Add MOSQ_OPT_DISABLE_SOCKETPAIR.

ChangeLog.txt

@@ -1,3 +1,13 @@
+2.1.0 - 2021-xx-xx
+==================
+
+Client library:
+- Add MOSQ_OPT_DISABLE_SOCKETPAIR to allow the disabling of the socketpair
+  feature that allows the network thread to be woken from select() by another
+  thread when e.g.  mosquitto_publish() is called. This reduces the number of
+  sockets used by each client by two.
+
+
 2.0.6 - 2021-01-xx
 ==================
 

include/mosquitto.h

@@ -138,6 +138,7 @@ enum mosq_opt_t {
 	MOSQ_OPT_TCP_NODELAY = 11,
 	MOSQ_OPT_BIND_ADDRESS = 12,
 	MOSQ_OPT_TLS_USE_OS_CERTS = 13,
+	MOSQ_OPT_DISABLE_SOCKETPAIR = 14,
 };
 
 
@@ -1540,6 +1541,14 @@ libmosq_EXPORT int mosquitto_opts_set(struct mosquitto *mosq, enum mosq_opt_t op
  *	MOSQ_OPT_TLS_USE_OS_CERTS - Set to 1 to instruct the client to load and
  *	          trust OS provided CA certificates for use with TLS connections.
  *	          Set to 0 (the default) to only use manually specified CA certs.
+ *
+ *	MOSQ_OPT_DISABLE_SOCKETPAIR - By default, each client connected will create
+ *            an internal pair of connected sockets to allow the network thread
+ *            to be notified and woken up if another thread calls
+ *            <mosquitto_publish> or other similar command. If you are
+ *            operating with an external loop, this is not necessary and
+ *            consumes an extra two sockets per client. Set this option to 1 to
+ *            disable the use of the socket pair.
  */
 libmosq_EXPORT int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t option, int value);
 

lib/mosquitto.c

@@ -213,11 +213,13 @@ int mosquitto_reinitialise(struct mosquitto *mosq, const char *id, bool clean_st
 	pthread_mutex_init(&mosq->mid_mutex, NULL);
 	mosq->thread_id = pthread_self();
 #endif
-	/* This must be after pthread_mutex_init(), otherwise the log mutex may be
-	 * used before being initialised. */
-	if(net__socketpair(&mosq->sockpairR, &mosq->sockpairW)){
-		log__printf(mosq, MOSQ_LOG_WARNING,
-				"Warning: Unable to open socket pair, outgoing publish commands may be delayed.");
+	if(mosq->disable_socketpair == false){
+		/* This must be after pthread_mutex_init(), otherwise the log mutex may be
+		* used before being initialised. */
+		if(net__socketpair(&mosq->sockpairR, &mosq->sockpairW)){
+			log__printf(mosq, MOSQ_LOG_WARNING,
+					"Warning: Unable to open socket pair, outgoing publish commands may be delayed.");
+		}
 	}
 
 	return MOSQ_ERR_SUCCESS;

lib/mosquitto_internal.h

@@ -332,6 +332,7 @@ struct mosquitto {
 	unsigned int reconnect_delay;
 	unsigned int reconnect_delay_max;
 	bool reconnect_exponential_backoff;
+	bool disable_socketpair;
 	char threaded;
 	struct mosquitto__packet *out_packet_last;
 	mosquitto_property *connect_properties;

lib/options.c

@@ -416,6 +416,10 @@ int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t option, int val
 	if(!mosq) return MOSQ_ERR_INVAL;
 
 	switch(option){
+		case MOSQ_OPT_DISABLE_SOCKETPAIR:
+			mosq->disable_socketpair = (bool)value;
+			break;
+
 		case MOSQ_OPT_PROTOCOL_VERSION:
 			if(value == MQTT_PROTOCOL_V31){
 				mosq->protocol = mosq_p_mqtt31;