Itcl - the [incr Tcl] extension

package require Thread

set thread_script {
     package require Iwidgets
     iwidgets::pushbutton .btn -text [thread::id]
     pack .btn
     after 4000 [list thread::release [thread::id]]
     thread::wait
}

for {set i 0} {$i<1000} {incr i} {
     thread::create -preserved $thread_script
     after 500
}

> That's why I'm convinced the problem is in Itcl rather than Itk,

Then please provide a reproducible example using Itcl only (without widgets, tk and whatever)... However I guess it is not Itk, but rather tk.

> and the stack trace points to Itcl as well.

Well, it maybe just an after effect, but...

> Probably, there is a mutex missing somewhere.

I never heard that tk is really thread-safe (vice versa, I always thought it is not)... You can surely use it in multi-threaded application, but have to guarantee that only one thread (mostly main thread) is communicating with tk and serves its events in the event cycle.

Sorry, but I'll close it as incorrect or external bug.

The pointer petrokaz gave to the itcl ticket

https://core.tcl-lang.org/itcl/tktview/4fff3fc277

is most likely the root of the evil. See that
ticket for my proposed fix, which I've tested with
petrokaz' example code snippet on Wine and
observed no crashes.

Regarding multi-threaded Tk: Windows should be fine,
Linux/X11/Mac not so much.

Although I don't think that even on Windows tk is thread-safe, [4fff3fc277] is indeed an issue... And your PoC looks good, so I'll cherry-pick it now. Thx, Christian.