libplugin: don't call callbacks if cmd completed before response.

This can particularly happen with commando:

```
 commando: FATAL SIGNAL 11 (version 06b36d3)
0x55609e953d51 send_backtrace
	common/daemon.c:33
0x55609e953dfb crashdump
	common/daemon.c:46
0x7f665e3b908f ???
	/build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/x86_64/sigaction.c:0
0x55609e9387a3 send_more_cmd
	plugins/commando.c:632
0x55609e93b270 handle_rpc_reply
	plugins/libplugin.c:669
0x55609e93bd50 rpc_read_response_one
	plugins/libplugin.c:842
0x55609e93be86 rpc_conn_read_response
	plugins/libplugin.c:862
0x55609e9f4f68 next_plan
	ccan/ccan/io/io.c:59
0x55609e9f5b70 do_plan
	ccan/ccan/io/io.c:407
0x55609e9f5bb2 io_ready
	ccan/ccan/io/io.c:417
0x55609e9f7ea5 io_loop
	ccan/ccan/io/poll.c:453
0x55609e93eb20 plugin_main
	plugins/libplugin.c:1676
0x55609e9397ab main
	plugins/commando.c:922
0x7f665e39a082 __libc_start_main
	../csu/libc-start.c:308
0x55609e93677d ???
	???:0
0xffffffffffffffff ???
	???:0
```

Reported-by: @adi2011
Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

plugins/libplugin.c

@@ -135,6 +135,22 @@ static void ld_rpc_send(struct plugin *plugin, struct json_stream *stream)
 	io_wake(plugin->io_rpc_conn);
 }
 
+
+/* When cmd for request is gone, we use this as noop callback */
+static struct command_result *ignore_cb(struct command *command,
+					const char *buf,
+					const jsmntok_t *result,
+					void *arg)
+{
+	return command_done();
+}
+
+static void disable_request_cb(struct command *cmd, struct out_req *out)
+{
+	out->errcb = NULL;
+	out->cb = ignore_cb;
+}
+
 /* FIXME: Move lightningd/jsonrpc to common/ ? */
 
 struct out_req *
@@ -160,6 +176,10 @@ jsonrpc_request_start_(struct plugin *plugin, struct command *cmd,
 	out->arg = arg;
 	uintmap_add(&plugin->out_reqs, out->id, out);
 
+	/* If command goes away, don't call callbacks! */
+	if (out->cmd)
+		tal_add_destructor2(out->cmd, disable_request_cb, out);
+
 	out->js = new_json_stream(NULL, cmd, NULL);
 	json_object_start(out->js, NULL);
 	json_add_string(out->js, "jsonrpc", "2.0");
@@ -646,6 +666,10 @@ static void handle_rpc_reply(struct plugin *plugin, const jsmntok_t *toks)
 			   json_tok_full_len(toks),
 			   json_tok_full(plugin->rpc_buffer, toks), id);
 
+	/* Remove destructor if one existed */
+	if (out->cmd)
+		tal_del_destructor2(out->cmd, disable_request_cb, out);
+
 	/* We want to free this if callback doesn't. */
 	tal_steal(tmpctx, out);
 	uintmap_del(&plugin->out_reqs, out->id);