Created attachment 250648 [details] [review]
Patch - fdsink: Make changing fd property on fdsink thread-safe

Changing the fd property of fdsink is not threadsafe.  This means that you cannot change the fd from any thread apart from the stream thread if the stream is currently running.

I've attached a patch that adds the guarantee that when
`g_object_set("fd", x, NULL)` returns the old fd will no longer be in use and may be closed, etc.

Because a lock is being held across a call to `write` this patch may cause
setting the fd to block for longer than previously unless the fd was
opened with O_NONBLOCK.  This applies to all fds on windows as I guess it
doesn't have an implementation of g_poll.  This doesn't seem so bad as due
to the previous lack of synchronisation aroud `fd` these programs would
have been relying on undefined behaviour anyway but it doesn't make me
entirely happy.

I'll see if I can also come up with a test that exacerbates this issue.