SERVER-30254 Make ThreadPool::join finish remaining tasks on a pristine thread instead of inline

2 files changed, 26 insertions, 2 deletions

diff --git a/src/mongo/util/concurrency/thread_pool.cpp b/src/mongo/util/concurrency/thread_pool.cpp
index 5ea024725e0..279d85bc85d 100644
--- a/src/mongo/util/concurrency/thread_pool.cpp
+++ b/src/mongo/util/concurrency/thread_pool.cpp
@@ -157,8 +157,10 @@ void ThreadPool::_join_inlock(stdx::unique_lock<stdx::mutex>* lk) {
     });
     _setState_inlock(joining);
     ++_numIdleThreads;
-    while (!_pendingTasks.empty()) {
-        _doOneTask(lk);
+    if (!_pendingTasks.empty()) {
+        lk->unlock();
+        _drainPendingTasks();
+        lk->lock();
     }
     --_numIdleThreads;
     ThreadList threadsToJoin;
@@ -172,6 +174,22 @@ void ThreadPool::_join_inlock(stdx::unique_lock<stdx::mutex>* lk) {
     _setState_inlock(shutdownComplete);
 }
 
+void ThreadPool::_drainPendingTasks() {
+    // Tasks cannot be run inline because they can create OperationContexts and the join() caller
+    // may already have one associated with the thread.
+    stdx::thread cleanThread = stdx::thread([&] {
+        const std::string threadName = str::stream() << _options.threadNamePrefix
+                                                     << _nextThreadId++;
+        setThreadName(threadName);
+        _options.onCreateThread(threadName);
+        while (!_pendingTasks.empty()) {
+            stdx::unique_lock<stdx::mutex> lock(_mutex);
+            _doOneTask(&lock);
+        }
+    });
+    cleanThread.join();
+}
+
 Status ThreadPool::schedule(Task task) {
     stdx::lock_guard<stdx::mutex> lk(_mutex);
     switch (_state) {
diff --git a/src/mongo/util/concurrency/thread_pool.h b/src/mongo/util/concurrency/thread_pool.h
index 982e11e250b..d88787fa484 100644
--- a/src/mongo/util/concurrency/thread_pool.h
+++ b/src/mongo/util/concurrency/thread_pool.h
@@ -186,6 +186,12 @@ private:
     void _join_inlock(stdx::unique_lock<stdx::mutex>* lk);
 
     /**
+     * Runs the remaining tasks on a new thread as part of the join process, blocking until
+     * complete. Caller must not hold the mutex!
+     */
+    void _drainPendingTasks();
+
+    /**
      * Executes one task from _pendingTasks. "lk" must own _mutex, and _pendingTasks must have at
      * least one entry.
      */