Don't generate srun hostlist for log for huge task count

If the overhead of determining the hostlist for a given task list
is too high, then report a hostlist of "Unknown" instead. If the
overhead is too high, then srun will become unresponsive and communications
will timeout/fail.
bug 3044

src/plugins/launch/slurm/launch_slurm.c

@@ -123,9 +123,9 @@ static char *_hostset_to_string(hostset_t hs)
 
 /* Convert an array of task IDs into a list of host names
  * RET: the string, caller must xfree() this value */
-static char *_task_ids_to_host_list(int ntasks, uint32_t taskids[])
+static char *_task_ids_to_host_list(int ntasks, uint32_t *taskids)
 {
-	int i;
+	int i, task_cnt = 0;
 	hostset_t hs;
 	char *hosts;
 	slurm_step_layout_t *sl;
@@ -133,6 +133,17 @@ static char *_task_ids_to_host_list(int ntasks, uint32_t taskids[])
 	if ((sl = launch_common_get_slurm_step_layout(local_srun_job)) == NULL)
 		return (xstrdup("Unknown"));
 
+	/* If overhead of determining the hostlist is too high then srun
+	 * communications will timeout and fail, so return "Unknown" instead.
+	 *
+	 * See slurm_step_layout_host_id() in src/common/slurm_step_layout.c
+	 * for details. */
+	for (i = 0; i < sl->node_cnt; i++) {
+		task_cnt += sl->tasks[i];
+	}
+	if (task_cnt > 100000)
+		return (xstrdup("Unknown"));
+
 	hs = hostset_create(NULL);
 	for (i = 0; i < ntasks; i++) {
 		char *host = slurm_step_layout_host_name(sl, taskids[i]);
@@ -166,7 +177,7 @@ static char *_task_array_to_string(int ntasks, uint32_t taskids[])
 		error("bit_alloc: memory allocation failure");
 		exit(error_exit);
 	}
-	for (i=0; i<ntasks; i++)
+	for (i = 0; i < ntasks; i++)
 		bit_set(tasks_bitmap, taskids[i]);
 	str = xmalloc(2048);
 	bit_fmt(str, 2048, tasks_bitmap);
@@ -322,8 +333,10 @@ static void _task_finish(task_exit_msg_t *msg)
 		}
 	}
 	if (build_task_string) {
-		tasks = _task_array_to_string(msg->num_tasks, msg->task_id_list);
-		hosts = _task_ids_to_host_list(msg->num_tasks, msg->task_id_list);
+		tasks = _task_array_to_string(msg->num_tasks,
+					      msg->task_id_list);
+		hosts = _task_ids_to_host_list(msg->num_tasks,
+					       msg->task_id_list);
 	}
 
 	slurm_mutex_lock(&launch_lock);