The Tor Browser: security/sandbox/chromium/base/threading/sequenced_worker

security/sandbox/chromium/base/threading/sequenced_worker_pool.h@925c144e1f1f (annotated)

security/sandbox/chromium/base/threading/sequenced_worker_pool.h

Fri, 16 Jan 2015 18:13:44 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Fri, 16 Jan 2015 18:13:44 +0100
branch: TOR_BUG_9701
changeset 14: 925c144e1f1f
permissions: -rw-r--r--

Integrate suggestion from review to improve consistency with existing code.

 // 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 <cstddef>
 #include <string>
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/task_runner.h"
 namespace tracked_objects {
 class Location;
 }  // namespace tracked_objects
 namespace base {
 class MessageLoopProxy;
 template <class T> class DeleteHelper;
 class SequencedTaskRunner;
 // 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.
 //
 // 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).
 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 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;
     }
    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();
   // When constructing a SequencedWorkerPool, there must be a
   // MessageLoop on the current thread unless you plan to deliberately
   // leak it.
   // Pass the maximum number of threads (they will be lazily created as needed)
   // and a prefix for the thread name to aid in debugging.
   SequencedWorkerPool(size_t max_threads,
                       const std::string& thread_name_prefix);
   // Like above, but with |observer| for testing.  Does not take
   // ownership of |observer|.
   SequencedWorkerPool(size_t max_threads,
                       const std::string& thread_name_prefix,
                       TestingObserver* observer);
   // Returns a unique token that can be used to sequence tasks posted to
   // PostSequencedWorkerTask(). Valid tokens are always nonzero.
   SequenceToken GetSequenceToken();
   // 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);
   // 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);
   // 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);
   // 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,
                       const Closure& task);
   // Same as PostWorkerTask 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 PostWorkerTask, i.e. the
   // task will be guaranteed to run to completion before shutdown
   // (BLOCK_SHUTDOWN semantics).
   bool PostDelayedWorkerTask(const tracked_objects::Location& from_here,
                              const Closure& task,
                              TimeDelta delay);
   // Same as PostWorkerTask but allows specification of the shutdown behavior.
   bool PostWorkerTaskWithShutdownBehavior(
       const tracked_objects::Location& from_here,
       const Closure& 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,
                                const Closure& 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,
                                     const Closure& 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,
       const Closure& task,
       TimeDelta delay);
   // Same as PostSequencedWorkerTask but allows specification of the shutdown
   // behavior.
   bool PostSequencedWorkerTaskWithShutdownBehavior(
       SequenceToken sequence_token,
       const tracked_objects::Location& from_here,
       const Closure& task,
       WorkerShutdown shutdown_behavior);
   // TaskRunner implementation. Forwards to PostDelayedWorkerTask().
   virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
                                const Closure& task,
                                TimeDelta delay) OVERRIDE;
   virtual bool RunsTasksOnCurrentThread() const OVERRIDE;
   // Returns true if the current thread is processing a task with the given
   // sequence_token.
   bool IsRunningSequenceOnCurrentThread(SequenceToken sequence_token) const;
   // 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.
   // This will not flush delayed tasks; delayed tasks get deleted.
   //
   // 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.
   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 from within tasks that execute during shutdown.
   // Only tasks designated as BLOCKING_SHUTDOWN will be allowed, and only if
   // posted by tasks that are not designated as CONTINUE_ON_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:
   virtual ~SequencedWorkerPool();
   virtual void OnDestruct() const OVERRIDE;
  private:
   friend class RefCountedThreadSafe<SequencedWorkerPool>;
   friend class DeleteHelper<SequencedWorkerPool>;
   class Inner;
   class Worker;
   const scoped_refptr<MessageLoopProxy> constructor_message_loop_;
   // Avoid pulling in too many headers by putting (almost) everything
   // into |inner_|.
   const scoped_ptr<Inner> inner_;
   DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPool);
 };
 }  // namespace base
 #endif  // BASE_THREADING_SEQUENCED_WORKER_POOL_H_

The Tor Browser / annotate

security/sandbox/chromium/base/threading/sequenced_worker_pool.h@925c144e1f1f (annotated)

security/sandbox/chromium/base/threading/sequenced_worker_pool.h