Avoid assertion violation when comparing with main-thread

* src/thread.c (unmark_main_thread): New function.
* src/lisp.h (unmark_main_thread): Prototype it.
* src/alloc.c (garbage_collect_1): Call it after sweeping.
(Bug#33073)

* test/src/thread-tests.el (threads-test-bug33073): New test.

src/alloc.c

@@ -6003,6 +6003,8 @@ garbage_collect_1 (void *end)
   VECTOR_UNMARK (&buffer_defaults);
   VECTOR_UNMARK (&buffer_local_symbols);
 
+  unmark_main_thread ();
+
   check_cons_list ();
 
   gc_in_progress = 0;

src/lisp.h

@@ -3975,6 +3975,7 @@ extern void syms_of_module (void);
 
 /* Defined in thread.c.  */
 extern void mark_threads (void);
+extern void unmark_main_thread (void);
 
 /* Defined in editfns.c.  */
 extern void insert1 (Lisp_Object);

src/thread.c

@@ -656,6 +656,12 @@ mark_threads (void)
   flush_stack_call_func (mark_threads_callback, NULL);
 }
 
+void
+unmark_main_thread (void)
+{
+  main_thread.header.size &= ~ARRAY_MARK_FLAG;
+}
+
 
 
 static void

test/src/thread-tests.el

@@ -347,4 +347,8 @@
     (should (= (length (all-threads)) 1))
     (should (equal (thread-last-error) '(error "Die, die, die!")))))
 
+(ert-deftest threads-test-bug33073 ()
+  (let ((th (make-thread 'ignore)))
+    (should-not (equal th main-thread))))
+
 ;;; threads.el ends here