Backport r60148 and r65481: sanity checks to avoid infinite loops.

Python/pystate.c

@@ -240,6 +240,7 @@ tstate_delete_common(PyThreadState *tstate)
 {
 	PyInterpreterState *interp;
 	PyThreadState **p;
+	PyThreadState *prev_p = NULL;
 	if (tstate == NULL)
 		Py_FatalError("PyThreadState_Delete: NULL tstate");
 	interp = tstate->interp;
@@ -252,6 +253,19 @@ tstate_delete_common(PyThreadState *tstate)
 				"PyThreadState_Delete: invalid tstate");
 		if (*p == tstate)
 			break;
+		/* Sanity check.  These states should never happen but if
+		 * they do we must abort.  Otherwise we'll end up spinning in
+		 * in a tight loop with the lock held.  A similar check is done
+		 * in thread.c find_key().  */
+		if (*p == prev_p)
+			Py_FatalError(
+				"PyThreadState_Delete: small circular list(!)"
+                                " and tstate not found.");
+		prev_p = *p;
+		if ((*p)->next == interp->tstate_head)
+			Py_FatalError(
+				"PyThreadState_Delete: circular list(!) and"
+                                " tstate not found.");
 	}
 	*p = tstate->next;
 	HEAD_UNLOCK();

Python/thread.c

@@ -264,15 +264,25 @@ static int nkeys = 0;  /* PyThread_create_key() hands out nkeys+1 next */
 static struct key *
 find_key(int key, void *value)
 {
-	struct key *p;
+	struct key *p, *prev_p;
 	long id = PyThread_get_thread_ident();
 
 	if (!keymutex)
 		return NULL;
 	PyThread_acquire_lock(keymutex, 1);
+	prev_p = NULL;
 	for (p = keyhead; p != NULL; p = p->next) {
 		if (p->id == id && p->key == key)
 			goto Done;
+		/* Sanity check.  These states should never happen but if
+		 * they do we must abort.  Otherwise we'll end up spinning in
+		 * in a tight loop with the lock held.  A similar check is done
+		 * in pystate.c tstate_delete_common().  */
+		if (p == prev_p)
+			Py_FatalError("tls find_key: small circular list(!)");
+		prev_p = p;
+		if (p->next == keyhead)
+			Py_FatalError("tls find_key: circular list(!)");
 	}
 	if (value == NULL) {
 		assert(p == NULL);