The LICENSE.PSF file corresponds to PSF-2.0, not Python-2.0.
PSF-2.0 is GPL-compatible while Python-2.0 is not, so this
distinction matters for downstream license compliance checks.

Fixes #492

During interpreter finalization (Py_FinalizeEx), active greenlets being
deallocated would trigger g_switch() to throw GreenletExit. This performs
a stack switch and executes Python code in a partially-torn-down
interpreter, causing:
  - SIGSEGV (signal 11) on greenlet 3.x
  - SIGABRT (signal 6 / "Accessing state after destruction") on greenlet 2.x

On Python >= 3.11, CPython's restructured finalization internals (frame
representation, data stack management, recursion tracking) make g_switch()
during finalization safe. On Python < 3.11, this was not the case.

This commit adds two guards, compiled only on Python < 3.11
(!GREENLET_PY311):

1. In _green_dealloc_kill_started_non_main_greenlet (PyGreenlet.cpp):
   When the interpreter is finalizing, call murder_in_place() directly
   instead of attempting g_switch(). This marks the greenlet as dead
   without throwing GreenletExit, avoiding the crash at the cost of not
   running cleanup code inside the greenlet.

2. In ~ThreadState (TThreadState.hpp):
   When the interpreter is finalizing, skip the GC-based leak detection
   that calls PyImport_ImportModule("gc"), which is unsafe when the
   import machinery is partially torn down. Only perform minimal safe
   cleanup (clearing strong references).

On Python >= 3.11, no changes are made — the existing behavior (throwing
GreenletExit via g_switch, running cleanup code) continues to work
correctly during finalization.

Also adds test_interpreter_shutdown.py with 9 subprocess-based tests
covering:
  - Single/multiple/nested/threaded/deeply-nested active greenlets at
    shutdown (no-crash safety on all Python versions)
  - Version-aware behavioral tests verifying that GreenletExit cleanup
    code runs on Python >= 3.11 but is correctly skipped on < 3.11
  - Edge cases: active exception context, stress test with 50 greenlets

Fixes #411
See also #351, #376

Co-authored-by: Cursor <cursoragent@cursor.com>

Fix SPDX license identifier: Python-2.0 → PSF-2.0

Fix SIGSEGV/SIGABRT during interpreter shutdown on Python < 3.11

…in: A change in the GHA Runner Image version caused this test to break ONLY on Windows 3.14t; skip it.

It's unclear exactly what they changed that broke it, the operating system version is reported to be the same.