Subj : src/xpdev/multisock.c
To   : Git commit to main/sbbs/master
From : Rob Swindell (on Debian Linux)
Date : Fri Aug 07 2026 15:55:33

https://gitlab.synchro.net/main/sbbs/-/commit/0db85afb4cff6c0b9efbd74c
Modified Files:
	src/xpdev/multisock.c
Log Message:
Set close-on-exec on listen sockets on POSIX (issue #1174)

A fork()ed child inherits every open descriptor, and external() closes
nothing but stdio and its pipes before execvp(), so every external program
inherits each server's listening sockets. An external that outlives a server
recycle then keeps those ports bound, and the rebind fails:

    mail !ERROR 98 binding SMTP Transfer Agent socket to port 25: Address
    already in use

This is not merely a failed rebind, which would be self-announcing. The
parent's own listen sockets are still open, so the kernel keeps completing
handshakes into a backlog that no longer has an accept() behind it: clients
connect and then wait forever for a protocol banner. It presents as a hung
server.

Mark the sockets close-on-exec where they are created, mirroring what the
Windows half of xpms_add() has done since 6816ce611 (carries-19-baby,
2026-06-24). Doing it at creation, rather than closing descriptors in the
child, is what covers
externals already running when a recycle happens: their copies were made
long before, and nothing done at recycle time can reach them.

fcntl() rather than SOCK_CLOEXEC: macOS has no SOCK_CLOEXEC, and passing it
to socket() on a kernel that predates support fails the call outright and
would take the listener with it. That leaves a window between socket() and
fcntl() in which a concurrent exec still inherits the descriptor, which is
worth accepting for sockets created during server start-up and recycle.

Accepted client sockets are left alone. They do not pin a listening port,
and on POSIX the passthru listener a socket door talks over is created by
open_socket() in main.cpp, not here, so it is unaffected either way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
--- SBBSecho 3.37-Linux
 * Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)

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