gh-116322: Add Py_mod_gil module slot (#116882)

This PR adds the ability to enable the GIL if it was disabled at
interpreter startup, and modifies the multi-phase module initialization
path to enable the GIL when loading a module, unless that module's spec
includes a slot indicating it can run safely without the GIL.

PEP 703 called the constant for the slot `Py_mod_gil_not_used`; I went
with `Py_MOD_GIL_NOT_USED` for consistency with gh-104148.

A warning will be issued up to once per interpreter for the first
GIL-using module that is loaded. If `-v` is given, a shorter message
will be printed to stderr every time a GIL-using module is loaded
(including the first one that issues a warning).

Python/Python-ast.c

@@ -17588,6 +17588,7 @@ astmodule_exec(PyObject *m)
 static PyModuleDef_Slot astmodule_slots[] = {
     {Py_mod_exec, astmodule_exec},
     {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };
 

Python/Python-tokenize.c

@@ -322,6 +322,7 @@ static PyMethodDef tokenize_methods[] = {
 static PyModuleDef_Slot tokenizemodule_slots[] = {
     {Py_mod_exec, tokenizemodule_exec},
     {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };
 

Python/_warnings.c

@@ -1498,6 +1498,7 @@ warnings_module_exec(PyObject *module)
 static PyModuleDef_Slot warnings_slots[] = {
     {Py_mod_exec, warnings_module_exec},
     {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };
 

Python/bltinmodule.c

@@ -3124,6 +3124,9 @@ _PyBuiltin_Init(PyInterpreterState *interp)
     mod = _PyModule_CreateInitialized(&builtinsmodule, PYTHON_API_VERSION);
     if (mod == NULL)
         return NULL;
+#ifdef Py_GIL_DISABLED
+    PyModule_ExperimentalSetGIL(mod, Py_MOD_GIL_NOT_USED);
+#endif
     dict = PyModule_GetDict(mod);
 
 #ifdef Py_TRACE_REFS

Python/import.c

@@ -4212,6 +4212,7 @@ imp_module_exec(PyObject *module)
 static PyModuleDef_Slot imp_slots[] = {
     {Py_mod_exec, imp_module_exec},
     {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };
 

Python/marshal.c

@@ -1952,6 +1952,7 @@ marshal_module_exec(PyObject *mod)
 static PyModuleDef_Slot marshalmodule_slots[] = {
     {Py_mod_exec, marshal_module_exec},
     {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
     {0, NULL}
 };
 

Python/sysmodule.c

@@ -3756,6 +3756,9 @@ _PySys_Create(PyThreadState *tstate, PyObject **sysmod_p)
     if (sysmod == NULL) {
         return _PyStatus_ERR("failed to create a module object");
     }
+#ifdef Py_GIL_DISABLED
+    PyModule_ExperimentalSetGIL(sysmod, Py_MOD_GIL_NOT_USED);
+#endif
 
     PyObject *sysdict = PyModule_GetDict(sysmod);
     if (sysdict == NULL) {