bpo-43962: Fix _PyInterpreterState_IDIncref() (GH-25683)

_PyInterpreterState_IDIncref() now calls
_PyInterpreterState_IDInitref() and always increments id_refcount.

Author: vstinner

Include/internal/pycore_interp.h

@@ -338,7 +338,7 @@ struct _xidregitem {
 PyAPI_FUNC(struct _is*) _PyInterpreterState_LookUpID(int64_t);
 
 PyAPI_FUNC(int) _PyInterpreterState_IDInitref(struct _is *);
-PyAPI_FUNC(void) _PyInterpreterState_IDIncref(struct _is *);
+PyAPI_FUNC(int) _PyInterpreterState_IDIncref(struct _is *);
 PyAPI_FUNC(void) _PyInterpreterState_IDDecref(struct _is *);
 
 #ifdef __cplusplus

Misc/NEWS.d/next/C API/2021-04-28-13-13-07.bpo-43962.9Jzs5X.rst

@@ -0,0 +1,5 @@
+_PyInterpreterState_IDIncref() now calls _PyInterpreterState_IDInitref() and
+always increments id_refcount. Previously, calling
+_xxsubinterpreters.get_current() could create an id_refcount inconsistency
+when a _xxsubinterpreters.InterpreterID object was deallocated. Patch by
+Victor Stinner.

Objects/interpreteridobject.c

@@ -24,15 +24,21 @@ newinterpid(PyTypeObject *cls, int64_t id, int force)
         }
     }
 
+    if (interp != NULL) {
+        if (_PyInterpreterState_IDIncref(interp) < 0) {
+            return NULL;
+        }
+    }
+
     interpid *self = PyObject_New(interpid, cls);
     if (self == NULL) {
+        if (interp != NULL) {
+            _PyInterpreterState_IDDecref(interp);
+        }
         return NULL;
     }
     self->id = id;
 
-    if (interp != NULL) {
-        _PyInterpreterState_IDIncref(interp);
-    }
     return self;
 }
 

Python/pystate.c

@@ -538,24 +538,25 @@ _PyInterpreterState_IDInitref(PyInterpreterState *interp)
 }
 
 
-void
+int
 _PyInterpreterState_IDIncref(PyInterpreterState *interp)
 {
-    if (interp->id_mutex == NULL) {
-        return;
+    if (_PyInterpreterState_IDInitref(interp) < 0) {
+        return -1;
     }
+
     PyThread_acquire_lock(interp->id_mutex, WAIT_LOCK);
     interp->id_refcount += 1;
     PyThread_release_lock(interp->id_mutex);
+    return 0;
 }
 
 
 void
 _PyInterpreterState_IDDecref(PyInterpreterState *interp)
 {
-    if (interp->id_mutex == NULL) {
-        return;
-    }
+    assert(interp->id_mutex != NULL);
+
     struct _gilstate_runtime_state *gilstate = &_PyRuntime.gilstate;
     PyThread_acquire_lock(interp->id_mutex, WAIT_LOCK);
     assert(interp->id_refcount != 0);