Fix construction races in QtNetwork

When two threads construct a QNetworkAccessManager at exactly the
same time on an SMP system, there are construction races for some
Q_GLOBAL_STATIC data. This is normal and expected - the losing
thread deletes its instance as part of the Q_GLOBAL_STATIC macro.

However, for two of the classes, destruction of the loser had
side effects. For QNetworkConfigurationMangerPrivate, there was
a crash because of uninitialised variable on the losing side.

For QNetworkAccessBackendFactoryData, a guard mechanism intended
to prevent the data being reconstructed by destructors of other
global static classes was being set by the loser.
To fix this, the bool is changed to a QAtomicInt. In the normal
case, it will have value 0->1 on startup and 1->0 on shutdown.
In the race case, it will have values 0->1->2->1 on startup and
1->0 on shutdown.

Task-Number: QTBUG-20343
Reviewed-By: mread

1 files changed, 10 insertions, 5 deletions

diff --git a/src/network/access/qnetworkaccessbackend.cpp b/src/network/access/qnetworkaccessbackend.cpp
index 2fae7d6..88c45d1 100644
--- a/src/network/access/qnetworkaccessbackend.cpp
+++ b/src/network/access/qnetworkaccessbackend.cpp
@@ -57,20 +57,25 @@
 
 QT_BEGIN_NAMESPACE
 
-static bool factoryDataShutdown = false;
 class QNetworkAccessBackendFactoryData: public QList<QNetworkAccessBackendFactory *>
 {
 public:
-    QNetworkAccessBackendFactoryData() : mutex(QMutex::Recursive) { }
+    QNetworkAccessBackendFactoryData() : mutex(QMutex::Recursive)
+    {
+        valid.ref();
+    }
     ~QNetworkAccessBackendFactoryData()
     {
         QMutexLocker locker(&mutex); // why do we need to lock?
-        factoryDataShutdown = true;
+        valid.deref();
     }
 
     QMutex mutex;
+    //this is used to avoid (re)constructing factory data from destructors of other global classes
+    static QAtomicInt valid;
 };
 Q_GLOBAL_STATIC(QNetworkAccessBackendFactoryData, factoryData)
+QAtomicInt QNetworkAccessBackendFactoryData::valid;
 
 QNetworkAccessBackendFactory::QNetworkAccessBackendFactory()
 {
@@ -80,7 +85,7 @@ QNetworkAccessBackendFactory::QNetworkAccessBackendFactory()
 
 QNetworkAccessBackendFactory::~QNetworkAccessBackendFactory()
 {
-    if (!factoryDataShutdown) {
+    if (QNetworkAccessBackendFactoryData::valid) {
         QMutexLocker locker(&factoryData()->mutex);
         factoryData()->removeAll(this);
     }
@@ -89,7 +94,7 @@ QNetworkAccessBackendFactory::~QNetworkAccessBackendFactory()
 QNetworkAccessBackend *QNetworkAccessManagerPrivate::findBackend(QNetworkAccessManager::Operation op,
                                                                  const QNetworkRequest &request)
 {
-    if (!factoryDataShutdown) {
+    if (QNetworkAccessBackendFactoryData::valid) {
         QMutexLocker locker(&factoryData()->mutex);
         QNetworkAccessBackendFactoryData::ConstIterator it = factoryData()->constBegin(),
                                                            end = factoryData()->constEnd();