Add function to remove active work list entries.

Later on, we can add re-use of the indexes, to prevent an ever-growing
array.

src/io/eventloop.c

@@ -173,3 +173,18 @@ MVMAsyncTask * MVM_io_eventloop_get_active_work(MVMThreadContext *tc, int work_i
         MVM_panic(1, "use of invalid eventloop work item index %d", work_idx);
     }
 }
+
+/* Removes an active work index from the active work list, enabling any
+ * memory associated with it to be collected. Replaces the work index with -1
+ * so that any future use of the task will be a failed lookup. */
+void MVM_io_eventloop_remove_active_work(MVMThreadContext *tc, int *work_idx_to_clear) {
+    int work_idx = *work_idx_to_clear;
+    if (work_idx >= 0 && work_idx < MVM_repr_elems(tc, tc->instance->event_loop_active)) {
+        *work_idx_to_clear = -1;
+        MVM_repr_bind_pos_o(tc, tc->instance->event_loop_active, work_idx, tc->instance->VMNull);
+        /* TODO: start to re-use the indices */
+    }
+    else {
+        MVM_panic(1, "cannot remove invalid eventloop work item index %d", work_idx);
+    }
+}

src/io/eventloop.h

@@ -21,3 +21,4 @@ void MVM_io_eventloop_send_cancellation_notification(MVMThreadContext *tc, MVMAs
 
 int MVM_io_eventloop_add_active_work(MVMThreadContext *tc, MVMObject *async_task);
 MVMAsyncTask * MVM_io_eventloop_get_active_work(MVMThreadContext *tc, int work_idx);
+void MVM_io_eventloop_remove_active_work(MVMThreadContext *tc, int *work_idx_to_clear);