Fix multicast join/leave when binding to QHostAddress::Any.

On OS X and Windows, this was not working, because the socket was being bound
in v6 mode (due to ::Any being for dual mode), but the address passed was a v4
address, meaning it took the wrong codepath. Linux, strangely, apparently works
anyway.

This is fixable in OS X (by using the v6 join path when bound in v6/dual mode),
but the same fix doesn't work on Windows, failing with WSAEADDRNOTAVAIL.

Don't allow this behaviour, and provide a sane error message telling the user
what to do instead.

Done-with: Shane Kearns
Task-number: QTBUG-25047
Change-Id: Iaf5bbee82e13ac92e11b60c558f5af9ce26f474b
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>

2 files changed, 17 insertions, 0 deletions

diff --git a/src/network/socket/qnativesocketengine.cpp b/src/network/socket/qnativesocketengine.cpp
index a34b19f7ef..8fac3613c0 100644
--- a/src/network/socket/qnativesocketengine.cpp
+++ b/src/network/socket/qnativesocketengine.cpp
@@ -637,6 +637,19 @@ bool QNativeSocketEngine::joinMulticastGroup(const QHostAddress &groupAddress,
     Q_CHECK_VALID_SOCKETLAYER(QNativeSocketEngine::joinMulticastGroup(), false);
     Q_CHECK_STATE(QNativeSocketEngine::joinMulticastGroup(), QAbstractSocket::BoundState, false);
     Q_CHECK_TYPE(QNativeSocketEngine::joinMulticastGroup(), QAbstractSocket::UdpSocket, false);
+
+    // if the user binds a socket to an IPv6 address (or QHostAddress::Any) and
+    // then attempts to join an IPv4 multicast group, this won't work on
+    // Windows. In order to make this cross-platform, we warn & fail on all
+    // platforms.
+    if (groupAddress.protocol() == QAbstractSocket::IPv4Protocol &&
+        (d->socketProtocol == QAbstractSocket::IPv6Protocol ||
+         d->socketProtocol == QAbstractSocket::AnyIPProtocol)) {
+        qWarning("QAbstractSocket: cannot bind to QHostAddress::Any (or an IPv6 address) and join an IPv4 multicast group");
+        qWarning("QAbstractSocket: bind to QHostAddress::AnyIPv4 instead if you want to do this");
+        return false;
+    }
+
     return d->nativeJoinMulticastGroup(groupAddress, iface);
 }
 
diff --git a/src/network/socket/qudpsocket.cpp b/src/network/socket/qudpsocket.cpp
index ec751c289e..23c1956ec5 100644
--- a/src/network/socket/qudpsocket.cpp
+++ b/src/network/socket/qudpsocket.cpp
@@ -178,6 +178,10 @@ QUdpSocket::~QUdpSocket()
     interface chosen by the operating system. The socket must be in BoundState,
     otherwise an error occurs.
 
+    Note that if you are attempting to join an IPv4 group, your socket must not
+    be bound using IPv6 (or in dual mode, using QHostAddress::Any). You must use
+    QHostAddress::AnyIPv4 instead.
+
     This function returns true if successful; otherwise it returns false
     and sets the socket error accordingly.