Relax check for message loop in SequencedWorkerPool

base/threading/sequenced_worker_pool.h

 // 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 <string>
@@ skipping 109 matching lines @@
 
   // Allows tests to perform certain actions.
   class TestingObserver {
    public:
     virtual ~TestingObserver() {}
     virtual void OnHasWork() = 0;
     virtual void WillWaitForShutdown() = 0;
     virtual void OnDestruct() = 0;
   };
 
+  // When constructing a SequencedWorkerPool, there must be a
+  // MessageLoop on the current thread unless you plan to deliberately
+  // leak it.

[joth, 2012/03/16 12:04:04]

FWIW reading this another approach occurred to me: we could make it explicit in
the API when an instance is intended to be leaked by requiring an extra
SequencedWorkerPool::LEAKY enum param to be passed, the absence of which would
invoke the current DCHECK for message loop (or even, allow the user to pass in
the desired destruct message loop).

Then in OnDestroy if the destruction_message_loop_ is NULL it can just bail out
(i.e. implementing the 'leak' semantics, even if the caller releases all
ref-counts onto the pool). (Optionally it could go into a half-dead state, where
it would DCHECK if any new tasks were posted).

+
   // Pass the maximum number of threads (they will be lazily created as needed)
-  // and a prefix for the thread name to ad in debugging.
+  // 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
@@ skipping 110 matching lines @@
   // 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_