UID THREAD REFS US-ASCII ALL slow / stalls forever on one folder.

[Jesper Schmitz Mouridsen]

Hi.

I recently upgraded Cyrus to 3.2.2 from 3.0.13.

Now threading "UID THREAD REFS US-ASCII ALL" on one specific folder with 
 >50K mails,

makes imapd process use 100 CPU% without any progress. truss[1] or dtrace

shows no output. The process seems totally stuck.

I installed in a FreeBSD 12.1 jail. Any hints beside the online docs of 
how to use gdb to

see what  is going one. I could not make imapd run under gdb even with 
the -D option

and debug_command setting or reading 
https://www.cyrusimap.org/imap/developer/developer-testing.html?highlight=debug

It is fast enough on other folders also with ~50k mails < 3 secs.

Any hints, highly appreciated :-)

[1] https://www.freebsd.org/cgi/man.cgi?truss

Regards

Jesper