Add TCP_NODELAY support to lib and clients.

Closes #1526. Thanks to Felix Moessbauer.

lib/mosquitto.h

@@ -113,6 +113,7 @@ enum mosq_opt_t {
 	MOSQ_OPT_TLS_ENGINE_KPASS_SHA1 = 8,
 	MOSQ_OPT_TLS_OCSP_REQUIRED = 9,
 	MOSQ_OPT_TLS_ALPN = 10,
+	MOSQ_OPT_TCP_NODELAY = 11,
 };
 
 
@@ -1432,6 +1433,12 @@ libmosq_EXPORT int mosquitto_opts_set(struct mosquitto *mosq, enum mosq_opt_t op
  *	value -  the option specific value.
  *
  * Options:
+ *	MOSQ_OPT_TCP_NODELAY -
+ *	          Set to 1 to disable Nagle's algorithm on client sockets. This has
+ *	          the effect of reducing latency of individual messages at the
+ *	          potential cost of increasing the number of packets being sent.
+ *	          Defaults to 0, which means Nagle remains enabled.
+ *
  *	MOSQ_OPT_PROTOCOL_VERSION -
  *	          Value must be set to either MQTT_PROTOCOL_V31,
  *	          MQTT_PROTOCOL_V311, or MQTT_PROTOCOL_V5. Must be set before the

lib/mosquitto_internal.h

@@ -337,6 +337,7 @@ struct mosquitto {
 #endif
 	uint8_t maximum_qos;
 	uint8_t retain_available;
+	bool tcp_nodelay;
 
 #ifdef WITH_BROKER
 	UT_hash_handle hh_id;

lib/net_mosq.c

@@ -25,6 +25,7 @@ Contributors:
 #ifndef WIN32
 #define _GNU_SOURCE
 #include <netdb.h>
+#include <netinet/tcp.h>
 #include <sys/socket.h>
 #include <unistd.h>
 #else
@@ -887,6 +888,13 @@ int net__socket_connect(struct mosquitto *mosq, const char *host, uint16_t port,
 
 	mosq->sock = sock;
 
+	if(mosq->tcp_nodelay){
+		int flag = 1;
+		if(setsockopt(mosq->sock, IPPROTO_TCP, TCP_NODELAY, &flag, sizeof(int)) != 0){
+			log__printf(mosq, MOSQ_LOG_WARNING, "Warning: Unable to set TCP_NODELAY.");
+		}
+	}
+
 #if defined(WITH_SOCKS) && !defined(WITH_BROKER)
 	if(!mosq->socks5_host)
 #endif

lib/options.c

@@ -447,6 +447,10 @@ int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t option, int val
 #endif
 			break;
 
+		case MOSQ_OPT_TCP_NODELAY:
+			mosq->tcp_nodelay = (bool)value;
+			break;
+
 		default:
 			return MOSQ_ERR_INVAL;
 	}