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// Copyright (c) 2012 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 BASE_THREADING_SEQUENCED_WORKER_POOL_H_
#define BASE_THREADING_SEQUENCED_WORKER_POOL_H_
#include <stddef.h>
#include <cstddef>
#include <memory>
#include <string>
#include "base/base_export.h"
#include "base/callback.h"
#include "base/compiler_specific.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/task_runner.h"
#include "base/task_scheduler/task_traits.h"
namespace tracked_objects {
class Location;
} // namespace tracked_objects
namespace base {
class SequencedTaskRunner;
template <class T> class DeleteHelper;
// A worker thread pool that enforces ordering between sets of tasks. It also
// allows you to specify what should happen to your tasks on shutdown.
//
// To enforce ordering, get a unique sequence token from the pool and post all
// tasks you want to order with the token. All tasks with the same token are
// guaranteed to execute serially, though not necessarily on the same thread.
// This means that:
//
// - No two tasks with the same token will run at the same time.
//
// - Given two tasks T1 and T2 with the same token such that T2 will
// run after T1, then T2 will start after T1 is destroyed.
//
// - If T2 will run after T1, then all memory changes in T1 and T1's
// destruction will be visible to T2.
//
// Example:
// SequencedWorkerPool::SequenceToken token = pool.GetSequenceToken();
// pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
// FROM_HERE, base::Bind(...));
// pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
// FROM_HERE, base::Bind(...));
//
// You can make named sequence tokens to make it easier to share a token
// across different components.
//
// You can also post tasks to the pool without ordering using PostWorkerTask.
// These will be executed in an unspecified order. The order of execution
// between tasks with different sequence tokens is also unspecified.
//
// You must call EnableForProcess() or
// EnableWithRedirectionToTaskSchedulerForProcess() before starting to post
// tasks to a process' SequencedWorkerPools.
//
// This class may be leaked on shutdown to facilitate fast shutdown. The
// expected usage, however, is to call Shutdown(), which correctly accounts
// for CONTINUE_ON_SHUTDOWN behavior and is required for BLOCK_SHUTDOWN
// behavior.
//
// Implementation note: This does not use a base::WorkerPool since that does
// not enforce shutdown semantics or allow us to specify how many worker
// threads to run. For the typical use case of random background work, we don't
// necessarily want to be super aggressive about creating threads.
//
// Note that SequencedWorkerPool is RefCountedThreadSafe (inherited
// from TaskRunner).
//
// Test-only code should wrap this in a base::SequencedWorkerPoolOwner to avoid
// memory leaks. See http://crbug.com/273800
class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
public:
// Defines what should happen to a task posted to the worker pool on
// shutdown.
enum WorkerShutdown {
// Tasks posted with this mode which have not run at shutdown will be
// deleted rather than run, and any tasks with this mode running at
// shutdown will be ignored (the worker thread will not be joined).
//
// This option provides a nice way to post stuff you don't want blocking
// shutdown. For example, you might be doing a slow DNS lookup and if it's
// blocked on the OS, you may not want to stop shutdown, since the result
// doesn't really matter at that point.
//
// However, you need to be very careful what you do in your callback when
// you use this option. Since the thread will continue to run until the OS
// terminates the process, the app can be in the process of tearing down
// when you're running. This means any singletons or global objects you
// use may suddenly become invalid out from under you. For this reason,
// it's best to use this only for slow but simple operations like the DNS
// example.
CONTINUE_ON_SHUTDOWN,
// Tasks posted with this mode that have not started executing at
// shutdown will be deleted rather than executed. However, any tasks that
// have already begun executing when shutdown is called will be allowed
// to continue, and will block shutdown until completion.
//
// Note: Because Shutdown() may block while these tasks are executing,
// care must be taken to ensure that they do not block on the thread that
// called Shutdown(), as this may lead to deadlock.
SKIP_ON_SHUTDOWN,
// Tasks posted with this mode will block shutdown until they're
// executed. Since this can have significant performance implications,
// use sparingly.
//
// Generally, this should be used only for user data, for example, a task
// writing a preference file.
//
// If a task is posted during shutdown, it will not get run since the
// workers may already be stopped. In this case, the post operation will
// fail (return false) and the task will be deleted.
BLOCK_SHUTDOWN,
};
// Opaque identifier that defines sequencing of tasks posted to the worker
// pool.
class BASE_EXPORT SequenceToken {
public:
SequenceToken() : id_(0) {}
~SequenceToken() {}
bool Equals(const SequenceToken& other) const {
return id_ == other.id_;
}
// Returns false if current thread is executing an unsequenced task.
bool IsValid() const {
return id_ != 0;
}
// Returns a string representation of this token. This method should only be
// used for debugging.
std::string ToString() const;
private:
friend class SequencedWorkerPool;
explicit SequenceToken(int id) : id_(id) {}
int id_;
};
// Allows tests to perform certain actions.
class TestingObserver {
public:
virtual ~TestingObserver() {}
virtual void OnHasWork() = 0;
virtual void WillWaitForShutdown() = 0;
virtual void OnDestruct() = 0;
};
// Gets the SequencedToken of the current thread.
// If current thread is not a SequencedWorkerPool worker thread or is running
// an unsequenced task, returns an invalid SequenceToken.
static SequenceToken GetSequenceTokenForCurrentThread();
// Returns the SequencedWorkerPool that owns this thread, or null if the
// current thread is not a SequencedWorkerPool worker thread.
//
// Always returns nullptr when SequencedWorkerPool is redirected to
// TaskScheduler.
//
// DEPRECATED. Use SequencedTaskRunnerHandle::Get() instead. Consequentially
// the only remaining use case is in sequenced_task_runner_handle.cc to
// implement that and will soon be removed along with SequencedWorkerPool:
// http://crbug.com/622400.
static scoped_refptr<SequencedWorkerPool> GetWorkerPoolForCurrentThread();
// Returns a unique token that can be used to sequence tasks posted to
// PostSequencedWorkerTask(). Valid tokens are always nonzero.
static SequenceToken GetSequenceToken();
// Enables posting tasks to this process' SequencedWorkerPools. Cannot be
// called if already enabled. This is not thread-safe; proper synchronization
// is required to use any SequencedWorkerPool method after calling this.
static void EnableForProcess();
// Same as EnableForProcess(), but tasks are redirected to the registered
// TaskScheduler. There must be a registered TaskScheduler when this is
// called. TODO(gab): Phase out SequencedWorkerPool entirely:
// http://crbug.com/533920.
static void EnableWithRedirectionToTaskSchedulerForProcess();
// Disables posting tasks to this process' SequencedWorkerPools. Calling this
// while there are active SequencedWorkerPools is not supported. This is not
// thread-safe; proper synchronization is required to use any
// SequencedWorkerPool method after calling this.
static void DisableForProcessForTesting();
// Returns true if posting tasks to this process' SequencedWorkerPool is
// enabled (with or without redirection to TaskScheduler).
static bool IsEnabled();
// When constructing a SequencedWorkerPool, there must be a
// ThreadTaskRunnerHandle on the current thread unless you plan to
// deliberately leak it.
// Constructs a SequencedWorkerPool which will lazily create up to
// |max_threads| and a prefix for the thread name to aid in debugging.
// |max_threads| must be greater than 1. |task_priority| will be used to hint
// base::TaskScheduler for an experiment in which all SequencedWorkerPool
// tasks will be redirected to it in processes where a base::TaskScheduler was
// instantiated.
SequencedWorkerPool(size_t max_threads,
const std::string& thread_name_prefix,
base::TaskPriority task_priority);
// Like above, but with |observer| for testing. Does not take ownership of
// |observer|.
SequencedWorkerPool(size_t max_threads,
const std::string& thread_name_prefix,
base::TaskPriority task_priority,
TestingObserver* observer);
// Returns the sequence token associated with the given name. Calling this
// function multiple times with the same string will always produce the
// same sequence token. If the name has not been used before, a new token
// will be created.
SequenceToken GetNamedSequenceToken(const std::string& name);
// Returns a SequencedTaskRunner wrapper which posts to this
// SequencedWorkerPool using the given sequence token. Tasks with nonzero
// delay are posted with SKIP_ON_SHUTDOWN behavior and tasks with zero delay
// are posted with BLOCK_SHUTDOWN behavior.
scoped_refptr<SequencedTaskRunner> GetSequencedTaskRunner(
SequenceToken token) WARN_UNUSED_RESULT;
// Returns a SequencedTaskRunner wrapper which posts to this
// SequencedWorkerPool using the given sequence token. Tasks with nonzero
// delay are posted with SKIP_ON_SHUTDOWN behavior and tasks with zero delay
// are posted with the given shutdown behavior.
scoped_refptr<SequencedTaskRunner> GetSequencedTaskRunnerWithShutdownBehavior(
SequenceToken token,
WorkerShutdown shutdown_behavior) WARN_UNUSED_RESULT;
// Returns a TaskRunner wrapper which posts to this SequencedWorkerPool using
// the given shutdown behavior. Tasks with nonzero delay are posted with
// SKIP_ON_SHUTDOWN behavior and tasks with zero delay are posted with the
// given shutdown behavior.
scoped_refptr<TaskRunner> GetTaskRunnerWithShutdownBehavior(
WorkerShutdown shutdown_behavior) WARN_UNUSED_RESULT;
// Posts the given task for execution in the worker pool. Tasks posted with
// this function will execute in an unspecified order on a background thread.
// Returns true if the task was posted. If your tasks have ordering
// requirements, see PostSequencedWorkerTask().
//
// This class will attempt to delete tasks that aren't run
// (non-block-shutdown semantics) but can't guarantee that this happens. If
// all worker threads are busy running CONTINUE_ON_SHUTDOWN tasks, there
// will be no workers available to delete these tasks. And there may be
// tasks with the same sequence token behind those CONTINUE_ON_SHUTDOWN
// tasks. Deleting those tasks before the previous one has completed could
// cause nondeterministic crashes because the task could be keeping some
// objects alive which do work in their destructor, which could voilate the
// assumptions of the running task.
//
// The task will be guaranteed to run to completion before shutdown
// (BLOCK_SHUTDOWN semantics).
//
// Returns true if the task was posted successfully. This may fail during
// shutdown regardless of the specified ShutdownBehavior.
bool PostWorkerTask(const tracked_objects::Location& from_here,
OnceClosure task);
// Same as PostWorkerTask but allows specification of the shutdown behavior.
bool PostWorkerTaskWithShutdownBehavior(
const tracked_objects::Location& from_here,
OnceClosure task,
WorkerShutdown shutdown_behavior);
// Like PostWorkerTask above, but provides sequencing semantics. This means
// that tasks posted with the same sequence token (see GetSequenceToken())
// are guaranteed to execute in order. This is useful in cases where you're
// doing operations that may depend on previous ones, like appending to a
// file.
//
// The task will be guaranteed to run to completion before shutdown
// (BLOCK_SHUTDOWN semantics).
//
// Returns true if the task was posted successfully. This may fail during
// shutdown regardless of the specified ShutdownBehavior.
bool PostSequencedWorkerTask(SequenceToken sequence_token,
const tracked_objects::Location& from_here,
OnceClosure task);
// Like PostSequencedWorkerTask above, but allows you to specify a named
// token, which saves an extra call to GetNamedSequenceToken.
bool PostNamedSequencedWorkerTask(const std::string& token_name,
const tracked_objects::Location& from_here,
OnceClosure task);
// Same as PostSequencedWorkerTask but allows a delay to be specified
// (although doing so changes the shutdown behavior). The task will be run
// after the given delay has elapsed.
//
// If the delay is nonzero, the task won't be guaranteed to run to completion
// before shutdown (SKIP_ON_SHUTDOWN semantics) to avoid shutdown hangs.
// If the delay is zero, this behaves exactly like PostSequencedWorkerTask,
// i.e. the task will be guaranteed to run to completion before shutdown
// (BLOCK_SHUTDOWN semantics).
bool PostDelayedSequencedWorkerTask(
SequenceToken sequence_token,
const tracked_objects::Location& from_here,
OnceClosure task,
TimeDelta delay);
// Same as PostSequencedWorkerTask but allows specification of the shutdown
// behavior.
bool PostSequencedWorkerTaskWithShutdownBehavior(
SequenceToken sequence_token,
const tracked_objects::Location& from_here,
OnceClosure task,
WorkerShutdown shutdown_behavior);
// TaskRunner implementation. Forwards to PostDelayedWorkerTask().
bool PostDelayedTask(const tracked_objects::Location& from_here,
OnceClosure task,
TimeDelta delay) override;
bool RunsTasksInCurrentSequence() const override;
// Blocks until all pending tasks are complete. This should only be called in
// unit tests when you want to validate something that should have happened.
// Does not wait for delayed tasks. If redirection to TaskScheduler is
// disabled, delayed tasks are deleted. If redirection to TaskScheduler is
// enabled, this will wait for all tasks posted to TaskScheduler (not just
// tasks posted to this SequencedWorkerPool).
//
// Note that calling this will not prevent other threads from posting work to
// the queue while the calling thread is waiting on Flush(). In this case,
// Flush will return only when there's no more work in the queue. Normally,
// this doesn't come up since in a test, all the work is being posted from
// the main thread.
//
// TODO(gab): Remove mentions of TaskScheduler in this comment if
// http://crbug.com/622400 fails.
void FlushForTesting();
// Spuriously signal that there is work to be done.
void SignalHasWorkForTesting();
// Implements the worker pool shutdown. This should be called during app
// shutdown, and will discard/join with appropriate tasks before returning.
// After this call, subsequent calls to post tasks will fail.
//
// Must be called from the same thread this object was constructed on.
void Shutdown() { Shutdown(0); }
// A variant that allows an arbitrary number of new blocking tasks to be
// posted during shutdown. The tasks cannot be posted within the execution
// context of tasks whose shutdown behavior is not BLOCKING_SHUTDOWN. Once
// the limit is reached, subsequent calls to post task fail in all cases.
// Must be called from the same thread this object was constructed on.
void Shutdown(int max_new_blocking_tasks_after_shutdown);
// Check if Shutdown was called for given threading pool. This method is used
// for aborting time consuming operation to avoid blocking shutdown.
//
// Can be called from any thread.
bool IsShutdownInProgress();
protected:
~SequencedWorkerPool() override;
void OnDestruct() const override;
private:
friend class RefCountedThreadSafe<SequencedWorkerPool>;
friend class DeleteHelper<SequencedWorkerPool>;
class Inner;
class PoolSequencedTaskRunner;
class Worker;
// Returns true if the current thread is processing a task with the given
// sequence_token.
bool IsRunningSequenceOnCurrentThread(SequenceToken sequence_token) const;
const scoped_refptr<SequencedTaskRunner> constructor_task_runner_;
// Avoid pulling in too many headers by putting (almost) everything
// into |inner_|.
const std::unique_ptr<Inner> inner_;
DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPool);
};
} // namespace base
#endif // BASE_THREADING_SEQUENCED_WORKER_POOL_H_