Add Lazy Creation and Thread Detachment Support in the Scheduler Worker Pool

base/task_scheduler/scheduler_worker_stack.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 BASE_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_
 
 #include <stddef.h>
 
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/macros.h"
 
 namespace base {
 namespace internal {
 
 class SchedulerWorker;
 
-// A stack of SchedulerWorkers. Supports removal of arbitrary
-// SchedulerWorkers. DCHECKs when a SchedulerWorker is inserted
-// multiple times. SchedulerWorkers are not owned by the stack. Push() is
-// amortized O(1). Pop(), Size() and Empty() are O(1). Remove is O(n). This
-// class is NOT thread-safe.
+// A stack of SchedulerWorkers. Supports removal of arbitrary SchedulerWorkers.
+// DCHECKs when a SchedulerWorker is inserted multiple times. SchedulerWorkers
+// are not owned by the stack. Push() is amortized O(1). Pop(), Peek(), Size()
+// and Empty() are O(1). Contains() and Remove() are O(n).
+// This class is NOT thread-safe.
 class BASE_EXPORT SchedulerWorkerStack {
  public:
   SchedulerWorkerStack();
   ~SchedulerWorkerStack();
 
   // Inserts |worker| at the top of the stack. |worker| must not already be on
   // the stack.
   void Push(SchedulerWorker* worker);
 
   // Removes the top SchedulerWorker from the stack and returns it.
   // Returns nullptr if the stack is empty.
   SchedulerWorker* Pop();
 
+  // Returns the top SchedulerWorker from the stack, nullptr if empty.
+  SchedulerWorker* Peek() const;
+
+  // Returns true if |worker| is already on the stack.
+  bool Contains(const SchedulerWorker* worker) const;
+
   // Removes |worker| from the stack.
   void Remove(const SchedulerWorker* worker);
 
   // Returns the number of SchedulerWorkers on the stack.
   size_t Size() const { return stack_.size(); }
 
   // Returns true if the stack is empty.
   bool IsEmpty() const { return stack_.empty(); }
 
  private:
   std::vector<SchedulerWorker*> stack_;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerStack);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_