This GDB was configured as "i386-unknown-openbsd2.9"...
Core was generated by `imap'.
Program terminated with signal 6, Abort trap.
Reading symbols from /usr/libexec/ld.so...done.
Reading symbols from /usr/lib/libc.so.26.2...done.
#0  0x400b16ff in _thread_sys_kill ()
(gdb) bt
#0  0x400b16ff in _thread_sys_kill ()
#1  0x400dd680 in abort ()
#2  0x410c7 in i_internal_panic_handler (
    fmt=0x3faac "file %s: line %d (%s): assertion failed: (%s)", 
    args=0xdfbfd768 "\237ú\003") at failures.c:376
#3  0x40c63 in i_panic (
    format=0x3faac "file %s: line %d (%s): assertion failed: (%s)")
    at failures.c:174
#4  0x401f6 in buffer_set_used_size (_buf=0x5a240, used_size=124)
    at buffer.c:303
#5  0x2870e in mail_index_read_map (index=0x68300, map=0x68380, 
    retry_r=0xdfbfd878) at mail-index.c:447
#6  0x2884e in mail_index_read_map_with_retry (index=0x68300, map=0x68380)
    at mail-index.c:503
#7  0x28a32 in mail_index_map (index=0x68300, force=0) at mail-index.c:599
#8  0x28d6f in mail_index_try_open (index=0x68300, lock_id_r=0xdfbfd910)
    at mail-index.c:749
#9  0x29203 in mail_index_open_files (index=0x68300, flags=13)
    at mail-index.c:897
#10 0x293fa in mail_index_open (index=0x68300, flags=13) at mail-index.c:964
#11 0x23d6e in index_storage_mailbox_init (storage=0x68080, box=0x58704, 
    index=0x68300, name=0x14ff0 "INBOX", flags=0) at index-storage.c:318
#12 0x151d4 in mbox_open (storage=0x68080, name=0x14ff0 "INBOX", flags=0)
    at mbox-storage.c:407
#13 0x153dd in mbox_mailbox_open (_storage=0x68080, name=0xdc39 "INBOX", 
    flags=0) at mbox-storage.c:469
#14 0x34db4 in mailbox_open (storage=0x68080, name=0xdc39 "INBOX", flags=0)
    at mail-storage.c:291
#15 0x4528 in _cmd_select_full (client=0x68100, readonly=0) at cmd-select.c:31
#16 0x46af in cmd_select (client=0x68100) at cmd-select.c:98
#17 0x5f5e in client_handle_input (client=0x68100) at client.c:323
#18 0x6031 in _client_input (context=0x68100) at client.c:367
#19 0x454fc in io_loop_handler_run (ioloop=0x65040) at ioloop-poll.c:184
#20 0x44d75 in io_loop_run (ioloop=0x65040) at ioloop.c:218
#21 0xd698 in main (argc=1, argv=0xdfbfdcc0, envp=0xdfbfdcc8) at main.c:218
(gdb) q