I have checked my 6 dovecot servers that where upgraded a week ago to
2.3.18 and none of them have segfaults.

Once is using squat, three using lucene, one using xapian, and two use
nothing. 4 are running on ubuntu 20.04, one on ubuntu 14.04, and one on alpine 3.15

Quoting Bryan Jacobs b@q3q.us:

Dear Dovecot Users,

Upgrading from Dovecot 2.3.14 to 2.3.18 on Arch gives me the
following dump immediately (100% of the time) on every LMTP handler
invocation. Of course I'm using Squat for FTS (yes, I understand
it's deprecated...).

plugin { fts = squat fts_squat = partial=4 full=4 fts_autoindex = yes sieve = file:/mnt/mail/%Ln/sieve;active=/mnt/mail/%Ln/.dovecot.sieve master_user = %u }

lmtp[592878]: segfault at 50 ip 00007f5355153eed sp 00007fff45b87ba0
error 4 in lib20_fts_plugin.so[7f5355153000+11000]

Code: 00 4c 8d 05 f9 17 01 00 48 8d 0d ae 19 01 00 31 c0 ba 53 01 00
00 48 8d 35 f2 17 01 00 48 8d 3d 49 01 01 00 ff 15 53 7f 03 00 <48>
8b 04 25 50 00 00 00 0f 0b be 2a 01 00 00 8d 3d cf 17 01 00

...

Feb 08 01:23:20 mail systemd-coredump[592880]: [🡕] Process 592878
(lmtp) of user 0 dumped core. Module linux-vdso.so.1 with build-id
0f9bd6468a3798a863048e6cd927830796809b46 Module libffi.so.8
with build-id f90d8b734f6de9b25faedb8cbfab7054dafc0a42
Module libp11-kit.so.0
with build-id cc372ea3c28c4d3dfc633b4d2e933c8584d2af16
Module
libcrypt.so.2 with build-id 5d45bdae438c16d52364b23d70bd5041ad070b0b Module librt.so.1
with build-id 75484da2d6f1515189eefa076e0a40328834cd16
Module
libnss_systemd.so.2 with build-id
56da60140e2f0e47044a141378608146f6fd9bb8 Module
libnss_files.so.2 with build-id
1a36dfc01d3a1010b2ee79766a24a8090a3266d5 Module
libdovecot-sieve.so.0 with build-id
a358da532b8ef09782b5a97a951df8ab5c8eb0db
Module lib90_sieve_plugin.so with
build-id eb52835c5c3d1034617e745995b950484faed909 Module
lib21_fts_squat_plugin.so with build-id
cc731fa9bf9cac2f890bbc1da598e958cb465d52 Module
libicudata.so.70 with build-id
e1dcc2a88cfaafed882d09c90c668af0eed4efed Module libgcc_s.so.1
with build-id 7f8508bb914546ada778809b64b99d234337d835 Module libm.so.6 with
build-id 2b8fd1f869ecab4e0b55e92f2f151897f6818acf Module libstdc++.so.6
with build-id 9b5eeeb149bf3c4efe787fb398b44f00507aec87 Module libicuuc.so.70
with build-id 2e245c2bf12f95fd8ab79b3a4be99524677cbd70 Module
libicui18n.so.70 with build-id
95af755fca990df26753a7d720b8e3ae24270e42 Module
lib20_fts_plugin.so with build-id
5b44deef3cd93f2cdac9a38ab3f5c4f1a2f48d40
Module lib10_quota_plugin.so with
build-id 5fa33175575dcf45f4aead56c701fe58a19f0df8 Module
libpthread.so.0 with build-id
07c8f95b4f3251d08550217ad8a1f31066229996
Module libcrypto.so.1.1 with build-id
4c926b672d97886b123e03a008387aecf0786de4
Module libssl.so.1.1 with
build-id 1024424ab33a3767da03f4fdb1fc1b02479f160f Module
libssl_iostream_openssl.so with build-id
ba2cf9015fbabf8459896bd584379f2f566aaa51
Module ld-linux-x86-64.so.2 with build-id
040cc3dd10461562f177df39e3be2f3704258c3c
Module libdl.so.2 with build-id
5abc547e7b0949f89f3c0e21ab0c8331a7440a8a Module libc.so.6 with
build-id 4b406737057708c0e4c642345a703c47a61c73dc Module
libdovecot.so.0 with build-id
3f68ab1f1e0bffceea1c130e6b46f799eea45522
Module
libdovecot-storage.so.0 with build-id
3bf1e7c355c0c6184165e209f67b3a6627355401 Module
libdovecot-lda.so.0 with build-id
54cc9d9face18809a95ccd5ee643330eed997d8b Module lmtp with
build-id 7cc9238bb78e35771572aa3e31bdecf1cda5d754 Stack trace of thread 592878: #0
0x00007f5355153eed n/a (lib20_fts_plugin.so + 0x9eed) #1
0x00007f535515db30 fts_mailbox_allocated (lib20_fts_plugin.so +
0x13b30) #2
0x00007f535595eb6d hook_mailbox_allocated (libdovecot-storage.so.0

• 0x60b6d) #3
  0x00007f53559598ec mailbox_alloc (libdovecot-storage.so.0 + 0x5b8ec) #4
  0x00007f5355959b5e mailbox_alloc_for_user (libdovecot-storage.so.0
• 0x5bb5e) #5
  0x00007f5355a6d790 mail_deliver_save_open (libdovecot-lda.so.0 +
  0x4790) #6
  0x00007f5355a6dbdc mail_deliver_save (libdovecot-lda.so.0 + 0x4bdc) #7
  0x00007f5355a6e39a mail_deliver (libdovecot-lda.so.0 + 0x539a) #8
  0x0000563d3a90bc2d lmtp_local_default_deliver (lmtp + 0x8c2d) #9
  0x0000563d3a90c39f lmtp_local_data (lmtp + 0x939f) #10
  0x0000563d3a90acc1 client_default_cmd_data (lmtp + 0x7cc1) #11
  0x0000563d3a90aa31 cmd_data_continue (lmtp + 0x7a31) #12
  0x00007f535579c3bf n/a (libdovecot.so.0 + 0x6d3bf) #13
  0x00007f535579cc46 n/a (libdovecot.so.0 + 0x6dc46) #14
  0x00007f53557a023e smtp_server_command_call_hooks (libdovecot.so.0
• 0x7123e) #15
  0x00007f53557a07f1 smtp_server_command_next_to_reply
  (libdovecot.so.0 + 0x717f1) #16
  0x00007f53557a5b5b n/a (libdovecot.so.0 + 0x76b5b) #17
  0x00007f535584469f io_loop_call_io (libdovecot.so.0 + 0x11569f) #18
  0x00007f5355844787 io_loop_handler_run (libdovecot.so.0 + 0x115787) #19
  0x00007f5355844919 io_loop_run (libdovecot.so.0 + 0x115919) #20
  0x00007f53557b31d8 master_service_run (libdovecot.so.0 + 0x841d8) #21
  0x0000563d3a909606 main (lmtp + 0x6606) #22
  0x00007f535557bb25 __libc_start_main (libc.so.6 + 0x27b25)

Thanks, Bryan Jacobs