tkinter runs a busy-wait loop on interactive Python

[mdehoon]

Bug report

Bug description:

In EventHook in Modules/_tkinter.c, we used to have the following block to run the event loop:

#if defined(WITH_THREAD) || defined(MS_WINDOWS)
        Py_BEGIN_ALLOW_THREADS
        if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
        tcl_tstate = event_tstate;
        
        result = Tcl_DoOneEvent(TCL_DONT_WAIT);
        
        tcl_tstate = NULL;
        if(tcl_lock)PyThread_release_lock(tcl_lock);
        if (result == 0)
            Sleep(Tkinter_busywaitinterval);
        Py_END_ALLOW_THREADS
#else
        result = Tcl_DoOneEvent(0);
#endif

in bpo-31370, support for threads-less builds was removed, and this block became

        Py_BEGIN_ALLOW_THREADS
        if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
        tcl_tstate = event_tstate;
        
        result = Tcl_DoOneEvent(TCL_DONT_WAIT);
        
        tcl_tstate = NULL;
        if(tcl_lock)PyThread_release_lock(tcl_lock);
        if (result == 0)
            Sleep(Tkinter_busywaitinterval);
        Py_END_ALLOW_THREADS

This is a bit unfortunate, as we now back to running a busy-wait loop. See Guido van Rossum's comment of May 23, 1998: 7bf15648

I guess this block could be

        if (self->threaded) {
            /* Allow other Python threads to run. */
            ENTER_TCL
            result = Tcl_DoOneEvent(0);
            LEAVE_TCL
        }
        else {
            Py_BEGIN_ALLOW_THREADS
            if(tcl_lock)PyThread_acquire_lock(tcl_lock, 1);
            tcl_tstate = tstate;
            result = Tcl_DoOneEvent(TCL_DONT_WAIT);
            tcl_tstate = NULL;
            if(tcl_lock)PyThread_release_lock(tcl_lock);
            if (result == 0)
                Sleep(Tkinter_busywaitinterval);
            Py_END_ALLOW_THREADS
        }

in analogy with the event loop in _tkinter_tkapp_mainloop_impl. As self->threaded is true in most builds, this would avoid the busy-wait loop.

I am happy to submit a PR, but I'll wait until PR #140147 is in to avoid unnecessary complications to this code.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS