Add PyInterpreterState_Main().

Author: ncoghlan

Include/pystate.h

@@ -293,6 +293,7 @@ PyAPI_FUNC(PyObject *) _PyThread_CurrentFrames(void);
 /* Routines for advanced debuggers, requested by David Beazley.
    Don't use unless you know what you are doing! */
 #ifndef Py_LIMITED_API
+PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Main(void);
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Head(void);
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Next(PyInterpreterState *);
 PyAPI_FUNC(PyThreadState *) PyInterpreterState_ThreadHead(PyInterpreterState *);

Python/pystate.c

@@ -46,6 +46,7 @@ static PyThread_type_lock head_mutex = NULL; /* Protects interp->tstate_head */
 /* The single PyInterpreterState used by this process'
    GILState implementation
 */
+/* TODO: Given interp_main, it may be possible to kill this ref */
 static PyInterpreterState *autoInterpreterState = NULL;
 static int autoTLSkey = -1;
 #else
@@ -55,6 +56,7 @@ static int autoTLSkey = -1;
 #endif
 
 static PyInterpreterState *interp_head = NULL;
+static PyInterpreterState *interp_main = NULL;
 
 /* Assuming the current thread holds the GIL, this is the
    PyThreadState for the current thread. */
@@ -102,6 +104,9 @@ PyInterpreterState_New(void)
 
         HEAD_LOCK();
         interp->next = interp_head;
+        if (interp_main == NULL) {
+            interp_main = interp;
+        }
         interp_head = interp;
         HEAD_UNLOCK();
     }
@@ -159,6 +164,11 @@ PyInterpreterState_Delete(PyInterpreterState *interp)
     if (interp->tstate_head != NULL)
         Py_FatalError("PyInterpreterState_Delete: remaining threads");
     *p = interp->next;
+    if (interp_main == interp) {
+        interp_main = NULL;
+        if (interp_head != NULL)
+            Py_FatalError("PyInterpreterState_Delete: remaining subinterpreters");
+    }
     HEAD_UNLOCK();
     PyMem_RawFree(interp);
 #ifdef WITH_THREAD
@@ -624,6 +634,12 @@ PyInterpreterState_Head(void)
     return interp_head;
 }
 
+PyInterpreterState *
+PyInterpreterState_Main(void)
+{
+    return interp_main;
+}
+
 PyInterpreterState *
 PyInterpreterState_Next(PyInterpreterState *interp) {
     return interp->next;