Add xrdp-waitforx to wait for X to start with RandR outputs

[derekschrock]

For some window managers (fvwm2 and fvwm3) if the X server isn't
running and has output it's possible for the window manager to fail or
reconfigure randr incorrectly.

With xrdp-waitfox:

• Install xrdp-waitfox to the BIN dir.
• sesman will run xrdp-waitfox as the logged in user.
• Set an alarm to exit after 30 seconds.
• Try to open env DISPLAY value's display (10 seconds).
• Test for RandR extension.
• Wait for outputs to appear (10 seconds).

Fixes #2436

[matt335672]

Thanks for this Derek.

[matt335672]

Using printf here raises an interesting question about some of the wrapper functions in common/os_calls.h.

Other bits of xrdp using printf() rather than g_printf()

Personally I can't see that we gain anything by using g_printf() over printf(). I think we should (over time) be removing the wrappers for standard C functions and simply use the <stdio.h> (etc) includes.

@metalefty - what's your take on this? Should we start a discussion?

[metalefty]

Regarding printf(), I personally think I would like to quit using wrapper function and use printf() directly.

[matt335672]

Thanks. I'll start a discussion about this.

[matt335672]

sleep() isn't a standard C function - can we use g_sleep(1000) here?

[derekschrock]

See d7d0866

[matt335672]

For consistency with the other signals these should probably be wrapped.

I'd suggest writing a function which does both the timer and setting the handler, as this could be moved to Windows later (not that this is likely to happen).

How does this look?

unsigned int g_set_alarm(void (*func)(int), unsigned int secs);

/*****************************************************************************/
/* does not work in win32 */
unsigned int
g_set_alarm(void (*func)(int), unsigned int secs)
{
#if defined(_WIN32)
    return 0;
#else
    /* Cancel any previous alarm to prevent a race */
    unsigned int rv = alarm(0);
    signal(SIGALRM, func);
    (void)alarm(secs);
    return rv;
#endif
}

[derekschrock]

See 8c44ae0 and d7d0866

[matt335672]

Can't use printf() in a signal handler. See signal-safety(7)

Given this is unlikely to be used on Windows, here's a suggested alternative.

/* Avoid printf() in signal handler (see signal-safety(7)) */
const char msg[] = "Timed out waiting for RandR outputs\n";
g_file_write(1, msg, g_strlen(msg));

[matt335672]

See above.

[derekschrock]

See d7d0866

[matt335672]

As far as the printf/g_printf discussion goes, this is effectively resolved. See #2499. We'll stick with printf() here I think.

[matt335672]

Thanks Derek.