I was experimenting with using Vivaldi Mail, and dovecot-imapd started
crashing repeatedly when Vivaldi was trying to sync and index my (huge)
mailbox.

Messages logged by Dovecot:

2026-08-14 10:26:43 imap(ian)<466672><Dy4ka/5Y9r1SRXa8>: Panic: file istream-header-filter.c: line 665 (i_stream_header_filter_snapshot_free): assertion failed: (snapshot->mstream->snapshot_pending)
2026-08-14 10:26:43 imap(ian)<466672><Dy4ka/5Y9r1SRXa8>: Error: Raw backtrace: #1 [unw_get_proc_name() failed: -10] -> #2 [unw_get_proc_name() failed: -10] -> #3 i_panic[0x7fe47ca655e6] -> #4 [unw_get_proc_name() failed: -10] -> #5 i_stream_snapshot_free[0x7fe47cb45c00] -> #6 i_stream_unref[0x7fe47cb45c70] -> #7 [unw_get_proc_name() failed: -10] -> #8 [unw_get_proc_name() failed: -10] -> #9 index_mail_get_special[0x7fe47cd494c0] -> #10 mail_get_special[0x7fe47ccc2700] -> #11 [unw_get_proc_name() failed: -10] -> #12 [unw_get_proc_name() failed: -10] -> #13 imap_fetch_more[0x561b58c5cf80] -> #14 cmd_fetch[0x561b58c50e40] -> #15 command_exec[0x561b58c5a080] -> #16 [unw_get_proc_name() failed: -10] -> #17 [unw_get_proc_name() failed: -10] -> #18 [unw_get_proc_name() failed: -10] -> #19 client_handle_input[0x561b58c60990] -> #20 client_input[0x561b58c60ed0] -> #21 io_loop_call_io[0x7fe47cb51750] -> #22 io_loop_handler_run_internal[0x7fe47cb534f0] -> #23 io_loop_handler_run[0x7fe47cb53680] -> #24 io_loop_run[0x7fe47cb53860] -> #25 master_service_run[0x7fe47caa6330] -> #26 main[0x561b58c4b880] -> #27 [unw_get_proc_name() failed: -10] -> #28 __libc_start_main[0x7fe47c835ce0] -> #29 _start[0x561b58c4bf30]

A friend kindly noted that upstream commit
https://github.com/dovecot/core/commit/3ff6022baa757583faf579251fc40 may
fix this problem.  I applied it to 2.4.1+dfsg1-6+deb13u6 and this did
indeed fix the problem.

Control: tags -1 + upstream fixed-upstream patch
Control: found -1 1:2.4.1+dfsg1-6
Control: fixed -1 1:2.4.2+dfsg1-1

Thank you; I've confirmed that the upstream patch applies cleanly, and
this is something we should be able to include in an upcoming point
release.

noah