gh-85283: Add PyInterpreterState_IsMain() function

This function can be used to convert the syslog extension to the
limited C API. The PyInterpreterState_Main() is not available in the
limited C API.

Author: vstinner

Diff for: Doc/c-api/init.rst

@@ -1126,6 +1126,15 @@ All of the following functions must be called after :c:func:`Py_Initialize`.
 
    .. versionadded:: 3.8
 
+.. c:function:: int PyInterpreterState_IsMain(PyInterpreterState *interp)
+
+   Return true (non-zero) if the interpreter is the main interpreter.
+   Return false (zero) otherwise.
+
+   See also :c:func:`PyInterpreterState_Main`.
+
+   .. versionadded:: 3.13
+
 .. c:type:: PyObject* (*_PyFrameEvalFunction)(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)
 
    Type of a frame evaluation function.

Diff for: Doc/whatsnew/3.13.rst

@@ -886,6 +886,10 @@ New Features
   (with an underscore prefix).
   (Contributed by Victor Stinner in :gh:`108014`.)
 
+* Add :c:func:`PyInterpreterState_IsMain` function to test if an interpreter
+  is the main interpreter.
+  (Contributed by Victor Stinner in :gh:`85283`.)
+
 Porting to Python 3.13
 ----------------------
 

Diff for: Include/pystate.h

@@ -37,6 +37,11 @@ PyAPI_FUNC(int64_t) PyInterpreterState_GetID(PyInterpreterState *);
 #endif
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03030000
 
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030d0000
+PyAPI_FUNC(int) PyInterpreterState_IsMain(PyInterpreterState *interp);
+#endif
+
+
 /* State unique per thread */
 
 /* New in 3.3 */

Diff for: Misc/NEWS.d/next/C API/2023-08-28-19-07-42.gh-issue-85283.BFzfPr.rst

@@ -0,0 +1,2 @@
+Add :c:func:`PyInterpreterState_IsMain` function to test if an interpreter
+is the main interpreter. Patch by Victor Stinner.

Diff for: Misc/stable_abi.toml

@@ -2452,3 +2452,5 @@
     added = '3.13'
 [function.PyLong_AsInt]
     added = '3.13'
+[function.PyInterpreterState_IsMain]
+    added = '3.13'

Diff for: Modules/syslogmodule.c

@@ -69,7 +69,8 @@ static char S_log_open = 0;
 static inline int
 is_main_interpreter(void)
 {
-    return (PyInterpreterState_Get() == PyInterpreterState_Main());
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    return PyInterpreterState_IsMain(interp);
 }
 
 static PyObject *

Diff for: Python/pystate.c

@@ -583,6 +583,12 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
 // lifecycle
 //----------
 
+int
+PyInterpreterState_IsMain(PyInterpreterState *interp)
+{
+    return _Py_IsMainInterpreter(interp);
+}
+
 /* Calling this indicates that the runtime is ready to create interpreters. */
 
 PyStatus