Disallow single-threaded SequencedWorkerPools.

BUG=646443

Review-Url: https://codereview.chromium.org/2330303003
Cr-Commit-Position: refs/heads/master@{#419496}

base/threading/sequenced_worker_pool.cc

@@ -652,7 +652,9 @@ SequencedWorkerPool::Inner::Inner(SequencedWorkerPool* worker_pool,
       cleanup_idlers_(0),
       cleanup_cv_(&lock_),
       testing_observer_(observer),
-      task_priority_(task_priority) {}
+      task_priority_(task_priority) {
+  DCHECK_GT(max_threads_, 1U);
+}
 
 SequencedWorkerPool::Inner::~Inner() {
   // You must call Shutdown() before destroying the pool.
@@ -848,22 +850,6 @@ SequencedWorkerPool::Inner::GetTaskSchedulerTaskRunner(
   if (!task_runner) {
     ExecutionMode execution_mode =
         sequence_token_id ? ExecutionMode::SEQUENCED : ExecutionMode::PARALLEL;
-
-    if (max_threads_ == 1U) {
-      // Tasks posted to single-threaded pools can assume thread affinity.
-      execution_mode = ExecutionMode::SINGLE_THREADED;
-
-      // Disallow posting tasks with different sequence tokens to single-
-      // threaded pools since the TaskScheduler can't force different sequences
-      // to run on the same thread.
-      DCHECK_LE(sequenced_task_runner_map_.size(), 1U);
-
-      // Disallow posting tasks without a sequence token to a single-threaded
-      // pool. No users do that currently and we don't want to support new use
-      // cases.
-      DCHECK(sequence_token_id);
-    }
-
     task_runner = CreateTaskRunnerWithTraits(traits, execution_mode);
   }
 

base/threading/sequenced_worker_pool.h

@@ -180,8 +180,6 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
   // Ideally, call this on the main thread of a process, before any other
   // threads are created and before any tasks are posted to that process'
   // SequencedWorkerPools.
-  // Note: SequencedWorkerPool instances with |max_threads == 1| will be special
-  // cased to send all of their work as ExecutionMode::SINGLE_THREADED.
   // TODO(gab): Remove this if http://crbug.com/622400 fails
   // (SequencedWorkerPool will be phased out completely otherwise).
   static void RedirectToTaskSchedulerForProcess();
@@ -198,11 +196,12 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
   // ThreadTaskRunnerHandle 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. |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.
+  // 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);