[MEDIUM] Implement "track [<backend>/]<server>"

This patch implements ability to set the current state of one server
by tracking another one. It:
 - adds two variables: *tracknext, *tracked to struct server
 - implements findserver(), similar to findproxy()
 - adds "track" keyword accepting both "proxy/server" and "server" (assuming current proxy)
 - verifies if both checks and tracking is not enabled at the same time
 - changes set_server_down() to notify tracking server
 - creates set_server_up(), set_server_disabled(), set_server_enabled() by
   moving the code from process_chk() and adding notifications
 - changes stats to show a name of tracked server instead of Chk/Dwn/Dwntime(html)
   or by adding new variable (csv)

Changes from the previuos version:
 - it is possibile to track independently of the declaration order
 - one extra comma bug is fixed
 - new condition to check if there is no disable-on-404 inconsistency

TODO

@@ -174,9 +174,6 @@ TODO for 1.3
     filters and backend, on which every entity could rely.
   - implement 'on uri <uri> <proxy>', 'on host <host> <proxy>'
   - remove the first now useless hop in hdr_idx
-  - implement "track XXX.YYY" for each server as an alternative to
-    health checks. This will automatically set the server state to
-    the same as server YYY of proxy XXX.
   - balance on URI hash (specify length or depth)
   - balance on any header hash (eg: host)
   - balance with redirections to real servers

doc/configuration.txt

@@ -3893,6 +3893,13 @@ source <addr>[:<port>] [usesrc { <addr2>[:<port2>] | client | clientip } ]
   as the backend "source" keyword, except that it only applies to the server
   referencing it. Please consult the "source" keyword for details.
 
+track [<proxy>/]<server>
+  This option enables ability to set the current state of the server by
+  tracking another one. Only a server with checks enabled can be tracked
+  so it is not possible for example to track a server that tracks another
+  one. If <proxy> is omitted the current one is used. If disable-on-404 is
+  used, it has to be enabled on both proxies.
+
 weight <weight>
   The "weight" parameter is used to adjust the server's weight relative to
   other servers. All servers will receive a load proportional to their weight

include/proto/proxy.h

@@ -36,6 +36,7 @@ void listen_proxies(void);
 const char *proxy_cap_str(int cap);
 const char *proxy_mode_str(int mode);
 struct proxy *findproxy(const char *name, int mode, int cap);
+struct server *findserver(const struct proxy *px, const char *name);
 int proxy_parse_timeout(const char **args, struct proxy *proxy,
 			struct proxy *defpx, char *err, int errlen);
 

include/types/server.h

@@ -91,6 +91,8 @@ struct server {
 	struct sockaddr_in tproxy_addr;		/* non-local address we want to bind to for connect() */
 #endif
 
+	struct server *tracknext, *tracked;	/* next server in a tracking list, tracked server */
+	char *trackit;				/* temporary variable to make assignment deferrable */
 	struct sockaddr_in check_addr;		/* the address to check, if different from <addr> */
 	short check_port;			/* the port to use for the health checks */
 	int health;				/* 0->rise-1 = bad; rise->rise+fall-1 = good */

src/cfgparse.c

@@ -1625,6 +1625,18 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int inv)
 				newsrv->slowstart = (val + 999) / 1000;
 				cur_arg += 2;
 			}
+			else if (!strcmp(args[cur_arg], "track")) {
+
+				if (!*args[cur_arg + 1]) {
+					Alert("parsing [%s:%d]: 'track' expects [<proxy>/]<server> as argument.\n",
+						file, linenum);
+					return -1;
+				}
+
+				newsrv->trackit = strdup(args[cur_arg + 1]);
+
+				cur_arg += 2;
+			}
 			else if (!strcmp(args[cur_arg], "check")) {
 				global.maxsock++;
 				do_check = 1;
@@ -1684,13 +1696,19 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int inv)
 				return -1;
 			}
 			else {
-				Alert("parsing [%s:%d] : server %s only supports options 'backup', 'cookie', 'redir', 'check', 'inter', 'fastinter', 'downinter', 'rise', 'fall', 'addr', 'port', 'source', 'minconn', 'maxconn', 'maxqueue', 'slowstart' and 'weight'.\n",
+				Alert("parsing [%s:%d] : server %s only supports options 'backup', 'cookie', 'redir', 'check', 'track', 'inter', 'fastinter', 'downinter', 'rise', 'fall', 'addr', 'port', 'source', 'minconn', 'maxconn', 'maxqueue', 'slowstart' and 'weight'.\n",
 				      file, linenum, newsrv->id);
 				return -1;
 			}
 		}
 
 		if (do_check) {
+			if (newsrv->trackit) {
+				Alert("parsing [%s:%d]: unable to enable checks and tracking at the same time!\n",
+					file, linenum);
+				return -1;
+			}
+
 			if (!newsrv->check_port && newsrv->check_addr.sin_port)
 				newsrv->check_port = newsrv->check_addr.sin_port;
 
@@ -2913,6 +2931,7 @@ int readcfgfile(const char *file)
 		/*
 		 * If this server supports a maxconn parameter, it needs a dedicated
 		 * tasks to fill the emptied slots when a connection leaves.
+		 * Also, resolve deferred tracking dependency if needed.
 		 */
 		newsrv = curproxy->srv;
 		while (newsrv != NULL) {
@@ -2950,6 +2969,65 @@ int readcfgfile(const char *file)
 				tv_eternity(&t->expire);
 				task_queue(t);
 			}
+
+			if (newsrv->trackit) {
+				struct proxy *px;
+				struct server *srv;
+				char *pname, *sname;
+
+				pname = newsrv->trackit;
+				sname = strrchr(pname, '/');
+
+				if (sname)
+					*sname++ = '\0';
+				else {
+					sname = pname;
+					pname = NULL;
+				}
+
+				if (pname) {
+					px = findproxy(pname, curproxy->mode, PR_CAP_BE);
+					if (!px) {
+						Alert("parsing %s, %s '%s', server '%s': unable to find required proxy '%s' for tracking.\n",
+							file, proxy_type_str(curproxy), curproxy->id,
+							newsrv->id, pname);
+						return -1;
+					}
+				} else
+					px = curproxy;
+
+				srv = findserver(px, sname);
+				if (!srv) {
+					Alert("parsing %s, %s '%s', server '%s': unable to find required server '%s' for tracking.\n",
+						file, proxy_type_str(curproxy), curproxy->id,
+						newsrv->id, sname);
+					return -1;
+				}
+
+				if (!(srv->state & SRV_CHECKED)) {
+					Alert("parsing %s, %s '%s', server '%s': unable to use %s/%s for "
+						"tracing as it does not have checks enabled.\n",
+						file, proxy_type_str(curproxy), curproxy->id,
+						newsrv->id, px->id, srv->id);
+					return -1;
+				}
+
+				if (curproxy != px &&
+					(curproxy->options & PR_O_DISABLE404) != (px->options & PR_O_DISABLE404)) {
+					Alert("parsing %s, %s '%s', server '%s': unable to use %s/%s for"
+						"tracing: disable-on-404 option inconsistency.\n",
+						file, proxy_type_str(curproxy), curproxy->id,
+						newsrv->id, px->id, srv->id);
+					return -1;
+				}
+
+				newsrv->tracked = srv;
+				newsrv->tracknext = srv->tracknext;
+				srv->tracknext = newsrv;
+
+				free(newsrv->trackit);
+			}
+
 			newsrv = newsrv->next;
 		}