waitforx changes

[matt335672]

Two changes to waitforx:-

1. Changed alarm message from "Timed out waiting for RandR outputs" to "Timed out waiting for X display". The former was confusing, and the times where this message was triggered were nothing to do with RandR
2. Don't try to open a display of the form ":n" directory (n >= 0). If the X server hasn't yet opened its local socket, the XOpenDisplay() call can go to the network and possibly block for a long time. Instead, use the (undocumented) "unix:n" display specification, which never goes to the network, and doesn't block.

We've had three reports now of waitforx seemingly blocking:-

• Unable to start X session #3164
• Timed out waiting for X server #3286
• ubuntu 24.04 ipv6 waitforx: Timed out waiting for RandR outputs #3293

I've not been able to get to the bottom of any of them satisfactorily, but I got a clue when looking at the last of these.

What I think is happening is this:-

1. xrdp-sesman starts Xorg
2. Xorg takes a while to initialise and open the UNIX socket for the display
3. waitforx runs before the socket has opened and tries to open a display of the form ":n". Let's say this is ":10".
4. Because the local socket is unavailable, the code in XOpenDisplay() goes to the network to try to connect to TCP port localhost:6010 for the X server.
5. Network code blocks, either resolving localhost or because the user has not unreasonably firewalled off X11 ports.
6. waitforx times out.

The fix is to use a display specification of (e.g.) unix:10 instead of :10. This never goes to the network, and so the scenario above is avoided.

However, the DISPLAY specification 'unix:n' is not documented.

It's clearly supported in the xcb code in _xcb_open(), which libX11 is now layered on:-

https://gitlab.freedesktop.org/xorg/lib/libxcb/-/blob/master/src/xcb_util.c?ref_type=heads#L247

There's evidence of it being supported back in 2007:-
https://gitlab.freedesktop.org/xorg/lib/libxcb/-/commit/09045eaac34973662aaa820a94ca8ed66d9dcb4e

I've tried the notation for both CentOS 7 and Ubuntu 16.04, which are the oldest distros I have available. These both support it.

Are people happy with this notation being used here? Is there a better way to get the same result?

[matt335672]

I'm going to merge this into devel, so we can give it more of a test. Works fine on Linux & FreeBSD

[matt335672]

Just spotted an issue - we need to support (e.g.) :10.0 as well as :10. I'll fix this before a merge