[TCLCORE] Tk and thread safety

[Christian W. <Chr...@t-...>]

Hello all,

few days ago an interesting discussion was on c.l.t. regarding Itcl
and Tk used to present an Itk button per dynamically created and
destroyed thread. It uncovered a long standing bug in Itcl regarding
thread safety, which got quickly fixed by sebres.

Now for the windowing system(s): it seems that Win32 is ready for this
use case, whereas X11 and macos are not. I'm not very familiar and
competent with macos, so let's focus on X11:

* There are two possible font renderers in the X11 port of Tk:
the plain old XDrawString() and libxft using Xrender and freetype.

* While the XDrawString() way is as thread safe as the whole Xlib,
the libxft is not at all. Maybe you remember that I initially proposed a
central mutex for various libxft calls in order to overcome this issue.

* The central mutex seems to work but only as long as there's no
dynamic regarding opening and closing display connections (which is
the essence of the use case discussed on c.l.t.). When display connections
are XOpenDisplay()'ed and XCloseDisplay()'ed randomly through the life
time of a Tk program I observed very early crashes at various places
in Xrender/Xft but not when using plain old XDrawString().

* After days of debugging I found a simple non-solution: on thread
exit all X resources pertaining to the Tk applications of the thread
are released, but the final XCloseDisplay() is not carried out. Instead
the display connection is hold back (parked) on a process wide
table to be re-used later. Final XCloseDisplay()s of all display
connections are carried out per process exit handler.

* Nevertheless, the XSetErrorHandler() is a mess and most likely
the final nail in Xlib's coffin, see https://www.remlab.net/op/xlib.shtml
so the best option would be to switch everything to XCB. But this
is a huuuge undertaking.

The result of these findings can be reviewed in this check-in

   https://www.androwish.org/home/info/9b0450622edec074

in the hope that it is useful for later adoption in Tk > 9.0.

Best regards,
Christian