Looks like the offloading of ixfr processing introduced a race.

Job #3735579 failed for 92fccb12:

WARNING: ThreadSanitizer: data race 
  Write of size 4 at 0x000000000001 by thread T0001:
    #0 axfr_apply lib/dns/xfrin.c:351:14 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #1 isc__work_cb lib/isc/work.c:30:2 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #2 uv__queue_work /usr/src/libuv-v1.46.0/src/threadpool.c:351:3 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)

  Previous write of size 4 at 0x000000000001 by thread T0002:
    #0 xfrin_recv_done lib/dns/xfrin.c:1976:15 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #1 tcp_recv_processall lib/dns/dispatch.c:748:3 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #2 tcp_recv lib/dns/dispatch.c:891:2 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #3 streamdns_on_complete_dnsmessage lib/isc/netmgr/streamdns.c (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #4 streamdns_on_dnsmessage_data_cb lib/isc/netmgr/streamdns.c:206:11
    #5 isc__dnsstream_assembler_callcb lib/isc/./include/isc/dnsstream.h:306:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #6 isc__dnsstream_assembler_handle_message lib/isc/./include/isc/dnsstream.h:353:10
    #7 isc__dnsstream_assembler_processing lib/isc/./include/isc/dnsstream.h:370:9 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #8 isc_dnsstream_assembler_incoming lib/isc/./include/isc/dnsstream.h:558:2
    #9 streamdns_handle_incoming_data lib/isc/netmgr/streamdns.c:242:2
    #10 streamdns_read_cb lib/isc/netmgr/streamdns.c:819:2 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #11 isc__job_cb lib/isc/job.c:78:3 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #12 uv__run_idle /usr/src/libuv-v1.46.0/src/unix/loop-watcher.c:68:1 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)
    #13 thread_body lib/isc/thread.c:85:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #14 thread_run lib/isc/thread.c:100:14

  Location is heap block of size 2440 at 0x000000000015 allocated by thread T0002:
    #0 malloc <null> (BuildId: c55e4ed47e9b3b4863e398d1728e3206a97e3b27)
    #1 mallocx lib/isc/./jemalloc_shim.h:67:14 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #2 mem_get lib/isc/mem.c:305:8
    #3 isc__mem_get lib/isc/mem.c:744:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #4 xfrin_create lib/dns/xfrin.c:1127:8 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #5 dns_xfrin_create lib/dns/xfrin.c:876:2
    #6 got_transfer_quota lib/dns/zone.c:17873:11 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #7 isc__async_cb lib/isc/async.c:111:3 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #8 uv__async_io /usr/src/libuv-v1.46.0/src/unix/async.c:176:5 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)
    #9 thread_body lib/isc/thread.c:85:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #10 thread_run lib/isc/thread.c:100:14

  Thread T0001 (running) created by thread T0002 at:
    #0 pthread_create <null> (BuildId: c55e4ed47e9b3b4863e398d1728e3206a97e3b27)
    #1 uv_thread_create_ex /usr/src/libuv-v1.46.0/src/unix/thread.c:172:9 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)
    #2 uv_once /usr/src/libuv-v1.46.0/src/unix/thread.c:433:7 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)
    #3 axfr_commit lib/dns/xfrin.c:390:2 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #4 xfr_rr lib/dns/xfrin.c:834:4 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #5 xfrin_recv_done lib/dns/xfrin.c:1890:5
    #6 tcp_recv_processall lib/dns/dispatch.c:748:3 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #7 tcp_recv lib/dns/dispatch.c:891:2 (BuildId: 918e08c40b4885dd265949877ff20bbfbd93fd9e)
    #8 streamdns_on_complete_dnsmessage lib/isc/netmgr/streamdns.c (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #9 streamdns_on_dnsmessage_data_cb lib/isc/netmgr/streamdns.c:206:11
    #10 isc__dnsstream_assembler_callcb lib/isc/./include/isc/dnsstream.h:306:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #11 isc__dnsstream_assembler_handle_message lib/isc/./include/isc/dnsstream.h:353:10
    #12 isc__dnsstream_assembler_processing lib/isc/./include/isc/dnsstream.h:370:9 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #13 isc__dnsstream_assembler_incoming_direct lib/isc/./include/isc/dnsstream.h:396:2
    #14 isc_dnsstream_assembler_incoming lib/isc/./include/isc/dnsstream.h:508:4 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #15 streamdns_handle_incoming_data lib/isc/netmgr/streamdns.c:242:2
    #16 streamdns_readcb lib/isc/netmgr/streamdns.c:526:2 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #17 isc___nm_readcb lib/isc/netmgr/netmgr.c:1764:2 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #18 isc__nm_readcb lib/isc/netmgr/netmgr.c:1779:3
    #19 isc__nm_tcp_read_cb lib/isc/netmgr/tcp.c:770:2 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #20 uv__read /usr/src/libuv-v1.46.0/src/unix/stream.c:1143:7 (BuildId: 1b5307d484d8cb99602a0a0749ea2f3df2d7d355)
    #21 thread_body lib/isc/thread.c:85:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #22 thread_run lib/isc/thread.c:100:14

  Thread T0002 'isc-loop-0001' (running) created by main thread at:
    #0 pthread_create <null> (BuildId: c55e4ed47e9b3b4863e398d1728e3206a97e3b27)
    #1 isc_thread_create lib/isc/thread.c:139:8 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #2 isc_loopmgr_run lib/isc/loop.c:448:3 (BuildId: e5f4080b4051faff5e28292316b2d614f59282be)
    #3 main bin/named/main.c:1580:2 (BuildId: c55e4ed47e9b3b4863e398d1728e3206a97e3b27)

SUMMARY: ThreadSanitizer: data race lib/dns/xfrin.c:351:14 in axfr_apply