Add set_tcp_nodelay option to disable Nagle's algorithm.

Bug: https://github.com/eclipse/mosquitto/issues/433

ChangeLog.txt

@@ -60,6 +60,8 @@ Broker:
 - Fix upgrade_outgoing_qos for retained message. Closes #534.
 - Fix CONNACK message not being sent for unauthorised connect on websockets.
   Closes #8.
+- Add set_tcp_nodelay option to allow Nagle's algorithm to be disabled on
+  client sockets. Closes #433.
 
 Client library:
 - Outgoing messages with QoS>1 are no longer retried after a timeout period.

man/mosquitto.conf.5.xml

@@ -585,6 +585,17 @@
 					<para>Reloaded on reload signal.</para>
 				</listitem>
 			</varlistentry>
+			<varlistentry>
+				<term><option>set_tcp_nodelay</option> [ true | false ]</term>
+				<listitem>
+					<para>If set to true, the TCP_NODELAY option will be set on
+							client sockets to disable Nagle's algorithm. This
+							has the effect of reducing latency of some messages
+							at potentially increasing the number of TCP packets
+							being sent. Defaults to false.</para>
+					<para>Reloaded on reload signal.</para>
+				</listitem>
+			</varlistentry>
 			<varlistentry>
 				<term><option>store_clean_interval</option> <replaceable>seconds</replaceable></term>
 				<listitem>

mosquitto.conf

@@ -131,6 +131,11 @@
 # This is a non-standard option explicitly disallowed by the spec.
 #upgrade_outgoing_qos false
 
+# 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.
+#set_tcp_nodelay false
+
 # =================================================================
 # Default listener
 # =================================================================

src/conf.c

@@ -188,6 +188,7 @@ static void config__init_reload(struct mosquitto__config *config)
 	mosquitto__free(config->psk_file);
 	config->psk_file = NULL;
 	config->queue_qos0_messages = false;
+	config->set_tcp_nodelay = false;
 	config->sys_interval = 10;
 	config->upgrade_outgoing_qos = false;
 	if(config->auth_plugins){
@@ -1575,6 +1576,8 @@ int config__read_file_core(struct mosquitto__config *config, bool reload, const
 #else
 					log__printf(NULL, MOSQ_LOG_WARNING, "Warning: Bridge support not available.");
 #endif
+				}else if(!strcmp(token, "set_tcp_nodelay")){
+					if(conf__parse_bool(&token, "set_tcp_nodelay", &config->set_tcp_nodelay, saveptr)) return MOSQ_ERR_INVAL;
 				}else if(!strcmp(token, "start_type")){
 #ifdef WITH_BRIDGE
 					if(reload) continue; // FIXME

src/mosquitto_broker_internal.h

@@ -210,6 +210,7 @@ struct mosquitto__config {
 	char *pid_file;
 	char *psk_file;
 	bool queue_qos0_messages;
+	bool set_tcp_nodelay;
 	int sys_interval;
 	bool upgrade_outgoing_qos;
 	char *user;

src/net.c

@@ -21,6 +21,7 @@ Contributors:
 #include <unistd.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>
+#include <netinet/tcp.h>
 #else
 #include <winsock2.h>
 #include <ws2tcpip.h>
@@ -120,6 +121,12 @@ int net__socket_accept(struct mosquitto_db *db, mosq_sock_t listensock)
 		return -1;
 	}
 #endif
+
+	if(db->config->set_tcp_nodelay){
+		int flag = 1;
+		setsockopt(new_sock, IPPROTO_TCP, TCP_NODELAY, &flag, sizeof(int));
+	}
+
 	new_context = context__init(db, new_sock);
 	if(!new_context){
 		COMPAT_CLOSE(new_sock);