blob: 9001884c97360658648098eadde93057d88aab25 [file] [log] [blame]
// 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.
#include "base/callback.h"
#include "base/location.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/memory/weak_ptr.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/cpp/system/system_export.h"
#include "mojo/public/cpp/system/watcher.h"
namespace base {
class SingleThreadTaskRunner;
namespace mojo {
// This provides a convenient thread-bound watcher implementation to safely
// watch a single handle, dispatching state change notifications to an arbitrary
// SingleThreadTaskRunner running on the same thread as the SimpleWatcher.
// SimpleWatcher exposes the concept of "arming" from the low-level Watcher API.
// In general, a SimpleWatcher must be "armed" in order to dispatch a single
// notification, and must then be rearmed before it will dispatch another. For
// more details, see the documentation for ArmingPolicy and the Arm() and
// ArmOrNotify() methods below.
class MOJO_CPP_SYSTEM_EXPORT SimpleWatcher {
// 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 watched handle has been closed. No further
// notifications will be fired, as this equivalent to an implicit
// CancelWatch().
// Note that unlike the first two conditions, this callback may be invoked
// with |MOJO_RESULT_CANCELLED| even while the SimpleWatcher is disarmed.
using ReadyCallback = base::Callback<void(MojoResult result)>;
// Selects how this SimpleWatcher is to be armed.
enum class ArmingPolicy {
// The SimpleWatcher is armed automatically on Watch() and rearmed again
// after every invocation of the ReadyCallback. There is no need to manually
// call Arm() on a SimpleWatcher using this policy. This mode is equivalent
// to calling ArmOrNotify() once after Watch() and once again after every
// dispatched notification in MANUAL mode.
// This provides a reasonable approximation of edge-triggered behavior,
// mitigating (but not completely eliminating) the potential for redundant
// notifications.
// NOTE: It is important when using AUTOMATIC policy that your ReadyCallback
// always attempt to change the state of the handle (e.g. read available
// messages on a message pipe.) Otherwise this will result in a potentially
// large number of avoidable redundant tasks.
// For perfect edge-triggered behavior, use MANUAL policy and manually Arm()
// the SimpleWatcher as soon as it becomes possible to do so again.
// The SimpleWatcher is never armed automatically. Arm() or ArmOrNotify()
// must be called manually before any non-cancellation notification can be
// dispatched to the ReadyCallback. See the documentation for Arm() and
// ArmNotify() methods below for more details.
SimpleWatcher(const tracked_objects::Location& from_here,
ArmingPolicy arming_policy,
scoped_refptr<base::SingleThreadTaskRunner> runner =
// Indicates if the SimpleWatcher is currently watching a handle.
bool IsWatching() const;
// Starts watching |handle|. A SimpleWatcher 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. |IsWatching()| returns |false|.)
// If |handle| is not a valid watchable (message or data pipe) handle or
// |signals| is not a valid set of signals to watch, this returns
// Otherwise |MOJO_RESULT_OK| is returned and the handle will be watched until
// either |handle| is closed, the SimpleWatcher is destroyed, or Cancel() is
// explicitly called.
// 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. Note
// that |callback| can be called for results other than
// |MOJO_RESULT_CANCELLED| only if the SimpleWatcher is currently armed. Use
// ArmingPolicy to configure how a SimpleWatcher is armed.
// |MOJO_RESULT_CANCELLED| may be dispatched even while the SimpleWatcher
// is disarmed, and no further notifications will be dispatched after that.
// Destroying the SimpleWatcher implicitly calls |Cancel()|.
MojoResult Watch(Handle handle,
MojoHandleSignals signals,
const ReadyCallback& callback);
// Cancels the current watch. Once this returns, the ReadyCallback previously
// passed to |Watch()| will never be called again for this SimpleWatcher.
// Note that when cancelled with an explicit call to |Cancel()| the
// ReadyCallback will not be invoked with a |MOJO_RESULT_CANCELLED| result.
void Cancel();
// Manually arms the SimpleWatcher.
// Arming the SimpleWatcher allows it to fire a single notification regarding
// some future relevant change in the watched handle's state. It's only valid
// to call Arm() while a handle is being watched (see Watch() above.)
// SimpleWatcher is always disarmed immediately before invoking its
// ReadyCallback and must be rearmed again before another notification can
// fire.
// If the watched handle already meets the watched signaling conditions -
// i.e., if it would have notified immediately once armed - the SimpleWatcher
// is NOT armed, and this call fails with a return value of
// |MOJO_RESULT_FAILED_PRECONDITION|. In that case, what would have been the
// result code for that immediate notification is instead placed in
// |*ready_result| if |ready_result| is non-null.
// If the watcher is successfully armed, this returns |MOJO_RESULT_OK| and
// |ready_result| is ignored.
MojoResult Arm(MojoResult* ready_result = nullptr);
// Manually arms the SimpleWatcher OR posts a task to invoke the ReadyCallback
// with the ready result of the failed arming attempt.
// This is meant as a convenient helper for a common usage of Arm(), and it
// ensures that the ReadyCallback will be invoked asynchronously again as soon
// as the watch's conditions are satisfied, assuming the SimpleWatcher isn't
// cancelled first.
// Unlike Arm() above, this can never fail.
void ArmOrNotify();
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;
class Context;
void OnHandleReady(int watch_id, MojoResult result);
base::ThreadChecker thread_checker_;
// The policy used to determine how this SimpleWatcher is armed.
const ArmingPolicy arming_policy_;
// The TaskRunner of this SimpleWatcher'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_;
ScopedWatcherHandle watcher_handle_;
// A thread-safe context object corresponding to the currently active watch,
// if any.
scoped_refptr<Context> context_;
// Fields below must only be accessed on the SimpleWatcher's owning thread.
// The handle currently under watch. Not owned.
Handle handle_;
// A simple counter to disambiguate notifications from multiple watch contexts
// in the event that this SimpleWatcher cancels and watches multiple times.
int watch_id_ = 0;
// The callback to call when the handle is signaled.
ReadyCallback callback_;
// Tracks if the SimpleWatcher has already notified of unsatisfiability. This
// is used to prevent redundant notifications in AUTOMATIC mode.
bool unsatisfiable_ = false;
// Tag used to ID memory allocations that originated from notifications in
// this watcher.
const char* heap_profiler_tag_ = nullptr;
base::WeakPtrFactory<SimpleWatcher> weak_factory_;
} // namespace mojo