Rework toggling the state of read notifier on unbuffered socket

To avoid infinite loop on unbuffered socket, previous implementation
always disabled the read notifications before emitting a readyRead()
signal. So, it's very likely that the socket will toggle the state of
notifier twice (on->off->on) in one notification cycle.

This patch prevents this unnecessary toggling by deferring the
notification disabling in canReadNotification() to the next
notification cycle.

Change-Id: Iebc5a7ad18a6f40ea1cf63e7f1b12f6c180cbf7a
Reviewed-by: Markus Goetz (Woboq GmbH) <markus@woboq.com>

src/network/socket/qabstractsocket.cpp

@@ -563,6 +563,7 @@ QAbstractSocketPrivate::QAbstractSocketPrivate()
       cachedSocketDescriptor(-1),
       readBufferMaxSize(0),
       isBuffered(false),
+      hasPendingData(false),
       connectTimer(0),
       disconnectTimer(0),
       hostLookupId(-1),
@@ -593,6 +594,7 @@ void QAbstractSocketPrivate::resetSocketLayer()
     qDebug("QAbstractSocketPrivate::resetSocketLayer()");
 #endif
 
+    hasPendingData = false;
     if (socketEngine) {
         socketEngine->close();
         socketEngine->disconnect();
@@ -683,8 +685,13 @@ bool QAbstractSocketPrivate::canReadNotification()
     qDebug("QAbstractSocketPrivate::canReadNotification()");
 #endif
 
-    if (!isBuffered)
-        socketEngine->setReadNotificationEnabled(false);
+    if (!isBuffered) {
+        if (hasPendingData) {
+            socketEngine->setReadNotificationEnabled(false);
+            return true;
+        }
+        hasPendingData = true;
+    }
 
     // If buffered, read data from the socket into the read buffer
     qint64 newBytes = 0;
@@ -2434,6 +2441,7 @@ qint64 QAbstractSocket::readData(char *data, qint64 maxSize)
         d->state = QAbstractSocket::UnconnectedState;
     } else {
         // Only do this when there was no error
+        d->hasPendingData = false;
         d->socketEngine->setReadNotificationEnabled(true);
     }
 

src/network/socket/qabstractsocket_p.h

@@ -143,6 +143,7 @@ public:
 
     qint64 readBufferMaxSize;
     bool isBuffered;
+    bool hasPendingData;
 
     QTimer *connectTimer;
     QTimer *disconnectTimer;

src/network/socket/qudpsocket.cpp

@@ -398,7 +398,8 @@ qint64 QUdpSocket::readDatagram(char *data, qint64 maxSize, QHostAddress *addres
         readBytes = d->socketEngine->readDatagram(data, maxSize);
     }
 
-    d_func()->socketEngine->setReadNotificationEnabled(true);
+    d->hasPendingData = false;
+    d->socketEngine->setReadNotificationEnabled(true);
     if (readBytes < 0)
         d->setErrorAndEmit(d->socketEngine->error(), d->socketEngine->errorString());
     return readBytes;