diff --git a/src/igc.c b/src/igc.c
index 3d1c2e5f80b..c7afa84b8c9 100644
--- a/src/igc.c
+++ b/src/igc.c
@@ -4084,6 +4084,13 @@ igc_process_messages (void)
   }
 }
 
+static void
+process_all_messages (void)
+{
+  while (process_one_message (global_igc))
+    ;
+}
+
 /* Discard entries for killed buffers from LIST and return the resulting
    list.  Used in window-{next,prev}-buffers.  */
 
@@ -4305,6 +4312,17 @@ DEFUN ("igc--collect", Figc__collect, Sigc__collect, 0, 0, 0,
   return Qnil;
 }
 
+DEFUN ("igc--process-messages", Figc__process_messages,
+       Sigc__process_messages, 0, 0, 0,
+       doc: /* Process all queued MPS messages.
+
+For internal use only. */)
+(void)
+{
+  process_all_messages ();
+  return Qnil;
+}
+
 size_t
 igc_hash (Lisp_Object key)
 {
@@ -5705,6 +5723,7 @@ syms_of_igc (void)
   defsubr (&Sigc_info);
   defsubr (&Sigc__roots);
   defsubr (&Sigc__collect);
+  defsubr (&Sigc__process_messages);
   defsubr (&Sigc__set_commit_limit);
   defsubr (&Sigc__set_pause_time);
   defsubr (&Sigc__add_extra_dependency);
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 32ea8de8a90..af2b1544d3c 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -1310,9 +1310,14 @@
              do (ft--check-entry w k1 v1 k2 v2))
     (should (= (length expected) (length actual)))))
 
-(defun ft--gc ()
+(defun ft--gc (weakness)
   (cond ((fboundp 'igc--collect)
-         (igc--collect))
+         (igc--collect)
+         (cl-ecase weakness
+           (key-or-value
+            (igc--process-messages)
+            (igc--collect))
+           ((key value key-and-value))))
         (t
          (garbage-collect))))
 
@@ -1326,7 +1331,7 @@
          (table (make-hash-table :weakness weakness))
          (f (lambda () (ft--populate-hashtable table (ft--nentries))))
          (pairs (thread-join (make-thread f))))
-    (ft--gc)
+    (ft--gc weakness)
     (ft--check-entries table pairs)))
 
 (ert-deftest ft-weak-key-removal () (ft--test-weak-removal 'key))