Mojo: Armed Watchers

mojo/edk/system/watch.h

Index: mojo/edk/system/watch.h
diff --git a/mojo/edk/system/watch.h b/mojo/edk/system/watch.h
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+// Copyright 2017 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_EDK_SYSTEM_WATCH_H_
+#define MOJO_EDK_SYSTEM_WATCH_H_
+
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
+#include "mojo/edk/system/atomic_flag.h"
+#include "mojo/edk/system/handle_signals_state.h"
+
+namespace mojo {
+namespace edk {
+
+class Dispatcher;
+class WatcherDispatcher;
+
+// Encapsulates the state associated with a single watch context within a
+// watcher.
+//
+// Every Watch has its own cancellation state, and is captured by RequestContext
+// notification finalizers to avoid redundant context resolution during
+// finalizer execution.
+class Watch : public base::RefCountedThreadSafe<Watch> {
+ public:
+  // Constructs a Watch which represents a watch within |watcher| associated
+  // with |context|, watching |dispatcher| for |signals|.
+  Watch(const scoped_refptr<WatcherDispatcher>& watcher,
+        const scoped_refptr<Dispatcher>& dispatcher,
+        uintptr_t context,
+        MojoHandleSignals signals);
+
+  // Notifies the Watch of a potential state change.
+  //
+  // If |allowed_to_call_callback| is true, this may add a notification
+  // finalizer to the current RequestContext to invoke the watcher's callback
+  // with this watch's context. See return values below.
+  //
+  // This is called directly by WatcherDispatcher whenever the Watch's observed
+  // dispatcher notifies the WatcherDispatcher of a state change.
+  //
+  // Returns |true| if the Watch entered or remains in a ready state as a result
+  // of the state change. If |allowed_to_call_callback| was true in this case,
+  // the Watch will have also attached a notification finalizer to the current
+  // RequestContext.
+  //
+  // Returns |false| if the
+  bool NotifyState(const HandleSignalsState& state,
+                   bool allowed_to_call_callback);
+
+  // Notifies the watch of cancellation ASAP. This will always be the last
+  // notification sent for the watch.
+  void Cancel();
+
+  // Finalizer method for RequestContexts. This method is invoked once for every
+  // notification finalizer added to a RequestContext by this object. This calls
+  // down into the WatcherDispatcher to do the actual notification call.
+  void InvokeCallback(MojoResult result,
+                      const HandleSignalsState& state,
+                      MojoWatcherNotificationFlags flags);
+
+  const scoped_refptr<Dispatcher>& dispatcher() const { return dispatcher_; }
+  uintptr_t context() const { return context_; }
+
+  MojoResult last_known_result() const {
+    AssertWatcherLockAcquired();
+    return last_known_result_;
+  }
+
+  MojoHandleSignalsState last_known_signals_state() const {
+    AssertWatcherLockAcquired();
+    return last_known_signals_state_;
+  }
+
+  bool ready() const {
+    AssertWatcherLockAcquired();
+    return last_known_result_ == MOJO_RESULT_OK ||
+           last_known_result_ == MOJO_RESULT_FAILED_PRECONDITION;
+  }
+
+ private:
+  friend class base::RefCountedThreadSafe<Watch>;
+
+  ~Watch();
+
+#if DCHECK_IS_ON()
+  void AssertWatcherLockAcquired() const;
+#else
+  void AssertWatcherLockAcquired() const {}
+#endif
+
+  const scoped_refptr<WatcherDispatcher> watcher_;
+  const scoped_refptr<Dispatcher> dispatcher_;
+  const uintptr_t context_;
+  const MojoHandleSignals signals_;
+
+  // The result code with which this Watch would notify if currently armed,
+  // based on the last known signaling state of |dispatcher_|. Guarded by the
+  // owning WatcherDispatcher's lock.
+  MojoResult last_known_result_ = MOJO_RESULT_UNKNOWN;
+
+  // The last known signaling state of |dispatcher_|. Guarded by the owning
+  // WatcherDispatcher's lock.
+  MojoHandleSignalsState last_known_signals_state_ = {0, 0};
+
+  // Guards |is_cancelled_| below and mutually excludes individual watch
+  // notification executions for this same watch context.
+  //
+  // Note that this should only be acquired from a RequestContext finalizer to
+  // ensure that no other internal locks are already held.
+  base::Lock notification_lock_;
+
+  // Guarded by |notification_lock_|.
+  bool is_cancelled_ = false;
+
+  DISALLOW_COPY_AND_ASSIGN(Watch);
+};
+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_WATCH_H_