dovecot-lmtp crashing when setting lmtp_proxy=yes

[Camilo Sperberg]

Hi all:

We are currently in the process of setting up dovecot proxy so that we can
deploy multiple machines in order to keep growing.

We are trying now to create an entry point, and from there send the traffic
to either the same machine or another node.
For this, we are using a MySQL database with information about which server
hosts which mailboxes, which turns out to be working great for sending out
mail and general authentification. This was really simple to set up, kudos
for that!

However, whenever I want to *receive* email, I just have to enable the
lmtp_proxy setting which makes the lmtp process segfault.

This happens everytime as I can reliable reproduce it + get a stack trace
of it. The process will crash on every incoming email.

I have attached:
- Complete output of dovecot-sysreport
- A core dump of the crashed process

Main information:
Using dovecot v2.3.14 (cee3cbc0d) on Ubuntu 18.04 LTS: Linux 5.4.0-1046-gcp
x86_64 Ubuntu 18.04.5 LTS. Our MTA is postfix.

Logs around the crashed process:
Jul 15 14:53:21 server postfix/postscreen[19402]: CONNECT from
[XX.XX.XX.XX]:36411 to [10.164.0.18]:25
Jul 15 14:53:22 server postfix/dnsblog[19406]: addr XX.XX.XX.XX listed by
domain dnsbl.sorbs.net as 127.0.0.6
Jul 15 14:53:27 server postfix/postscreen[19402]: PASS NEW
[XX.XX.XX.XX]:36411
Jul 15 14:53:27 server postfix/smtpd[19408]: connect from mail-XX.google.com
[XX.XX.XX.XX]
Jul 15 14:53:28 server postfix/smtpd[19408]: B765F401DB: client=
mail-XX.google.com[XX.XX.XX.XX]
Jul 15 14:53:28 server postfix/cleanup[19411]: B765F401DB: message-id=<
XX+5j3RpXXXXXOK7GHdiDSGSPXXXX at mail.gmail.com>
Jul 15 14:53:29 server postfix/qmgr[1414]: B765F401DB: from=<
unreal4u at gmail.com>, size=2815, nrcpt=1 (queue active)
Jul 15 14:53:29 server dovecot: lmtp(19413): Connect from local
Jul 15 14:53:29 server kernel: [1738624.857507] lmtp[19413]: segfault at 0
ip 00007fd486cd0a37 sp 00007ffc12a6c118 error 4 in
libdovecot.so.0.0.0[7fd486ba1000+1aa000]
Jul 15 14:53:29 server kernel: [1738624.857514] Code: eb de 90 48 85 d2 75
1e eb 33 66 0f 1f 84 00 00 00 00 00 84 c0 74 18 48 83 c7 01 48 83 c6 01 88
47 ff 48 83 ea 01 48 83 fa 01 <0f> b6 06 77 e4 c6 07 00 31 c0 80 3e 00 0f
95 c0 f7 d8 c3 b8 ff ff
Jul 15 14:53:29 server dovecot: lmtp(19413): Fatal: master: service(lmtp):
child 19413 killed with signal 11 (core dumped)
Jul 15 14:53:29 server postfix/lmtp[19412]: B765F401DB: to=<YYY at YYYY.YY>,
relay=ZZZ.ZZZ.ZZ[private/dovecot-lmtp], delay=0.88,
delays=0.84/0.01/0.01/0.03, dsn=4.4.2, status=deferred (lost connection
with ZZZ.ZZZ.ZZ[private/dovecot-lmtp] while sending RCPT TO)
Jul 15 14:53:29 server postfix/smtpd[19408]: disconnect from
mail-XX.google.com[XX.XX.XX.XX] ehlo=2 starttls=1 mail=1 rcpt=1 data=1
quit=1 commands=7

Am I missing something in the conf? Did I encounter a bug? If you need more
information, please let me know.

Greetings,
Camilo Sperberg
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