Strange linking problem.

[William Johnston]

I have this element I'm working on (misbfixcoordinates) which has four 
pads (vidoesink, misbsink, videosrc, misbsrc). And when I try to link it 
up it has a very odd behavior. Whichever two pads I link first succeed 
and whichever two pads I link second fail. It doesn't matter what the 
order is; the first two always link and the last two always fail to link.

So focusing on misbfixcoordinates:misbsink, if it's in the first two 
link requests it links. The important line in the debug messages is:

0:00:02.648363881  8774 0x55dc16de5c00 DEBUG GST_CAPS 
gstutils.c:3064:gst_pad_query_caps:<misbfixcoordinates1:misbsink> query 
returned meta/klv

And if it in the last two link requests it fails. And again the 
important debug message is:

0:00:02.681719335  9214 0x556613e16c00 DEBUG GST_CAPS 
gstutils.c:3064:gst_pad_query_caps:<misbfixcoordinates1:misbsink> query 
returned EMPTY

So now I know why it fails to link. If the pad link request is after the 
first two link requests then the pad forgets it capabilities. Why is that?