Mojo: Armed Watchers

mojo/edk/system/request_context.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_EDK_SYSTEM_REQUEST_CONTEXT_H_
 #define MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_
 
 #include "base/containers/stack_container.h"
 #include "base/macros.h"
 #include "mojo/edk/system/handle_signals_state.h"
 #include "mojo/edk/system/system_impl_export.h"
-#include "mojo/edk/system/watcher.h"
+#include "mojo/edk/system/watch.h"
 
 namespace base {
 template<typename T> class ThreadLocalPointer;
 }
 
 namespace mojo {
 namespace edk {
 
 // A RequestContext is a thread-local object which exists for the duration of
 // a single system API call. It is constructed immediately upon EDK entry and
@@ skipping 19 matching lines @@
   explicit RequestContext(Source source);
   ~RequestContext();
 
   // Returns the current thread-local RequestContext.
   static RequestContext* current();
 
   Source source() const { return source_; }
 
   // Adds a finalizer to this RequestContext corresponding to a watch callback
   // which should be triggered in response to some handle state change. If
-  // the Watcher hasn't been cancelled by the time this RequestContext is
+  // the WatcherDispatcher hasn't been closed by the time this RequestContext is
   // destroyed, its WatchCallback will be invoked with |result| and |state|
   // arguments.
-  void AddWatchNotifyFinalizer(scoped_refptr<Watcher> watcher,
+  void AddWatchNotifyFinalizer(scoped_refptr<Watch> watch,
                                MojoResult result,
                                const HandleSignalsState& state);
 
-  // Adds a finalizer to this RequestContext which cancels a watch.
-  void AddWatchCancelFinalizer(scoped_refptr<Watcher> watcher);
+  // Adds a finalizer to this RequestContext corresponding to a watch callback
+  // which should be triggered to notify of watch cancellation. This appends to
+  // a separate finalizer list from AddWatchNotifyFinalizer, as pending
+  // cancellations must always preempt other pending notifications.
+  void AddWatchCancelFinalizer(scoped_refptr<Watch> watch);
 
  private:
   // Is this request context the current one?
   bool IsCurrent() const;
 
   struct WatchNotifyFinalizer {
-    WatchNotifyFinalizer(scoped_refptr<Watcher> watcher,
+    WatchNotifyFinalizer(scoped_refptr<Watch> watch,
                          MojoResult result,
                          const HandleSignalsState& state);
     WatchNotifyFinalizer(const WatchNotifyFinalizer& other);
     ~WatchNotifyFinalizer();
 
-    scoped_refptr<Watcher> watcher;
+    scoped_refptr<Watch> watch;
     MojoResult result;
     HandleSignalsState state;
   };
 
-  // Chosen by fair dice roll.
-  //
-  // TODO: We should measure the distribution of # of finalizers typical to
-  // any RequestContext and adjust this number accordingly. It's probably
-  // almost always 1, but 4 seems like a harmless upper bound for now.
-  static const size_t kStaticWatchFinalizersCapacity = 4;
+  // NOTE: This upper bound was chosen somewhat arbitrarily after observing some
+  // rare worst-case behavior in Chrome. A vast majority of RequestContexts only
+  // ever accumulate 0 or 1 finalizers.
+  static const size_t kStaticWatchFinalizersCapacity = 8;
 
   using WatchNotifyFinalizerList =
       base::StackVector<WatchNotifyFinalizer, kStaticWatchFinalizersCapacity>;
   using WatchCancelFinalizerList =
-      base::StackVector<scoped_refptr<Watcher>, kStaticWatchFinalizersCapacity>;
+      base::StackVector<scoped_refptr<Watch>, kStaticWatchFinalizersCapacity>;
 
   const Source source_;
 
   WatchNotifyFinalizerList watch_notify_finalizers_;
   WatchCancelFinalizerList watch_cancel_finalizers_;
 
   // Pointer to the TLS context. Although this can easily be accessed via the
   // global LazyInstance, accessing a LazyInstance has a large cost relative to
   // the rest of this class and its usages.
   base::ThreadLocalPointer<RequestContext>* tls_context_;
 
   DISALLOW_COPY_AND_ASSIGN(RequestContext);
 };
 
 }  // namespace edk
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_