Fix up SequencedWorkerPool in preparation for making it a TaskRunner

base/threading/sequenced_worker_pool.h

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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_
 #pragma once
 
+#include <cstddef>
+#include <list>
+#include <map>
+#include <set>
 #include <string>
+#include <vector>
 
+#include "base/atomicops.h"
+#include "base/base_export.h"
+#include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/linked_ptr.h"
 #include "base/memory/ref_counted.h"
+#include "base/synchronization/condition_variable.h"

[brettw, 2012/02/24 05:43:34]

One of the reasons I liked the inner class was that lots of code will use this,
and it keeps the dependencies under control. This is quite a lot of big
dependencies.

+#include "base/synchronization/lock.h"
 #include "base/tracked_objects.h"
-#include "base/base_export.h"
 
 namespace base {
 
 // 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.
 //
@@ skipping 13 matching lines @@
 //
 // This class is designed to be leaked on shutdown to allow the
 // CONTINUE_ON_SHUTDOWN behavior to be implemented. To enforce the
 // BLOCK_SHUTDOWN behavior, you must call Shutdown() which will wait until
 // the necessary tasks have completed.
 //
 // 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.
-class BASE_EXPORT SequencedWorkerPool {
+class BASE_EXPORT SequencedWorkerPool
+    : public RefCountedThreadSafe<SequencedWorkerPool> {
  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
@@ skipping 20 matching lines @@
     // 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. See NewSequenceToken().
+  // pool.
   class SequenceToken {
    public:
-    explicit SequenceToken() : id_(0) {}
+    SequenceToken() : id_(0) {}
     ~SequenceToken() {}
 
     bool Equals(const SequenceToken& other) const {
       return id_ == other.id_;
     }
 
    private:
     friend class SequencedWorkerPool;
 
-    SequenceToken(int id) : id_(id) {}
+    explicit SequenceToken(int id) : id_(id) {}
 
     int id_;
   };
 
   // Allows tests to perform certain actions.
   class TestingObserver {
    public:
     virtual ~TestingObserver() {}
     virtual void WillWaitForShutdown() = 0;
   };
 
   // Pass the maximum number of threads (they will be lazily created as needed)
   // and a prefix for the thread name to ad in debugging.
   SequencedWorkerPool(size_t max_threads,
                       const std::string& thread_name_prefix);
-  ~SequencedWorkerPool();
 
   // Returns a unique token that can be used to sequence tasks posted to
   // PostSequencedWorkerTask(). Valid tokens are alwys 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);
@@ skipping 12 matching lines @@
   // 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 base::Closure& task);
+                      const Closure& task);
 
   // Same as PostWorkerTask but allows specification of the shutdown behavior.
   bool PostWorkerTaskWithShutdownBehavior(
       const tracked_objects::Location& from_here,
-      const base::Closure& task,
+      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 base::Closure& task);
+                               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 base::Closure& task);
+                                    const Closure& task);
 
   // Same as PostSequencedWorkerTask but allows specification of the shutdown
   // behavior.
   bool PostSequencedWorkerTaskWithShutdownBehavior(
       SequenceToken sequence_token,
       const tracked_objects::Location& from_here,
-      const base::Closure& task,
+      const Closure& task,
       WorkerShutdown shutdown_behavior);
 
   // 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.
   //
   // 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 sine in a test, all the work is being posted from
   // the main thread.
   void FlushForTesting();
 
   // 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.
   void Shutdown();
 
   // Called by tests to set the testing observer. This is NULL by default
   // and ownership of the pointer is kept with the caller.
   void SetTestingObserver(TestingObserver* observer);
 
  private:
-  class Inner;
+  friend class RefCountedThreadSafe<SequencedWorkerPool>;
   class Worker;
 
-  friend class Inner;
-  friend class Worker;
+  struct SequencedTask {
+    SequencedTask();
+    ~SequencedTask();
 
-  scoped_refptr<Inner> inner_;
+    int sequence_token_id;
+    WorkerShutdown shutdown_behavior;
+    tracked_objects::Location location;
+    Closure task;
+  };
+
+  ~SequencedWorkerPool();
+
+  // This function accepts a name and an ID. If the name is null, the
+  // token ID is used. This allows us to implement the optional name lookup
+  // from a single function without having to enter the lock a separate time.
+  bool PostTaskHelper(const std::string* optional_token_name,
+                      SequenceToken sequence_token,
+                      WorkerShutdown shutdown_behavior,
+                      const tracked_objects::Location& from_here,
+                      const Closure& task);
+
+  // Runs the worker loop on the background thread.
+  void ThreadLoop(Worker* this_worker);
+
+  // Called from within the lock, this converts the given token name into a
+  // token ID, creating a new one if necessary.
+  int LockedGetNamedTokenID(const std::string& name);
+
+  // The calling code should clear the given delete_these_oustide_lock
+  // vector the next time the lock is released. See the implementation for
+  // a more detailed description.
+  bool GetWork(SequencedTask* task,
+               std::vector<Closure>* delete_these_outside_lock);
+
+  // Peforms init and cleanup around running the given task. WillRun...
+  // returns the value from PrepareToStartAdditionalThreadIfNecessary.
+  // The calling code should call FinishStartingAdditionalThread once the
+  // lock is released if the return values is nonzero.
+  int WillRunWorkerTask(const SequencedTask& task);
+  void DidRunWorkerTask(const SequencedTask& task);
+
+  // Returns true if there are no threads currently running the given
+  // sequence token.
+  bool IsSequenceTokenRunnable(int sequence_token_id) const;
+
+  // Checks if all threads are busy and the addition of one more could run an
+  // additional task waiting in the queue. This must be called from within
+  // the lock.
+  //
+  // If another thread is helpful, this will mark the thread as being in the
+  // process of starting and returns the index of the new thread which will be
+  // 0 or more. The caller should then call FinishStartingAdditionalThread to
+  // complete initialization once the lock is released.
+  //
+  // If another thread is not necessary, returne 0;
+  //
+  // See the implementedion for more.
+  int PrepareToStartAdditionalThreadIfHelpful();
+
+  // The second part of thread creation after
+  // PrepareToStartAdditionalThreadIfHelpful with the thread number it
+  // generated. This actually creates the thread and should be called outside
+  // the lock to avoid blocking important work starting a thread in the lock.
+  void FinishStartingAdditionalThread(int thread_number);
+
+  // Checks whether there is work left that's blocking shutdown. Must be
+  // called inside the lock.
+  bool CanShutdown() const;
+
+  // The last sequence number used. Managed by GetSequenceToken, since this
+  // only does threadsafe increment operations, you do not need to hold the
+  // lock.
+  volatile subtle::Atomic32 last_sequence_number_;
+
+  // This lock protects |everything in this class|. Do not read or modify
+  // anything without holding this lock. Do not block while holding this
+  // lock.
+  Lock lock_;
+
+  // Condition variable used to wake up worker threads when a task is runnable.
+  ConditionVariable cond_var_;
+
+  // The maximum number of worker threads we'll create.
+  size_t max_threads_;
+
+  std::string thread_name_prefix_;
+
+  // Associates all known sequence token names with their IDs.
+  std::map<std::string, int> named_sequence_tokens_;
+
+  // Owning pointers to all threads we've created so far. Since we lazily
+  // create threads, this may be less than max_threads_ and will be initially
+  // empty.
+  std::vector<linked_ptr<Worker> > threads_;
+
+  // Set to true when we're in the process of creating another thread.
+  // See PrepareToStartAdditionalThreadIfHelpful for more.
+  bool thread_being_created_;
+
+  // Number of threads currently waiting for work.
+  size_t waiting_thread_count_;
+
+  // Number of threads currently running tasks that have the BLOCK_SHUTDOWN
+  // flag set.
+  size_t blocking_shutdown_thread_count_;
+
+  // In-order list of all pending tasks. These are tasks waiting for a thread
+  // to run on or that are blocked on a previous task in their sequence.
+  //
+  // We maintain the pending_task_count_ separately for metrics because
+  // list.size() can be linear time.
+  std::list<SequencedTask> pending_tasks_;
+  size_t pending_task_count_;
+
+  // Number of tasks in the pending_tasks_ list that are marked as blocking
+  // shutdown.
+  size_t blocking_shutdown_pending_task_count_;
+
+  // Lists all sequence tokens currently executing.
+  std::set<int> current_sequences_;
+
+  // Set when Shutdown is called and no further tasks should be
+  // allowed, though we may still be running existing tasks.
+  bool shutdown_called_;
+
+  TestingObserver* testing_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPool);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_SEQUENCED_WORKER_POOL_H_