QThreadPool: obey the docs that say we always use at least 1 thread

Even if the user (usually accidentally) sets a thread count of zero or
negative. The reporter in the bug report did
QThread::idealThreadCount() - 1 on a 1 CPU system...

Drive-by add to the documentation and the missing #include.

Fixes: QTBUG-93007
Change-Id: I6cdea00671e8479b9c50fffd167807d14e030154
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
(cherry picked from commit 885eff053797d56f2e295558d0a71b030fbb1a69)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 13 insertions, 8 deletions

diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index 4aebbcc8cb..7eb1b601ff 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/src/corelib/thread/qthreadpool.cpp
@@ -176,7 +176,7 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
     }
 
     // can't do anything if we're over the limit
-    if (activeThreadCount() >= maxThreadCount)
+    if (activeThreadCount() >= maxThreadCount())
         return false;
 
     if (waitingThreads.count() > 0) {
@@ -249,7 +249,7 @@ void QThreadPoolPrivate::tryToStartMoreThreads()
 bool QThreadPoolPrivate::tooManyThreadsActive() const
 {
     const int activeThreadCount = this->activeThreadCount();
-    return activeThreadCount > maxThreadCount && (activeThreadCount - reservedThreads) > 1;
+    return activeThreadCount > maxThreadCount() && (activeThreadCount - reservedThreads) > 1;
 }
 
 /*!
@@ -571,7 +571,7 @@ bool QThreadPool::tryStart(std::function<void()> functionToRun)
 
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
-    if (!d->allThreads.isEmpty() && d->activeThreadCount() >= d->maxThreadCount)
+    if (!d->allThreads.isEmpty() && d->activeThreadCount() >= d->maxThreadCount())
         return false;
 
     QRunnable *runnable = QRunnable::create(std::move(functionToRun));
@@ -612,7 +612,9 @@ void QThreadPool::setExpiryTimeout(int expiryTimeout)
 
 /*! \property QThreadPool::maxThreadCount
 
-    \brief the maximum number of threads used by the thread pool.
+    \brief the maximum number of threads used by the thread pool. This property
+    will default to the value of QThread::idealThreadCount() at the moment the
+    QThreadPool object is created.
 
     \note The thread pool will always use at least 1 thread, even if
     \a maxThreadCount limit is zero or negative.
@@ -623,7 +625,7 @@ void QThreadPool::setExpiryTimeout(int expiryTimeout)
 int QThreadPool::maxThreadCount() const
 {
     Q_D(const QThreadPool);
-    return d->maxThreadCount;
+    return d->requestedMaxThreadCount;
 }
 
 void QThreadPool::setMaxThreadCount(int maxThreadCount)
@@ -631,10 +633,10 @@ void QThreadPool::setMaxThreadCount(int maxThreadCount)
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
 
-    if (maxThreadCount == d->maxThreadCount)
+    if (maxThreadCount == d->requestedMaxThreadCount)
         return;
 
-    d->maxThreadCount = maxThreadCount;
+    d->requestedMaxThreadCount = maxThreadCount;
     d->tryToStartMoreThreads();
 }
 
diff --git a/src/corelib/thread/qthreadpool_p.h b/src/corelib/thread/qthreadpool_p.h
index 4d73a480b5..a2c7d917a4 100644
--- a/src/corelib/thread/qthreadpool_p.h
+++ b/src/corelib/thread/qthreadpool_p.h
@@ -55,6 +55,7 @@
 #include "QtCore/qmutex.h"
 #include "QtCore/qthread.h"
 #include "QtCore/qwaitcondition.h"
+#include "QtCore/qthreadpool.h"
 #include "QtCore/qset.h"
 #include "QtCore/qqueue.h"
 #include "private/qobject_p.h"
@@ -158,6 +159,8 @@ public:
     void tryToStartMoreThreads();
     bool tooManyThreadsActive() const;
 
+    int maxThreadCount() const
+    { return qMax(requestedMaxThreadCount, 1); }    // documentation says we start at least one
     void startThread(QRunnable *runnable = nullptr);
     void reset();
     bool waitForDone(int msecs);
@@ -174,7 +177,7 @@ public:
     QWaitCondition noActiveThreads;
 
     int expiryTimeout = 30000;
-    int maxThreadCount = QThread::idealThreadCount();
+    int requestedMaxThreadCount = QThread::idealThreadCount();  // don't use this directly
     int reservedThreads = 0;
     int activeThreads = 0;
     uint stackSize = 0;