QThreadPool: fix race at time of thread expiry.

The current synchronization mechanism was racy: decrementing waitingThreads
and then hoping that the wakeOne will wake a thread before its expiry
timeout happens. In other words, on timeout, a just-assigned task would
never run. And then no other task would run, if maxThreadCount is reached.

Fixed by using a queue of waiting threads (rather than just a count), and by
moving the wait condition into the thread itself, so we know precisely
which one we're waking up, and we can remove it from the set of waiting threads
before waking it up, and therefore it can determine on wakeup whether it
has work to do (caller removed it from the queue) or it expired (it's still
in the queue). This is reliable, whereas the return value from QWaitCondition::wait
isn't reliable, when the main thread has already decided that this thread
has work to do.

Task-number: QTBUG-3786
Backport from qtbase/a9b6a78e54670a70b96c122b10ad7bd64d166514

Change-Id: Ic766ff67dea7a8bb8f1bc893943060ee5428d782
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/tests/auto/qthreadpool/tst_qthreadpool.cpp b/tests/auto/qthreadpool/tst_qthreadpool.cpp
index 61343c1..c40bd40 100644
--- a/tests/auto/qthreadpool/tst_qthreadpool.cpp
+++ b/tests/auto/qthreadpool/tst_qthreadpool.cpp
@@ -98,6 +98,7 @@ private slots:
     void destruction();
     void threadRecycling();
     void expiryTimeout();
+    void expiryTimeoutRace();
     void exceptions();
     void maxThreadCount();
     void setMaxThreadCount_data();
@@ -333,7 +334,7 @@ class ExpiryTimeoutTask : public QRunnable
 {
 public:
     QThread *thread;
-    int runCount;
+    QAtomicInt runCount;
     QSemaphore semaphore;
 
     ExpiryTimeoutTask()
@@ -345,7 +346,7 @@ public:
     void run()
     {
         thread = QThread::currentThread();
-        ++runCount;
+        runCount.ref();
         semaphore.release();
     }
 };
@@ -364,7 +365,7 @@ void tst_QThreadPool::expiryTimeout()
     // run the task
     threadPool.start(&task);
     QVERIFY(task.semaphore.tryAcquire(1, 10000));
-    QCOMPARE(task.runCount, 1);
+    QCOMPARE(int(task.runCount), 1);
     QVERIFY(!task.thread->wait(100));
     // thread should expire
     QThread *firstThread = task.thread;
@@ -373,7 +374,7 @@ void tst_QThreadPool::expiryTimeout()
     // run task again, thread should be restarted
     threadPool.start(&task);
     QVERIFY(task.semaphore.tryAcquire(1, 10000));
-    QCOMPARE(task.runCount, 2);
+    QCOMPARE(int(task.runCount), 2);
     QVERIFY(!task.thread->wait(100));
     // thread should expire again
     QVERIFY(task.thread->wait(10000));
@@ -386,6 +387,22 @@ void tst_QThreadPool::expiryTimeout()
     QCOMPARE(threadPool.expiryTimeout(), expiryTimeout);
 }
 
+void tst_QThreadPool::expiryTimeoutRace() // QTBUG-3786
+{
+    ExpiryTimeoutTask task;
+
+    QThreadPool threadPool;
+    threadPool.setMaxThreadCount(1);
+    threadPool.setExpiryTimeout(50);
+    const int numTasks = 20;
+    for (int i = 0; i < numTasks; ++i) {
+        threadPool.start(&task);
+        QTest::qSleep(50); // exactly the same as the expiry timeout
+    }
+    QCOMPARE(int(task.runCount), numTasks);
+    QVERIFY(threadPool.waitForDone(2000));
+}
+
 #ifndef QT_NO_EXCEPTIONS
 class ExceptionTask : public QRunnable
 {