bug#71223: 30.0.50; stack overflow after very fast opening and closing of frames

[Stefan Monnier]

OK, I think I understand the code well enough now that I'm confident my
current patch is an overall improvement, and pushed it to `master`.

I think the problem is not completely specific to server.el's process
filter, so it might be be worthwhile to add some more generic support
for this kind of situation (e.g. ways to force some process filters to
be run sequentially rather than in a nested fashion, as well as ways for
process filters to know if there's already more work pending so as to
better handle bursts of concurrent requests).  I also still have some
FIXMEs in the code (one for `server--message-sit-for` which I think
deserves to be better implemented and moved to core, and another for the
question of the 5s vs 1s).

But w.r.t this bug report, I think we're done, so I'm closing it.


        Stefan