After upgrading libvmod-netmapper on cp4028 to tackle T266567, I've reloaded the varnish-frontend instance to ensure it wouldn't crash. It did crash due to the following liburcu6 assertion:
Oct 28 10:26:39 cp4028 varnishd: Child (49027) said varnishd: urcu-qsbr.c:470: rcu_register_thread_qsbr: Assertion `!URCU_TLS(rcu_reader).registered' failed. Oct 28 10:26:40 cp4028 varnishd: Child (49027) died signal=6 Oct 28 10:26:40 cp4028 varnishd: Child (49027) Panic at: Wed, 28 Oct 2020 10:26:40 GMT Wrong turn at cache/cache_main.c:284: Signal 6 (Aborted) received at 0x700000bf83 si_code -6 version = varnish-6.0.6 revision 29a1a8243dbef3d973aec28dc90403188c1dc8e7, vrt api = 7.1
Luckily I tried the reload on one host before upgrading all other libvmod-netmappers, or multiple instances would have crashed upon the first pool/depool. Canceling the fleet-wide libvmod-netmapper upgrade.