sched: ast_sched_del may return prematurely due to spurious wakeup

When deleting a scheduled item if the item in question is currently
executing the ast_sched_del function waits until it has completed.
This is accomplished using ast_cond_wait. Unfortunately the
ast_cond_wait function can suffer from spurious wakeups so the
predicate needs to be checked after it returns to make sure it has
really woken up as a result of being signaled.

This change adds a loop around the ast_cond_wait to make sure that
it only exits when the executing task has really completed.

ASTERISK-25355 #close

Change-Id: I51198270eb0b637c956c61aa409f46283432be61

main/sched.c

@@ -497,7 +497,9 @@ int _ast_sched_del(struct ast_sched_context *con, int id, const char *file, int
 		/* Wait for executing task to complete so that caller of ast_sched_del() does not
 		 * free memory out from under the task.
 		 */
-		ast_cond_wait(&s->cond, &con->lock);
+		while (con->currently_executing && (id == con->currently_executing->id)) {
+			ast_cond_wait(&s->cond, &con->lock);
+		}
 		/* Do not sched_release() here because ast_sched_runq() will do it */
 	}