Set receive-maximum to not exceed the -C message count.

This is for mosquitto_sub and mosquitto_rr, to avoid potentially lost
messages.

Closes #2134. Thanks to Frantisek Fuka.

ChangeLog.txt

@@ -1,3 +1,11 @@
+2.0.10 - 2021-xx-xx
+==================
+
+Clients:
+- Set `receive-maximum` to not exceed the `-C` message count in mosquitto_sub
+  and mosquitto_rr, to avoid potentially lost messages. Closes #2134.
+
+
 2.0.9 - 2021-03-11
 ==================
 

client/client_shared.c

@@ -1321,6 +1321,13 @@ int client_opts_set(struct mosquitto *mosq, struct mosq_config *cfg)
 	if(cfg->tcp_nodelay){
 		mosquitto_int_option(mosq, MOSQ_OPT_TCP_NODELAY, 1);
 	}
+
+	if(cfg->msg_count > 0 && cfg->msg_count < 20){
+		/* 20 is the default "receive maximum"
+		 * If we don't set this, then we can receive > msg_count messages
+		 * before we quit.*/
+		mosquitto_int_option(mosq, MOSQ_OPT_RECEIVE_MAXIMUM, cfg->msg_count);
+	}
 	return MOSQ_ERR_SUCCESS;
 }