[Bug 253277] x11/xtrans: Don't unlink existing UNIX sockets => allows multiple X sessions from sddm

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253277

--- Comment #2 from Olivier Certner <olivier.freebsd at free.fr> ---
Hi,

As for the high-level problem I'm fixing, I wrote this description:

> Typically, sddm exactly triggers the problem. So when you try to open
> another session through it, the first session's sockets are crushed
> and it becomes stale.

What do you find unclear in it exactly?

As for the low-level description, let me recapitulate with more context. As you
most probably know, there are two ways that Xorg selects its display at
startup:
- Either you supply an explicit display on the command-line (e.g., ":1").
- Or you do not, and either "-displayfd" has been specified, in which case Xorg
will try to assign some automatically, or it has not, in which case :0 is used
by default.
("-displayfd" has other effects, but this is a separate discussion, relevant to
the other bug #253278.)

The problem is with the automatic assignment of display. It works by trying to
grab the first display available (from :0 when using UNIX sockets), which it
does by simply unlinking an existing socket file and then binding a new socket
to the same file. When the server is run as root, this operation always
succeeds, and any existing socket for :0 ("/tmp/.X11-UNIX/X0") that would exist
is removed. This leaves any preexisting X session on :0 stale (e.g., launching
new apps make them go to the new session).

And SDDM triggers that: It passes "-displayfd", and no display directive,
relying on the server to do display assignmnent, which it doesn't do correctly.

Now, I agree that this problem is probably not FreeBSD specific (it affects
platforms/distributions running Xorg as root). If you prefer, I'll report it
upstream and we'll see how it goes.

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