curl: Report only ready sockets

curl: Report only ready sockets

Instead of reporting all sockets to cURL, only report the one that has
caused curl_multi_do_locked() to be called.  This lets us get rid of the
QLIST_FOREACH_SAFE() list, which was actually wrong: SAFE foreaches are
only safe when the current element is removed in each iteration.  If it
possible for the list to be concurrently modified, we cannot guarantee
that only the current element will be removed.  Therefore, we must not
use QLIST_FOREACH_SAFE() here.

Fixes: ff5ca1664af85b24a4180d595ea6873fd3deac57
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190910124136.10565-6-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 9abaf9fc474c3dd53e8e119326abc774c977c331)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>