Revert of Mojo: Armed Watchers

mojo/public/cpp/system/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_SYSTEM_WATCHER_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_checker.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "mojo/public/c/system/types.h"
-#include "mojo/public/c/system/watcher.h"
 #include "mojo/public/cpp/system/handle.h"
 #include "mojo/public/cpp/system/system_export.h"
 
 namespace mojo {
 
-// A strongly-typed representation of a |MojoHandle| for a watcher.
-class WatcherHandle : public Handle {
+// A Watcher watches a single Mojo handle for signal state changes.
+//
+// NOTE: Watchers may only be used on threads which have a running MessageLoop.
+class MOJO_CPP_SYSTEM_EXPORT Watcher {
  public:
-  WatcherHandle() = default;
-  explicit WatcherHandle(MojoHandle value) : Handle(value) {}
+  // A callback to be called any time a watched handle changes state in some
+  // interesting way. The |result| argument indicates one of the following
+  // conditions depending on its value:
+  //
+  //   |MOJO_RESULT_OK|: One or more of the signals being watched is satisfied.
+  //
+  //   |MOJO_RESULT_FAILED_PRECONDITION|: None of the signals being watched can
+  //       ever be satisfied again.
+  //
+  //   |MOJO_RESULT_CANCELLED|: The handle has been closed and the watch has
+  //       been cancelled implicitly.
+  using ReadyCallback = base::Callback<void(MojoResult result)>;
 
-  // Copying and assignment allowed.
+  Watcher(const tracked_objects::Location& from_here,
+          scoped_refptr<base::SingleThreadTaskRunner> runner =
+              base::ThreadTaskRunnerHandle::Get());
+
+  // NOTE: This destructor automatically calls |Cancel()| if the Watcher is
+  // still active.
+  ~Watcher();
+
+  // Indicates if the Watcher is currently watching a handle.
+  bool IsWatching() const;
+
+  // Starts watching |handle|. A Watcher may only watch one handle at a time,
+  // but it is safe to call this more than once as long as the previous watch
+  // has been cancelled (i.e. |is_watching()| returns |false|.)
+  //
+  // If no signals in |signals| can ever be satisfied for |handle|, this returns
+  // |MOJO_RESULT_FAILED_PRECONDITION|.
+  //
+  // If |handle| is not a valid watchable (message or data pipe) handle, this
+  // returns |MOJO_RESULT_INVALID_ARGUMENT|.
+  //
+  // Otherwise |MOJO_RESULT_OK| is returned and the handle will be watched until
+  // closure or cancellation.
+  //
+  // Once the watch is started, |callback| may be called at any time on the
+  // current thread until |Cancel()| is called or the handle is closed.
+  //
+  // Destroying the Watcher implicitly calls |Cancel()|.
+  MojoResult Start(Handle handle,
+                   MojoHandleSignals signals,
+                   const ReadyCallback& callback);
+
+  // Cancels the current watch. Once this returns, the callback previously
+  // passed to |Start()| will never be called again for this Watcher.
+  void Cancel();
+
+  Handle handle() const { return handle_; }
+  ReadyCallback ready_callback() const { return callback_; }
+
+ // Sets the tag used by the heap profiler.
+ // |tag| must be a const string literal.
+ void set_heap_profiler_tag(const char* heap_profiler_tag) {
+   heap_profiler_tag_ = heap_profiler_tag;
+ }
+
+ private:
+  void OnHandleReady(MojoResult result);
+
+  static void CallOnHandleReady(uintptr_t context,
+                                MojoResult result,
+                                MojoHandleSignalsState signals_state,
+                                MojoWatchNotificationFlags flags);
+
+  base::ThreadChecker thread_checker_;
+
+  // The TaskRunner of this Watcher's owning thread. This field is safe to
+  // access from any thread.
+  const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  // Whether |task_runner_| is the same as base::ThreadTaskRunnerHandle::Get()
+  // for the thread.
+  const bool is_default_task_runner_;
+
+  // A persistent weak reference to this Watcher which can be passed to the
+  // Dispatcher any time this object should be signalled. Safe to access (but
+  // not to dereference!) from any thread.
+  base::WeakPtr<Watcher> weak_self_;
+
+  // Fields below must only be accessed on the Watcher's owning thread.
+
+  // The handle currently under watch. Not owned.
+  Handle handle_;
+
+  // The callback to call when the handle is signaled.
+  ReadyCallback callback_;
+
+  // Tag used to ID memory allocations that originated from notifications in
+  // this watcher.
+  const char* heap_profiler_tag_ = nullptr;
+
+  base::WeakPtrFactory<Watcher> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(Watcher);
 };
 
-static_assert(sizeof(WatcherHandle) == sizeof(Handle),
-              "Bad size for C++ WatcherHandle");
-
-typedef ScopedHandleBase<WatcherHandle> ScopedWatcherHandle;
-static_assert(sizeof(ScopedWatcherHandle) == sizeof(WatcherHandle),
-              "Bad size for C++ ScopedWatcherHandle");
-
-MOJO_CPP_SYSTEM_EXPORT MojoResult
-CreateWatcher(MojoWatcherCallback callback,
-              ScopedWatcherHandle* watcher_handle);
-
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_