QSslSocket: stabilize triggering for write

QSslSocket::writeData() accumulates outgoing data. It might be called
multiple times during the event processing (most likely from the long
loops which serialize the data).

As this function produces a notification event on each call, it's
possible to get a huge number of slot invocations on the next event
loop run, when we are interested in a single flush.

So, this patch protects the code against uncontrolled signal emission
that results in the lesser resource usage.

Change-Id: If7cf5b0e239abf0bd88a0dfaa8c1183cbd49e5ed
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Markus Goetz (Woboq GmbH) <markus@woboq.com>

src/network/ssl/qsslsocket.cpp

@@ -2015,7 +2015,10 @@ qint64 QSslSocket::writeData(const char *data, qint64 len)
     d->writeBuffer.append(data, len);
 
     // make sure we flush to the plain socket's buffer
-    QMetaObject::invokeMethod(this, "_q_flushWriteBuffer", Qt::QueuedConnection);
+    if (!d->flushTriggered) {
+        d->flushTriggered = true;
+        QMetaObject::invokeMethod(this, "_q_flushWriteBuffer", Qt::QueuedConnection);
+    }
 
     return len;
 }
@@ -2034,6 +2037,7 @@ QSslSocketPrivate::QSslSocketPrivate()
     , allowRootCertOnDemandLoading(true)
     , plainSocket(0)
     , paused(false)
+    , flushTriggered(false)
 {
     QSslConfigurationPrivate::deepCopyDefaultConfiguration(&configuration);
 }
@@ -2056,6 +2060,7 @@ void QSslSocketPrivate::init()
     ignoreAllSslErrors = false;
     shutdown = false;
     pendingClose = false;
+    flushTriggered = false;
 
     // we don't want to clear the ignoreErrorsList, so
     // that it is possible setting it before connecting
@@ -2512,6 +2517,10 @@ void QSslSocketPrivate::_q_channelBytesWrittenSlot(int channel, qint64 written)
 void QSslSocketPrivate::_q_flushWriteBuffer()
 {
     Q_Q(QSslSocket);
+
+    // need to notice if knock-on effects of this flush (e.g. a readReady() via transmit())
+    // make another necessary, so clear flag before calling:
+    flushTriggered = false;
     if (!writeBuffer.isEmpty())
         q->flush();
 }

src/network/ssl/qsslsocket_p.h

@@ -217,6 +217,7 @@ private:
 protected:
     bool verifyErrorsHaveBeenIgnored();
     bool paused;
+    bool flushTriggered;
 };
 
 QT_END_NAMESPACE