QSslSocket: respect read buffer's max size (SecureTransport)

1. QSslSocketBackendPrivate::transmit was ignoring 'readBufferMaxSize';
as a result, we can have a user trying to set read buffer's size
to a small value (and more important - reading slowly in a small
chunks from this socket), but SSL itself socket reading 'too fast',
potentially growing its internal buffer to a huge size. This also
results in auto-tests failing - whenever we're trying to limit read
rate in some test.

2. Update qsslsocket auto-test.

Task-number: QTBUG-43388
Task-number: QTBUG-55170
Change-Id: Iedece26df0ac5b3b7cad62cc8c98aedc28e7ca5b
Reviewed-by: Richard J. Moore <rich@kde.org>

src/network/ssl/qsslsocket_mac.cpp

@@ -668,7 +668,7 @@ void QSslSocketBackendPrivate::transmit()
 
     if (connectionEncrypted) {
         QVarLengthArray<char, 4096> data;
-        while (context) {
+        while (context && (!readBufferMaxSize || buffer.size() < readBufferMaxSize)) {
             size_t readBytes = 0;
             data.resize(4096);
             const OSStatus err = SSLRead(context, data.data(), data.size(), &readBytes);

tests/auto/network/ssl/qsslsocket/tst_qsslsocket.cpp

@@ -229,6 +229,7 @@ private slots:
     void ecdhServer();
     void verifyClientCertificate_data();
     void verifyClientCertificate();
+    void readBufferMaxSize();
     void setEmptyDefaultConfiguration(); // this test should be last
 
 #ifndef QT_NO_OPENSSL
@@ -3043,6 +3044,68 @@ void tst_QSslSocket::verifyClientCertificate()
     QCOMPARE(client->isEncrypted(), works);
 }
 
+void tst_QSslSocket::readBufferMaxSize()
+{
+#ifdef QT_SECURETRANSPORT
+    // QTBUG-55170:
+    // SecureTransport back-end was ignoring read-buffer
+    // size limit, resulting (potentially) in a constantly
+    // growing internal buffer.
+    // The test's logic is: we set a small read buffer size on a client
+    // socket (to some ridiculously small value), server sends us
+    // a bunch of bytes , we ignore readReady signal so
+    // that socket's internal buffer size stays
+    // >= readBufferMaxSize, we wait for a quite long time
+    // (which previously would be enough to read completely)
+    // and we check socket's bytesAvaiable to be less than sent.
+    QFETCH_GLOBAL(bool, setProxy);
+    if (setProxy)
+        return;
+
+    SslServer server;
+    QVERIFY(server.listen());
+
+    QEventLoop loop;
+
+    QSslSocketPtr client(new QSslSocket);
+    socket = client.data();
+    connect(socket, SIGNAL(error(QAbstractSocket::SocketError)), &loop, SLOT(quit()));
+    connect(socket, SIGNAL(sslErrors(QList<QSslError>)), this, SLOT(ignoreErrorSlot()));
+    connect(socket, SIGNAL(encrypted()), &loop, SLOT(quit()));
+
+    client->connectToHostEncrypted(QHostAddress(QHostAddress::LocalHost).toString(),
+                                   server.serverPort());
+
+    // Wait for 'encrypted' first:
+    QTimer::singleShot(5000, &loop, SLOT(quit()));
+    loop.exec();
+
+    QCOMPARE(client->state(), QAbstractSocket::ConnectedState);
+    QCOMPARE(client->mode(), QSslSocket::SslClientMode);
+
+    client->setReadBufferSize(10);
+    const QByteArray message(int(0xffff), 'a');
+    server.socket->write(message);
+
+    QTimer::singleShot(5000, &loop, SLOT(quit()));
+    loop.exec();
+
+    int readSoFar = client->bytesAvailable();
+    QVERIFY(readSoFar > 0 && readSoFar < message.size());
+    // Now, let's check that we still can read the rest of it:
+    QCOMPARE(client->readAll().size(), readSoFar);
+
+    client->setReadBufferSize(0);
+
+    QTimer::singleShot(1500, &loop, SLOT(quit()));
+    loop.exec();
+
+    QCOMPARE(client->bytesAvailable() + readSoFar, message.size());
+#else
+    // Not needed, QSslSocket works correctly with other back-ends.
+#endif
+}
+
 void tst_QSslSocket::setEmptyDefaultConfiguration() // this test should be last, as it has some side effects
 {
     // used to produce a crash in QSslConfigurationPrivate::deepCopyDefaultConfiguration, QTBUG-13265