bug report: dovecot-imapd core dump

[Tamas Papp]

Typical log message:

Oct  7 11:39:36 hostname dovecot: imap(username): Fatal: master: service(imap): child 14864 killed with signal 11 (core dumped)

What triggers the problem:

User moving multiple messages to a subfolder via IMAP (from another
account, on another server, client: Thunderbird). Moving single messages
either does not trigger the problem, or with much lower probability
(could not reproduce).

Version:
$ dpkg -l 'dovecot*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                  Version                 Architecture            Description
+++-=====================================-=======================-=======================-================================================================================
un  dovecot-common                        <none>                  <none>                  (no description available)
ii  dovecot-core                          1:2.2.9-1ubuntu5        amd64                   secure POP3/IMAP server - core files
un  dovecot-gssapi                        <none>                  <none>                  (no description available)
ii  dovecot-imapd                         1:2.2.9-1ubuntu5        amd64                   secure POP3/IMAP server - IMAP daemon
un  dovecot-ldap                          <none>                  <none>                  (no description available)
un  dovecot-lmtpd                         <none>                  <none>                  (no description available)
un  dovecot-managesieved                  <none>                  <none>                  (no description available)
un  dovecot-mysql                         <none>                  <none>                  (no description available)
un  dovecot-pgsql                         <none>                  <none>                  (no description available)
un  dovecot-pop3d                         <none>                  <none>                  (no description available)
un  dovecot-sieve                         <none>                  <none>                  (no description available)
un  dovecot-solr                          <none>                  <none>                  (no description available)
un  dovecot-sqlite                        <none>                  <none>                  (no description available)
tamas at szob:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 15.04
Release:        15.04
Codename:       vivid

Also tried 2.2.18 from Ubuntu wiley, same results.

Backtrace (I am not sure this is useful, extraced core from Ubuntu's Apport):

(gdb) bt full
#0  0x00007f404e58e3b6 in mail_index_keywords_unref () from /usr/lib/dovecot/libdovecot-storage.so.0
No symbol table info available.
#1  0x00007f404ea254ad in _start ()
No symbol table info available.

Any help would be appreciated, I am happy to provide more info if necessary.

Tamas