Fix session-expiry-interval for v5 clients using -c.

Default behaviour for v5 clients using `-c` is now to use infinite length
sessions, as with v3 clients.

Closes #1546. Thanks to Kiran Pradeep.

ChangeLog.txt

@@ -42,6 +42,8 @@ Clients:
   option.
 - Add `-x` to all clients to all the session-expiry-interval property to be
   easily set for MQTT v5 clients.
+- Default behaviour for v5 clients using `-c` is now to use infinite length
+  sessions, as with v3 clients. Closes #1546.
 
 
 1.6.8 - 20191128

client/args.txt

@@ -45,7 +45,7 @@ v - RR,SUB         (verbose)
 W - SUB            (timeout)
 w
 X
-x
+x - PUB,RR,SUB     (session-expiry-interval)
 Y
 y
 Z

client/client_shared.c

@@ -155,6 +155,7 @@ void init_config(struct mosq_config *cfg, int pub_or_sub)
 	}else{
 		cfg->protocol_version = MQTT_PROTOCOL_V311;
 	}
+	cfg->session_expiry_interval = -1; /* -1 means unset here, the user can't set it to -1. */
 }
 
 void client_config_cleanup(struct mosq_config *cfg)
@@ -366,11 +367,13 @@ int client_config_load(struct mosq_config *cfg, int pub_or_sub, int argc, char *
 	}
 #endif
 
-	if(cfg->protocol_version != 5 && cfg->clean_session == false && (cfg->id_prefix || !cfg->id)){
+	if(cfg->protocol_version == 5){
-		fprintf(stderr, "Error: You must provide a client id if you are using the -c option.\n");
+		if(cfg->clean_session == false && cfg->session_expiry_interval == -1){
-		return 1;
+			/* User hasn't set session-expiry-interval, but has cleared clean
+			 * session so default to persistent session. */
+			cfg->session_expiry_interval = UINT32_MAX;
 		}
-	if(cfg->protocol_version == 5 && cfg->session_expiry_interval > 0){
+		if(cfg->session_expiry_interval > 0){
 			if(cfg->session_expiry_interval == UINT32_MAX && (cfg->id_prefix || !cfg->id)){
 				fprintf(stderr, "Error: You must provide a client id if you are using an infinite session expiry interval.\n");
 				return 1;
@@ -380,6 +383,12 @@ int client_config_load(struct mosq_config *cfg, int pub_or_sub, int argc, char *
 				fprintf(stderr, "Error adding property session-expiry-interval\n");
 			}
 		}
+	}else{
+		if(cfg->clean_session == false && (cfg->id_prefix || !cfg->id)){
+			fprintf(stderr, "Error: You must provide a client id if you are using the -c option.\n");
+			return 1;
+		}
+	}
 
 	if(pub_or_sub == CLIENT_SUB){
 		if(cfg->topic_count == 0){