QHostInfo: fix a race condition on QHostInfoCache::enabled

In auto-test-enabled builds, QHostInfoCache can be enabled and
disabled using qt_qhostinfo_enable_cache() at any time. We cannot rule
out that users use this function, or, indeed, that the auto-test never
gets a threading stress-test. Under the assumption, then, that
QHostInfoCache::enabled can be set by any thread at any time, and is
read by any thread using QHostInfo::lookupHost(), we're presented with
a data race, thus UB.

Fix by making the accesses to QHostInfoCache::enabed atomic. Relaxed
operations are suffcient, as the bool is the only data of interest in
these situations. In particular, access to the cache itself is
protected by the cache's mutex.

We use std::atomic<bool> because QAtomicInteger<bool> doesn't exist on
all implementations, but std::atomic<bool> must. Commit a0faf9e236 set
the precedent that it works.

Change-Id: Ia1766753bb54c5fe8d8447b51a49a96a7a853eef
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/kernel/qhostinfo.cpp

@@ -952,23 +952,10 @@ void qt_qhostinfo_cache_inject(const QString &hostname, const QHostInfo &resolut
 QHostInfoCache::QHostInfoCache() : max_age(60), enabled(true), cache(128)
 {
 #ifdef QT_QHOSTINFO_CACHE_DISABLED_BY_DEFAULT
-    enabled = false;
+    enabled.store(false, std::memory_order_relaxed);
 #endif
 }
 
-bool QHostInfoCache::isEnabled()
-{
-    return enabled;
-}
-
-// this function is currently only used for the auto tests
-// and not usable by public API
-void QHostInfoCache::setEnabled(bool e)
-{
-    enabled = e;
-}
-
-
 QHostInfo QHostInfoCache::get(const QString &name, bool *valid)
 {
     QMutexLocker locker(&this->mutex);

src/network/kernel/qhostinfo_p.h

@@ -73,6 +73,7 @@
 #include <QNetworkSession>
 #include <QSharedPointer>
 
+#include <atomic>
 
 QT_BEGIN_NAMESPACE
 
@@ -176,10 +177,12 @@ public:
     void put(const QString &name, const QHostInfo &info);
     void clear();
 
-    bool isEnabled();
-    void setEnabled(bool e);
+    bool isEnabled() { return enabled.load(std::memory_order_relaxed); }
+    // this function is currently only used for the auto tests
+    // and not usable by public API
+    void setEnabled(bool e) { enabled.store(e, std::memory_order_relaxed); }
 private:
-    bool enabled;
+    std::atomic<bool> enabled;
     struct QHostInfoCacheElement {
         QHostInfo info;
         QElapsedTimer age;