MINOR: tcp: Return TCP statistics like RTT and RTT variance

This patch adds 4 new sample fetches which returns the RTT of the
established connexion and the RTT variance. The established connection
can be between the client and HAProxy, and between HAProxy and the
server. This is very useful for statistics. A great use case is the
estimation of the TCP connection time of the client. Note that the
RTT of the server side is not so interesting because we already have
the connect() time.

doc/configuration.txt

@@ -12737,6 +12737,22 @@ dst_port : integer
   a same server, or to pass the destination port information to a server using
   an HTTP header.
 
+fc_rtt(<unit>) : integer
+  Returns the Round Trip Time (RTT) measured by the kernel for the client
+  connection. <unit> is facultative, by default the unit is milliseconds. <unit>
+  can be set to "ms" for milliseconds or "us" for microseconds. If the server
+  connection is not established, if the connection is not TCP or if the
+  operating system does not support TCP_INFO, for example Linux kernels before
+  2.4, the sample fetch fails.
+
+fc_rttvar(<unit>) : integer
+  Returns the Round Trip Time (RTT) variance measured by the kernel for the
+  client connection. <unit> is facultative, by default the unit is milliseconds.
+  <unit> can be set to "ms" for milliseconds or "us" for microseconds. If the
+  server connection is not established, if the connection is not TCP or if the
+  operating system does not support TCP_INFO, for example Linux kernels before
+  2.4, the sample fetch fails.
+
 fe_id : integer
   Returns an integer containing the current frontend's id. It can be used in
   backends to check from which backend it was called, or to stick all users
@@ -13117,7 +13133,6 @@ srv_id : integer
   While it's almost only used with ACLs, it may be used for logging or
   debugging.
 
-
 7.3.4. Fetching samples at Layer 5
 ----------------------------------
 

src/proto_tcp.c

@@ -2313,6 +2313,84 @@ smp_fetch_dport(const struct arg *args, struct sample *smp, const char *kw, void
 	return 1;
 }
 
+#ifdef TCP_INFO
+
+/* Returns some tcp_info data is its avalaible. "dir" must be set to 0 if
+ * the client connection is require, otherwise it is set to 1. "val" represents
+ * the required value. Use 0 for rtt and 1 for rttavg. "unit" is the expected unit
+ * by default, the rtt is in us. Id "unit" is set to 0, the unit is us, if it is
+ * set to 1, the untis are milliseconds.
+ * If the function fails it returns 0, otherwise it returns 1 and "result" is filled.
+ */
+static inline int get_tcp_info(const struct arg *args, struct sample *smp,
+                               int dir, int val)
+{
+	struct connection *conn;
+	struct tcp_info info;
+	socklen_t optlen;
+
+	/* strm can be null. */
+	if (!smp->strm)
+		return 0;
+
+	/* get the object associated with the stream interface.The
+	 * object can be other thing than a connection. For example,
+	 * it be a appctx. */
+	conn = objt_conn(smp->strm->si[dir].end);
+	if (!conn)
+		return 0;
+
+	/* The fd may not be avalaible for the tcp_info struct, and the
+	  syscal can fail. */
+	optlen = sizeof(info);
+	if (getsockopt(conn->t.sock.fd, SOL_TCP, TCP_INFO, &info, &optlen) == -1)
+		return 0;
+
+	/* extract the value. */
+	smp->data.type = SMP_T_SINT;
+	switch (val) {
+	case 0:  smp->data.u.sint = info.tcpi_rtt;    break;
+	case 1:  smp->data.u.sint = info.tcpi_rttvar; break;
+	default: return 0;
+	}
+
+	/* Convert the value as expected. */
+	if (args) {
+		if (args[0].type == ARGT_STR) {
+			if (strcmp(args[0].data.str.str, "us") == 0) {
+				/* Do nothing. */
+			} else if (strcmp(args[0].data.str.str, "ms") == 0) {
+				smp->data.u.sint = (smp->data.u.sint + 500) / 1000;
+			} else
+				return 0;
+		} else if (args[0].type == ARGT_STOP) {
+			smp->data.u.sint = (smp->data.u.sint + 500) / 1000;
+		} else
+			return 0;
+	}
+
+	return 1;
+}
+
+/* get the mean rtt of a client connexion */
+static int
+smp_fetch_fc_rtt(const struct arg *args, struct sample *smp, const char *kw, void *private)
+{
+	if (!get_tcp_info(args, smp, 0, 0))
+		return 0;
+	return 1;
+}
+
+/* get the variance of the mean rtt of a client connexion */
+static int
+smp_fetch_fc_rttvar(const struct arg *args, struct sample *smp, const char *kw, void *private)
+{
+	if (!get_tcp_info(args, smp, 0, 1))
+		return 0;
+	return 1;
+}
+#endif
+
 #ifdef IPV6_V6ONLY
 /* parse the "v4v6" bind keyword */
 static int bind_parse_v4v6(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
@@ -2540,6 +2618,10 @@ static struct sample_fetch_kw_list sample_fetch_keywords = {ILH, {
 	{ "dst_port", smp_fetch_dport, 0, NULL, SMP_T_SINT, SMP_USE_L4CLI },
 	{ "src",      smp_fetch_src,   0, NULL, SMP_T_IPV4, SMP_USE_L4CLI },
 	{ "src_port", smp_fetch_sport, 0, NULL, SMP_T_SINT, SMP_USE_L4CLI },
+#ifdef TCP_INFO
+	{ "fc_rtt",    smp_fetch_fc_rtt,    ARG1(0,STR), NULL, SMP_T_SINT, SMP_USE_L4CLI },
+	{ "fc_rttvar", smp_fetch_fc_rttvar, ARG1(0,STR), NULL, SMP_T_SINT, SMP_USE_L4CLI },
+#endif
 	{ /* END */ },
 }};