preventing abort on "package require Tk"

[Michael Kirkham]

This has vexed me for some time, and we're a ways behind on Tcl/Tk
revisions so perhaps it's fixed already in a different way, but I finally
found a workable solution for us, and thought I would share.

Problem:

If you have an Aqua build of Tk and attempt to load it dynamically using
"package require Tk" (or Tcl_PkgRequire), and you are not logged in at the
console, then the require doesn't just fail and return an error, but
causes the application to terminate with SIGABRT.

After much digging, I found that you can use SessionGetInfo() from the
Security framework to determine if a GUI environment was available, and it
also seems to work for detecting both Aqua and X11:

     #include <Security/AuthSession.h>

     OSStatus error;
     SecuritySessionId mySession;
     SessionAttributeBits sessionInfo;

     error = SessionGetInfo(callerSecuritySession, &mySession, &sessionInfo);

Need to link with -framework Security.  If successful, sessionInfo will have
sessionHasGraphicAccess set if the user is logged into the console.
It also appears to be set if you're logged in remotely via ssh and tunneling
to a local X server yet not logged into the console, but for Tk's purposes
it probably only needs to be checked in the Aqua TkpInit (where, if it's not
set, it would return TCL_ERROR).

Source:

http://developer.apple.com/documentation/MacOSX/Conceptual/BPMultipleUsers/Concepts/FastUserSwitching.html

[Though that page has the wrong header file in the snippet and I found the
flag while digging through the headers.]

Caveat:

Assuming you are logged in at the console (or are root), without this check
it's possible to log into the machine remotely via ssh, run wish (or
tclsh and require Tk) and the Tk GUI will be sent to the console.  I find
that the bit is NOT set if you are logged in remotely via ssh and not
able to tunnel X to a local X server, even if you are logged in at the
console, so using this check you can't remotely send windows to the console.

Whether the ability to do so without the check would be considered a
feature or not, I can't say, but maybe to preserve that ability while still
allowing developers to catch Tk failing to load (due to no console -or- X)
could be done with a flag in the env or tcl_platform arrays.

--
Michael Kirkham
President & CEO
Muonics, Inc.
http://www.muonics.com/

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