lightningd: in second shutdown_plugin, trigger EOF in plugin stdin

So a db_write plugin can still self-terminate when its ready for it.

Credit to ZmnSCPxj for the EOF idea, see issue ElementsProject#4785

lightningd/plugin.c

@@ -2167,8 +2167,9 @@ void shutdown_plugins(struct lightningd *ld, bool first_call)
 			continue;
 		}
 
-		/* First call: Send shutdown notification while jsonrpc still exists */
+		/* First call: */
 		if (first_call) {
+			/* Send shutdown notifications to all while jsonrpc still exists */
 			notified = notify_plugin_shutdown(ld, p);
 
 			/* Always keep db_write plugins alive in first call */
@@ -2181,10 +2182,16 @@ void shutdown_plugins(struct lightningd *ld, bool first_call)
 			else
 				tal_free(p);
 
-		/* Second call: Only wait if plugin got notified in first call */
+		/* Second call: */
 		} else {
-			p->plugin_state = SHUTDOWN;
-			if (!plugin_subscriptions_contains(p, "shutdown"))
+			/* Only wait if the plugin got notified in first call */
+			if (plugin_subscriptions_contains(p, "shutdown")) {
+				p->plugin_state = SHUTDOWN;
+
+				/* io_close triggers EOF at plugin's stdin read-loop, so it can
+				 * still self-terminate when its ready (within 30s). */
+				io_close(p->stdin_conn);
+			} else
 				tal_free(p);
 		}
 	}

lightningd/plugin.h

@@ -240,12 +240,17 @@ void plugin_kill(struct plugin *plugin, enum log_level loglevel,
 		 const char *fmt, ...);
 
 /**
- * Tell plugins we're shutting down, and free them.
+ * Tell plugins we're shutting down, maybe wait for them to self-terminate and
+ * free them.
  *
- * @skip_db_write_plugins - keep db_write plugins alive, i.e. the ones that
- * registered the db_write hook
+ * @first_call=true: keeps db_write plugins alive, i.e. the ones that
+ * registered the db_write hook, but they still receive a shutdown notification.
+ *
+ * @first_call=false: shutdown remaining plugins: if it subscribed to shutdown
+ * then trigger EOF on stdin and wait for it to self-terminate, otherwise kill
+ * it immediately.
  */
-void shutdown_plugins(struct lightningd *ld, bool first);
+void shutdown_plugins(struct lightningd *ld, bool first_call);
 
 /**
  * Returns the plugin which registers the command with name {cmd_name}