QThreadPool: fix counting of waiting threads

QTBUG-21051 has a testcase where activeThreadCount() could actually
end up at -1 (converted to an autotest in this commit).
The reason was: start() calls tryStart() which returns false due to
too many active threads (reserveThread() causes this), so it calls
enqueueTask() - which actually wakes up the waiting thread, but
it didn't decrement the number of waiting threads.

Note that tryStart() is "if I can grab a waiting thread, enqueue task and wake it"
while start(), in case tryStart() fails, wants to "enqueue, and then if I can grab
a waiting thread, wake it". This is why enqueue shouldn't wake; waking must happen
only if we can grab a thread (d->waitingThreads > 0).

Task-number: QTBUG-21051
Backport from qtbase/dacf9961da86751a59da0e84bc943fe0d1c8d95b

Change-Id: I1e437da27b733a72b48ff1b6f2b78f81a7ed129b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/corelib/concurrent/qthreadpool.cpp b/src/corelib/concurrent/qthreadpool.cpp
index 5435aef..26a5337 100644
--- a/src/corelib/concurrent/qthreadpool.cpp
+++ b/src/corelib/concurrent/qthreadpool.cpp
@@ -187,6 +187,7 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
         // recycle an available thread
         --waitingThreads;
         enqueueTask(task);
+        runnableReady.wakeOne();
         return true;
     }
 
@@ -218,7 +219,6 @@ void QThreadPoolPrivate::enqueueTask(QRunnable *runnable, int priority)
     QList<QPair<QRunnable *, int> >::iterator at =
         qUpperBound(queue.begin(), queue.end(), priority);
     queue.insert(at, qMakePair(runnable, priority));
-    runnableReady.wakeOne();
 }
 
 int QThreadPoolPrivate::activeThreadCount() const
@@ -471,8 +471,14 @@ void QThreadPool::start(QRunnable *runnable, int priority)
 
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
-    if (!d->tryStart(runnable))
+    if (!d->tryStart(runnable)) {
         d->enqueueTask(runnable, priority);
+
+        if (d->waitingThreads > 0) {
+            --d->waitingThreads;
+            d->runnableReady.wakeOne();
+        }
+    }
 }
 
 /*!