Merged revisions 88719 via svnmerge from

https://origsvn.digium.com/svn/asterisk/branches/1.4

........
r88719 | russell | 2007-11-05 14:40:01 -0600 (Mon, 05 Nov 2007) | 7 lines

Merge changes from asterisk/team/kpfleming/SRV-priority-handling

Previously, the SRV record support in Asterisk was broken.  There was no
guarantee on what record Asterisk would choose to actually use.  This set of
changes improves the situation by ensuring that Asterisk will choose the
highest priority record.

........


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@88740 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/srv.h

@@ -26,9 +26,10 @@
 /*!
   \file srv.h
   \brief Support for DNS SRV records, used in to locate SIP services.
-  \note Note: The Asterisk DNS SRV record support is broken, it only
-	supports the first DNS SRV record and will give no load 
-	balancing or failover support.
+  \note Note: This SRV record support is very minimal; it will only
+        return the first (lowest priority) answer that is received, and
+	has no provisions for the 'weight' elements of the records or
+	retrying if the first returned result fails.
 */
 
 /*! Lookup entry in SRV records Returns 1 if found, 0 if not found, -1 on hangup 

main/srv.c

@@ -52,90 +52,196 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/dns.h"
 #include "asterisk/options.h"
 #include "asterisk/utils.h"
+#include "asterisk/linkedlists.h"
 
 #ifdef __APPLE__
 #undef T_SRV
 #define T_SRV 33
 #endif
 
-struct srv {
+struct srv_entry {
 	unsigned short priority;
 	unsigned short weight;
-	unsigned short portnum;
-} __attribute__ ((__packed__));
+	unsigned short port;
+	unsigned int weight_sum;
+	AST_LIST_ENTRY(srv_entry) list;
+	char host[1];
+};
 
-static int parse_srv(char *host, int hostlen, int *portno, unsigned char *answer, int len, unsigned char *msg)
+struct srv_context {
+	unsigned int have_weights:1;
+	AST_LIST_HEAD_NOLOCK(srv_entries, srv_entry) entries;
+};
+
+static int parse_srv(unsigned char *answer, int len, unsigned char *msg, struct srv_entry **result)
 {
+	struct srv {
+		unsigned short priority;
+		unsigned short weight;
+		unsigned short port;
+	} __attribute__ ((__packed__)) *srv = (struct srv *) answer;
+
 	int res = 0;
-	struct srv *srv = (struct srv *)answer;
 	char repl[256] = "";
+	struct srv_entry *entry;
 
-	if (len < sizeof(struct srv)) {
-		printf("Length too short\n");
+	if (len < sizeof(*srv))
 		return -1;
-	}
-	answer += sizeof(struct srv);
-	len -= sizeof(struct srv);
 
-	if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) < 0) {
+	answer += sizeof(*srv);
+	len -= sizeof(*srv);
+
+	if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) <= 0) {
 		ast_log(LOG_WARNING, "Failed to expand hostname\n");
 		return -1;
 	}
-	if (res && strcmp(repl, ".")) {
-		ast_verb(3, "parse_srv: SRV mapped to host %s, port %d\n", repl, ntohs(srv->portnum));
-		if (host) {
-			ast_copy_string(host, repl, hostlen);
-			host[hostlen-1] = '\0';
-		}
-		if (portno)
-			*portno = ntohs(srv->portnum);
-		return 0;
-	}
-	return -1;
-}
 
-struct srv_context {
-	char *host;
-	int hostlen;
-	int *port;
-};
+	/* the magic value "." for the target domain means that this service
+	   is *NOT* available at the domain we searched */
+	if (!strcmp(repl, "."))
+		return -1;
+
+	if (!(entry = ast_calloc(1, sizeof(*entry) + strlen(repl))))
+		return -1;
+	
+	entry->priority = ntohs(srv->priority);
+	entry->weight = ntohs(srv->weight);
+	entry->port = ntohs(srv->port);
+	strcpy(entry->host, repl);
+
+	*result = entry;
+	
+	return 0;
+}
 
 static int srv_callback(void *context, unsigned char *answer, int len, unsigned char *fullanswer)
 {
-	struct srv_context *c = (struct srv_context *)context;
+	struct srv_context *c = (struct srv_context *) context;
+	struct srv_entry *entry = NULL;
+	struct srv_entry *current;
 
-	if (parse_srv(c->host, c->hostlen, c->port, answer, len, fullanswer)) {
-		ast_log(LOG_WARNING, "Failed to parse srv\n");
+	if (parse_srv(answer, len, fullanswer, &entry))
 		return -1;
+
+	if (entry->weight)
+		c->have_weights = 1;
+
+	AST_LIST_TRAVERSE_SAFE_BEGIN(&c->entries, current, list) {
+		/* insert this entry just before the first existing
+		   entry with a higher priority */
+		if (current->priority <= entry->priority)
+			continue;
+
+		AST_LIST_INSERT_BEFORE_CURRENT(&c->entries, entry, list);
+		entry = NULL;
+		break;
 	}
+	AST_LIST_TRAVERSE_SAFE_END;
 
-	if (!ast_strlen_zero(c->host))
-		return 1;
+	/* if we didn't find a place to insert the entry before an existing
+	   entry, then just add it to the end */
+	if (entry)
+		AST_LIST_INSERT_TAIL(&c->entries, entry, list);
 
 	return 0;
 }
 
+/* Do the bizarre SRV record weight-handling algorithm
+   involving sorting and random number generation...
+
+   See RFC 2782 if you want know why this code does this
+*/
+static void process_weights(struct srv_context *context)
+{
+	struct srv_entry *current;
+	struct srv_entries newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
+
+	while (AST_LIST_FIRST(&context->entries)) {
+		unsigned int random_weight;
+		unsigned int weight_sum;
+		unsigned short cur_priority = AST_LIST_FIRST(&context->entries)->priority;
+		struct srv_entries temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
+		weight_sum = 0;
+
+		AST_LIST_TRAVERSE_SAFE_BEGIN(&context->entries, current, list) {
+			if (current->priority != cur_priority)
+				break;
+
+			AST_LIST_REMOVE_CURRENT(&context->entries, list);
+			AST_LIST_INSERT_TAIL(&temp_list, current, list);
+		}
+		AST_LIST_TRAVERSE_SAFE_END;
+
+		while (AST_LIST_FIRST(&temp_list)) {
+			weight_sum = 0;
+			AST_LIST_TRAVERSE(&temp_list, current, list)
+				current->weight_sum = weight_sum += current->weight;
+
+			/* if all the remaining entries have weight == 0,
+			   then just append them to the result list and quit */
+			if (weight_sum == 0) {
+				AST_LIST_APPEND_LIST(&newlist, &temp_list, list);
+				break;
+			}
+
+			random_weight = 1 + (unsigned int) ((float) weight_sum * (ast_random() / ((float) RAND_MAX + 1.0)));
+
+			AST_LIST_TRAVERSE_SAFE_BEGIN(&temp_list, current, list) {
+				if (current->weight < random_weight)
+					continue;
+
+				AST_LIST_REMOVE_CURRENT(&temp_list, list);
+				AST_LIST_INSERT_TAIL(&newlist, current, list);
+			}
+			AST_LIST_TRAVERSE_SAFE_END;
+		}
+
+	}
+
+	/* now that the new list has been ordered,
+	   put it in place */
+
+	AST_LIST_APPEND_LIST(&context->entries, &newlist, list);
+}
+
 int ast_get_srv(struct ast_channel *chan, char *host, int hostlen, int *port, const char *service)
 {
-	struct srv_context context;
+	struct srv_context context = { .entries = AST_LIST_HEAD_NOLOCK_INIT_VALUE };
+	struct srv_entry *current;
 	int ret;
 
-	context.host = host;
-	context.hostlen = hostlen;
-	context.port = port;
-
 	if (chan && ast_autoservice_start(chan) < 0)
 		return -1;
 
 	ret = ast_search_dns(&context, service, C_IN, T_SRV, srv_callback);
 
+	if (context.have_weights)
+		process_weights(&context);
+
 	if (chan)
 		ret |= ast_autoservice_stop(chan);
 
-	if (ret <= 0) {
+	/* TODO: there could be a "." entry in the returned list of
+	   answers... if so, this requires special handling */
+
+	/* the list of entries will be sorted in the proper selection order
+	   already, so we just need the first one (if any) */
+
+	if ((ret > 0) && (current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
+		ast_copy_string(host, current->host, hostlen);
+		*port = current->port;
+		ast_free(current);
+		if (option_verbose > 3) {
+			ast_verbose(VERBOSE_PREFIX_3 "ast_get_srv: SRV lookup for '%s' mapped to host %s, port %d\n",
+				    service, host, *port);
+		}
+	} else {
 		host[0] = '\0';
 		*port = -1;
-		return ret;
 	}
+
+	while ((current = AST_LIST_REMOVE_HEAD(&context.entries, list)))
+		ast_free(current);
+
 	return ret;
 }