[Bug Report] 2.4.3 on macOS: anvil panics at anvil-connection.c:727 on every IMAP client handshake

   Hi Dovecot developers,
   I'm hitting a reproducible anvil panic on macOS when any IMAP client tries
   to connect. Every client handshake triggers an `i_unreached()` assertion
   in
   `anvil-connection.c:727`, causing the anvil child to be killed with
   SIGABRT.
   Dovecot keeps restarting anvil, but every subsequent connection attempt
   triggers the same panic, so the service effectively cannot accept logins.
   ## Environment
   - Dovecot: 2.4.3 (e278b1e09b), Pigeonhole 2.4.3 (0bf9ee48) -- from
   Homebrew
   - OS: macOS 15.6 (Sequoia, Darwin 24.6.0, arm64/Apple Silicon, APFS)
   - Mail client: Mozilla Thunderbird 149.0.2
   - Dovecot started via `sudo brew services start dovecot` (launchd)
   ## Reproducing
   1. Fresh macOS install of Dovecot 2.4.3 via Homebrew.
   2. Configure a minimal single-user IMAP setup with `passdb static` /
      `userdb static`, SSL on port 993 with a self-signed cert, Maildir
      storage (config at end of this mail).
   3. Start Dovecot as root via launchd (`sudo brew services start dovecot`).
   4. Connect any IMAP client to localhost:993 (I used Thunderbird 149 with a
      permanent cert exception for the self-signed certificate).
   5. The client hangs / reports "server closed the connection" / sees no
      folders.
   6. `dovecot.log` fills with repeated entries like the following:
   ```
   anvil: Warning: conn unix:anvil (uid=0): Handshake with duplicate
   service=imap-login pid=-1 - replacing the old connection
   anvil: Panic: file anvil-connection.c: line 727: unreached
   anvil: Error: Raw backtrace: 0 libdovecot.0.dylib backtrace_append + 72 ->
   1 libdovecot.0.dylib backtrace_get + 44 -> 2 libdovecot.0.dylib
   default_fatal_finish + 72 -> 3 libdovecot.0.dylib default_error_handler ->
   4 libdovecot.0.dylib i_internal_error_handler -> 5 libdovecot.0.dylib
   i_fatal -> 6 libdovecot.0.dylib default_write -> 7 anvil
   anvil_connection_input_line -> 8 libdovecot.0.dylib
   connection_input_read_stream + 196 -> 9 libdovecot.0.dylib
   connection_input_default + 72 -> 10 libdovecot.0.dylib io_loop_call_io +
   116 -> 11 libdovecot.0.dylib io_loop_handler_run_internal + 284 -> 12
   libdovecot.0.dylib io_loop_handler_run + 220 -> 13 libdovecot.0.dylib
   io_loop_run + 88 -> 14 libdovecot.0.dylib master_service_run + 292 -> 15
   anvil main + 368 -> 16 dyld start + 6076
   anvil: Fatal: master: service(anvil): child NNNN killed with signal 6
   (core dumps disabled)
   ```
   The key signature is the two-line sequence:
   ```
   Warning: conn unix:anvil (uid=0): Handshake with duplicate
   service=imap-login pid=-1 - replacing the old connection
   Panic: file anvil-connection.c: line 727: unreached
   ```
   Every IMAP client handshake produces this. The `pid=-1` in the handshake
   warning looks suspicious -- it appears anvil is either not receiving a
   valid pid from imap-login or is registering a second connection with the
   same service name before the first is cleaned up.
   ## Workarounds attempted (none succeeded)
   - User-level setup with default_login_user=andreas -> same panic
   - Root-level setup with default_login_user=andreas -> same panic
   - Root-level setup with default_login_user=_www (unprivileged macOS
     service account with uid=70) -> same panic
   - Adding `default_vsz_limit = unlimited` (required separately on macOS
     because setrlimit(RLIMIT_DATA) fails with EINVAL on Darwin) -> did not
     affect anvil
   - Plaintext IMAP on 143 (no TLS) vs. IMAPS on 993 with self-signed cert ->
     same panic in both
   - Restarting Dovecot via launchd (`sudo brew services stop/start dovecot`)
     -> the panic recurs on the next connection
   ## `doveconf -n` output
   ```
   # 2.4.3 (e278b1e09b): /opt/homebrew/etc/dovecot/dovecot.conf
   # Pigeonhole version 2.4.3 (0bf9ee48)
   # OS: Darwin 24.6.0 arm64  apfs
   # Hostname: macbook-air-von-andreas.local
   dovecot_config_version = 2.4.3
   auth_mechanisms = plain login
   default_internal_group = _www
   default_internal_user = _www
   default_login_user = _www
   default_vsz_limit = unlimited
   dovecot_storage_version = 2.4.3
   info_log_path = /opt/homebrew/var/log/dovecot/dovecot-info.log
   listen = 127.0.0.1
   log_path = /opt/homebrew/var/log/dovecot/dovecot.log
   mail_driver = maildir
   mail_gid = staff
   mail_home = /Users/andreas/mail/%{user}
   mail_path = /Users/andreas/mail/%{user}
   mail_uid = andreas
   protocols = imap
   ssl_server {
     cert_file = /opt/homebrew/etc/dovecot/ssl/dovecot.crt
     key_file = /opt/homebrew/etc/dovecot/ssl/dovecot.key
   }
   service imap-login {
     inet_listener imap {
       port = 143
     }
     inet_listener imaps {
       port = 993
       ssl = yes
     }
   }
   passdb static {
     password = # hidden
   }
   userdb static {
     fields {
       gid = staff
       home = /Users/andreas/mail/%{user}
       uid = andreas
     }
   }
   namespace inbox {
     mailbox_list_layout = fs
     inbox = yes
     prefix =
     separator = /
   }
   ```
   ## What I would expect
   The anvil connection from imap-login should register with a valid pid
   (not -1) or anvil should tolerate the re-registration gracefully instead
   of hitting an i_unreached() assertion.
   ## Note on `dovecot-sysreport`
   Running `dovecot-sysreport` as the normal user fails with permission
   errors on `/opt/homebrew/var/run/dovecot/*` sockets (log-errors, master).
   The `grep -P` usage inside the script also fails on BSD grep (macOS).
   Let me know if a sysreport archive would be useful -- I can re-run with
   sudo and adjust grep if needed.
   Happy to provide further reproduction details, additional log output, or
   test any fix suggestions.
   Thanks for Dovecot.
   Andreas Marth
   [email protected]
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