This leads to spurious negotiation failures because the configured method can
change over time and caps queries (and thus transform_caps) are happening
independently from configuring caps. Instead prefer the transformed caps of the
current method, but always also return the transformed caps for all other
methods. Previously all other methods would've only been used if the current
method returned empty caps. If a different method is needed later when
configuring the caps, it will be and was selected regardless.
Later during caps fixation, prefer the caps of the current method too for the
fixated caps if possible in any way.
This should preserve the desired behaviour of preferring the current method if
possible but to change to a different method if nothing else is possible, while
also returning consistent (and not too narrow) caps every time.
The way how the current method was checked was also racy as the current method
might change at any moment during caps query handling, and apart from
inconsistent results also a NULL pointer dereference was possible here. Use the
GST_OBJECT_LOCK to protect access to the current method like in other places.
This part of the code was introduced in f349cdccf5
and tried to be fixed multiple times over the years without addressing the root
cause of caps queries and caps configuration happening independently from each
other, e.g. in !2687 and !2699.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8431>
fc026fcfbe