tkinter: Tk on macOS replaces Python's SIGINT, SIGHUP and SIGTERM handlers, so Ctrl-C exits instead of raising KeyboardInterrupt

Bug report

Since Tk 8.6.11, TkpInit() on macOS installs its own C-level handler for SIGINT, SIGHUP and SIGTERM, which calls Tcl_Exit(1) (tkMacOSXInit.c, Tk ticket f3e96942ed). It does this unconditionally, so Python's handler is replaced as soon as a Tk instance is created. signal.getsignal() still returns the Python handler, but it is never called.

import signal, time, tkinter
print(signal.getsignal(signal.SIGINT))  # <built-in function default_int_handler>
root = tkinter.Tk()
time.sleep(60)  # press Ctrl-C

On macOS the process exits silently with status 1 instead of raising KeyboardInterrupt. Python-level handlers set with signal.signal() for SIGTERM and SIGHUP are disabled the same way.

This also affects IDLE: with #157662, which interrupts user code with a real SIGINT, Ctrl-C in the Shell would terminate the user process on macOS once the user's code has created a Tk window. On the macOS CI the test_idle worker exits with status 1 for the same reason, since GUI tests earlier in the same process created Tk windows.

The fix on the Tk side is on branch mac-signal-handlers, but installed Tk builds will not have it for a long time. _tkinter should save the handlers of these signals before Tk_Init() and restore them after it.

Linked PRs

• gh-157673

Start in _tkinter at the Tk_Init() call and trace how the macOS signal handlers are affected. Reproduce the issue with the provided tkinter script, then verify that creating a Tk instance preserves the Python handlers and Ctrl-C raises KeyboardInterrupt.