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udpsrc: Disable allocated port reuse for unicast

Jonas Danielsson requested to merge jonas.danielsson/gstreamer:jonasdn/udpsrc-reuse-default-issue-3411 into main

The reuse property end up setting the SO_REUSEADDR socket option for the UDP socket. This setting have surprising effects.

On Linux systems the man page (socket(7)) states:

SO_REUSEADDR
    Indicates that the rules used in validating addresses supplied
    in a bind(2) call should allow reuse of local addresses. For
    AF_INET sockets this means that a socket may bind, except when
    there is an active listening socket bound to the address.

But since UDP does not listen this ends up meaning that when an ephemeral port is allocated (setting the port to 0) the kernel is free to reuse any other UDP port that has SO_REUSEADDR set.

Tests checking the likelyhood of port conflict when using multiple udpsrc shows port conflicts starting to occur after ~100-300 udpsrc with port allocation enabled. See issue #3411 (closed) for more details.

Changing the default value of a property is not a small thing we risk breaking application that rely on the current default value. But since the effects of having reuse default TRUE on can also have damaging and hard-to-debug consequences, it might be worth to consider.

Having SO_REUSEADDR enabled for multicast, might have some use cases but for unicast, with dynamic port allocation, it does not make sense.

When not using an multicast address we will disable port reuse if the port property is set to 0 (=allocate) and warn the user that we did so.

Closes #3411 (closed)

Edited by Jonas Danielsson

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