`test.test_interpreters.test_api.TestInterpreterIsRunning.test_main` Fails in Embedded App

[ericsnowcurrently]

Bug report

(See https://discuss.python.org/t/clarification-on-pep-734-subinterpreters-with-embedded-c-api/45889/1)

The test fails if CPython (main/3.13) is embedded without using Py_Main(). This is because Py_Main() calls _PyInterpreterState_SetRunningMain(). which is a new internal API that embedders are unlikely to even know about.

The failure can be reproduced in a regular build by commenting out the _PyInterpreterState_SetRunningMain() and _PyInterpreterState_SetNotRunningMain() calls in Modules/main.c:

(expand)

diff --git a/Modules/main.c b/Modules/main.c
index df2ce55024..5c0cb8222d 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -612,7 +612,7 @@ pymain_run_python(int *exitcode)

     pymain_header(config);

-    _PyInterpreterState_SetRunningMain(interp);
+//    _PyInterpreterState_SetRunningMain(interp);
     assert(!PyErr_Occurred());

     if (config->run_command) {
@@ -638,7 +638,7 @@ pymain_run_python(int *exitcode)
     *exitcode = pymain_exit_err_print();

 done:
-    _PyInterpreterState_SetNotRunningMain(interp);
+//    _PyInterpreterState_SetNotRunningMain(interp);
     Py_XDECREF(main_importer_path);
 }

FTR, I added the "running"-tracking API (1dd9dee) to support PEP 734's Interpreter.is_running(). The PEP hasn't been accepted yet, but we still use the implementation to exercise subinterpreters in the test suite.

Possible solutions:

• always assume the main interpreter is running (in the main thread)
• make calls to the “running”-tracking API implicit to calls to the PyRun_*() family (and similar)
• infer the “running”-tracking API should have been called in certain situations
• make it public API
• stop tracking if an interpreter is “running” (i.e. drop the API and the related state)

I plan on implementing a short-term fix right away. If we keep "running"-tracking then we'll definitely at least want to make the API public.

CC @freakboy3742

Linked PRs

• gh-115482: Assume the Main Interpreter is Always Running "main" #115484