Fix QThreadPool::maxThreadCount() usage

The docs claim that QThreadPool always creates at least one thread.
However, the user can (usually by mistake) request zero or a negative
number of threads.
The maxThreadCount() function is simply returning the value, that was
requested by the user.
Since it's a public API, it is used in several places in QtConcurrent,
where it is assumed that the value is always positive. This can lead
to a crash if the user sets zero as a maxThreadCount.

Update all such places with std::max(maxThreadCount(), 1).
Prefer this approach over changing the return value of
maxThreadCount(), because its behavior is documented and tested.

Amends 885eff053797d56f2e295558d0a71b030fbb1a69.

Fixes: QTBUG-120335
Pick-to: 6.7 6.6 6.5 6.2
Change-Id: Id3b2087cec7fbc7a2d42febca6586f2dacffe444
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp b/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
index 94248404da..e66edb1b60 100644
--- a/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
+++ b/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
@@ -185,6 +185,17 @@ void tst_QtConcurrentMap::map()
         QCOMPARE(list, NonTemplateSequence({ 2, 4, 6 }));
     }
 
+    // custom pool with invalid number of threads
+    {
+        QList<int> list;
+        list << 1 << 2 << 3;
+        QThreadPool pool;
+        pool.setMaxThreadCount(0); // explicitly set incorrect value
+        // This should not crash
+        QtConcurrent::map(&pool, list, MultiplyBy2InPlace()).waitForFinished();
+        QCOMPARE(list, QList<int>() << 2 << 4 << 6);
+    }
+
 #if 0
     // not allowed: map() with immutable sequences makes no sense
     {
@@ -1075,6 +1086,17 @@ void tst_QtConcurrentMap::mappedReducedThreadPool()
                                                           intCube, intSumReduce);
         QCOMPARE(result, sumOfCubes);
     }
+
+    {
+        // pool with invalid number of threads
+        QThreadPool pool;
+        pool.setMaxThreadCount(0); // explicitly set incorrect value
+
+        // This should not crash
+        NonTemplateSequence list { 1, 2, 3 };
+        auto future = QtConcurrent::mappedReduced(&pool, list, multiplyBy2, intSumReduce);
+        QCOMPARE(future.result(), 12);
+    }
 }
 
 void tst_QtConcurrentMap::mappedReducedWithMoveOnlyCallable()