Add -E option to mosquitto_sub.

This causes the client to exit immediately after its subscriptions are
acknowledged by the broker, and can be used to create a durable client
session without requiring messages to be delivered.

Closes #952.

ChangeLog.txt

@@ -1,10 +1,15 @@
 1.6 - 2018xxxx
 ==============
 
-Library features:
+Client library features:
 - Add mosquitto_subscribe_multiple() for sending subscriptions to multiple
   topics in one command.
 
+Client features:
+- Add -E to mosquitto_sub, which causes it to exit immediately after having
+  its subscriptions acknowledged. Use with -c to create a durable client
+  session without requiring a message to be received.
+
 
 1.5 - 20180502
 ==============

client/client_shared.c

@@ -479,6 +479,11 @@ int client_config_line_proc(struct mosq_config *cfg, int pub_or_sub, int argc, c
 				}
 			}
 			i++;
+		}else if(!strcmp(argv[i], "-E")){
+			if(pub_or_sub == CLIENT_PUB){
+				goto unknown_option;
+			}
+			cfg->exit_after_sub = true;
 		}else if(!strcmp(argv[i], "-F")){
 			if(pub_or_sub == CLIENT_PUB){
 				goto unknown_option;

client/client_shared.h

@@ -74,6 +74,7 @@ struct mosq_config {
 	bool clean_session;
 	char **topics; /* sub */
 	int topic_count; /* sub */
+	bool exit_after_sub; /* sub */
 	bool no_retain; /* sub */
 	bool retained_only; /* sub */
 	char **filter_outs; /* sub */

client/sub_client.c

@@ -96,9 +96,8 @@ void my_connect_callback(struct mosquitto *mosq, void *obj, int result, int flag
 	cfg = (struct mosq_config *)obj;
 
 	if(!result){
-		for(i=0; i<cfg->topic_count; i++){
-			mosquitto_subscribe(mosq, NULL, cfg->topics[i], cfg->qos);
-		}
+		mosquitto_subscribe_multiple(mosq, NULL, cfg->topic_count, (const char **)cfg->topics, cfg->qos);
+
 		for(i=0; i<cfg->unsub_topic_count; i++){
 			mosquitto_unsubscribe(mosq, NULL, cfg->unsub_topics[i]);
 		}
@@ -123,6 +122,10 @@ void my_subscribe_callback(struct mosquitto *mosq, void *obj, int mid, int qos_c
 		if(!cfg->quiet) printf(", %d", granted_qos[i]);
 	}
 	if(!cfg->quiet) printf("\n");
+
+	if(cfg->exit_after_sub){
+		mosquitto_disconnect(mosq);
+	}
 }
 
 void my_log_callback(struct mosquitto *mosq, void *obj, int level, const char *str)
@@ -139,7 +142,7 @@ void print_usage(void)
 	printf("mosquitto_sub version %s running on libmosquitto %d.%d.%d.\n\n", VERSION, major, minor, revision);
 	printf("Usage: mosquitto_sub {[-h host] [-p port] [-u username [-P password]] -t topic | -L URL [-t topic]}\n");
 	printf("                     [-c] [-k keepalive] [-q qos]\n");
-	printf("                     [-C msg_count] [-R] [--retained-only] [-T filter_out] [-U topic ...]\n");
+	printf("                     [-C msg_count] [-E] [-R] [--retained-only] [-T filter_out] [-U topic ...]\n");
 	printf("                     [-F format]\n");
 #ifndef WIN32
 	printf("                     [-W timeout_secs]\n");
@@ -168,6 +171,7 @@ void print_usage(void)
 	printf(" -c : disable 'clean session' (store subscription and pending messages when client disconnects).\n");
 	printf(" -C : disconnect and exit after receiving the 'msg_count' messages.\n");
 	printf(" -d : enable debug messages.\n");
+	printf(" -E : Exit once all subscriptions have been acknowledged by the broker.\n");
 	printf(" -F : output format.\n");
 	printf(" -h : mqtt host to connect to. Defaults to localhost.\n");
 	printf(" -i : id to use for this client. Defaults to mosquitto_sub_ appended with the process id.\n");

man/mosquitto_sub.1.xml

@@ -36,6 +36,7 @@
 			<arg><option>-c</option></arg>
 			<arg><option>-C</option> <replaceable>msg count</replaceable></arg>
 			<arg><option>-d</option></arg>
+			<arg><option>-E</option></arg>
 			<arg><option>-i</option> <replaceable>client_id</replaceable></arg>
 			<arg><option>-I</option> <replaceable>client id prefix</replaceable></arg>
 			<arg><option>-k</option> <replaceable>keepalive time</replaceable></arg>
@@ -209,6 +210,18 @@
 					<para>Enable debug messages.</para>
 				</listitem>
 			</varlistentry>
+			<varlistentry>
+				<term><option>-E</option></term>
+				<listitem>
+					<para>If this option is given,
+						<command>mosquitto_sub</command> will exit immediately
+						that all of its subscriptions have been acknowledged by
+						the broker. In conjunction with <option>-c</option>
+						this allows a durable client session to be initialised
+						on the broker for future use without requiring any
+						messages to be received.</para>
+				</listitem>
+			</varlistentry>
 			<varlistentry>
 				<term><option>-F</option></term>
 				<listitem>