This class is not thread safe.
+class SyncHandleWatcher : public base::MessageLoop::DestructionObserver {
+ public:
+  // Returns a thread-local object.
+  static SyncHandleWatcher* current();
+
+  using HandleCallback = base::Callback<void(MojoResult)>;
+  bool RegisterHandle(const Handle& handle,
+                      MojoHandleSignals handle_signals,
+                      const HandleCallback& callback);
+
+  void UnregisterHandle(const Handle& handle);
+
+  // Waits on all the registered handles and runs callbacks synchronously for
+  // those ready handles.
+  // The method:
+  //   - returns true when |*should_stop| is set to true;
+  //   - returns false when either |caller_handle| is unregistered or any error
+  //     occurs.
+  bool WatchAllHandles(const Handle& caller_handle, const bool* should_stop);
+
+ private:
+  struct HandleHasher {
+    size_t operator()(const Handle& handle) const {
+      return std::hash<uint32_t>()(static_cast<uint32_t>(handle.value()));
+    }
+  };
+  using HandleMap = std::unordered_map<Handle, HandleCallback, HandleHasher>;
+
+  SyncHandleWatcher();
+  ~SyncHandleWatcher() override;
+
+  // base::MessageLoop::DestructionObserver implementation:
+  void WillDestroyCurrentMessageLoop() override;
+
+  HandleMap handles_;
+
+  ScopedHandle wait_set_handle_;
+
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(SyncHandleWatcher);
+};
+
+}  // namespace internal
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_SYNC_HANDLE_WATCHER_H_