adb.c:2693: INSIST(((void *)((find)->plink.prev) != (void *)(-1))) failed, back trace

The following crash was triggered on main (at commit 746b3ac8) during a respdiff test which ran over the weekend:

https://gitlab.isc.org/isc-projects/bind9/-/jobs/2451222

19-Apr-2022 00:11:54.354 REFUSED unexpected RCODE resolving 'essersmith.com/A/IN': 208.82.119.221#53
19-Apr-2022 00:11:54.386 REFUSED unexpected RCODE resolving 'essersmith.com/A/IN': 45.82.72.253#53
19-Apr-2022 00:11:54.386 query client=0x7f9123af5400 thread=0x7f91311ff700(essersmith.com/A): query_gotanswer: unexpected error: failure
19-Apr-2022 00:11:54.502 loop detected resolving 'ns02.3g.claro.net.br/A'
19-Apr-2022 00:11:54.502 loop detected resolving 'ns03.3g.claro.net.br/A'
19-Apr-2022 00:11:54.502 loop detected resolving 'ns04.3g.claro.net.br/A'
19-Apr-2022 00:11:54.506 adb.c:2693: INSIST(((void *)((find)->plink.prev) != (void *)(-1))) failed, back trace

(The log file is bind-qa/bind9/respdiff/rspworkdir/testbind/named.run. There is also a core dump in that directory.)

The above code location is in dns_adb_cancelfind():

2691         UNLOCK(&find->lock);                                                    
2692         LOCK(&nbucket->lock);                                                   
2693     >>> ISC_LIST_UNLINK(adbname->finds, find, plink);                           
2694         UNLOCK(&nbucket->lock);                                                 
2695         LOCK(&find->lock);                                                      

This is not a shutdown crash, it happened while respdiff was still running.

Marking confidential for the time being as a crash is involved.