cachedb_redis: add ASK redirect handling and fix hash slot calculation

Add support for Redis ASK redirects during cluster resharding.  When
a slot is being migrated between nodes, Redis returns an ASK response
instead of MOVED.  Unlike MOVED (permanent redirect), ASK is a
one-time redirect that requires sending the ASKING command to the
target node before retrying the original query.

The implementation:
 - Detects ASK responses alongside existing MOVED handling
 - Sends ASKING command to the target node before retrying
 - Reuses the MOVED redirect infrastructure (endpoint lookup,
   reconnection, retry logic)

Also refactor parse_moved_reply() into parse_redirect_reply() that
accepts the prefix as a parameter, with inline wrappers
parse_moved_reply() and parse_ask_reply() for backward compatibility.

Fix the hash slot calculation in redisHash(): replace the bitwise AND
operator with modulo using the standard Redis cluster slot count
(16384).  The previous code used '& con->slots_assigned' which only
produces correct results when slots_assigned is a power-of-2 minus 1.
Using the spec-defined constant is always correct.

Partially addresses #2811

Author: dondetir

modules/cachedb_redis/cachedb_redis_dbase.c

@@ -564,48 +564,83 @@ static int _redis_run_command(cachedb_con *connection, redisReply **rpl, str *ke
 					reply,reply?(unsigned)reply->len:7,reply?reply->str:"FAILURE",
 					node->context->errstr);
 
-				if (match_prefix(reply->str, reply->len, MOVED_PREFIX, MOVED_PREFIX_LEN)) {
-    					// It's a MOVED response
+				if (match_prefix(reply->str, reply->len, MOVED_PREFIX, MOVED_PREFIX_LEN) ||
+						match_prefix(reply->str, reply->len, ASK_PREFIX, ASK_PREFIX_LEN)) {
+					int is_ask = match_prefix(reply->str, reply->len,
+						ASK_PREFIX, ASK_PREFIX_LEN);
 					redis_moved *moved_info = pkg_malloc(sizeof(redis_moved));
-						if (!moved_info) {
-						LM_ERR("cachedb_redis: Unable to allocate redis_moved struct, no more pkg memory\n");
-							freeReplyObject(reply);
-							reply = NULL;
-							goto try_next_con;
-					} else {
-							if (parse_moved_reply(reply, moved_info) < 0) {
-							LM_ERR("cachedb_redis: Unable to parse MOVED reply\n");
-							pkg_free(moved_info);
-							moved_info = NULL;
-								freeReplyObject(reply);
-							goto try_next_con;
-						}
-
-						LM_DBG("cachedb_redis: MOVED slot: [%d] endpoint: [%.*s] port: [%d]\n", moved_info->slot, moved_info->endpoint.len, moved_info->endpoint.s, moved_info->port);
-						node = get_redis_connection_by_endpoint(con, moved_info);
+					if (!moved_info) {
+						LM_ERR("Unable to allocate redis_moved struct,"
+							" no more pkg memory\n");
+						freeReplyObject(reply);
+						reply = NULL;
+						goto try_next_con;
+					}
 
+					if ((is_ask ?
+							parse_ask_reply(reply, moved_info) :
+							parse_moved_reply(reply, moved_info)) < 0) {
+						LM_ERR("Unable to parse %s reply\n",
+							is_ask ? "ASK" : "MOVED");
 						pkg_free(moved_info);
-						moved_info = NULL;
 						freeReplyObject(reply);
-						reply = NULL;
+						goto try_next_con;
+					}
 
-						if (node == NULL) {
-							LM_ERR("Unable to locate connection by endpoint\n");
-							last_err = -10;
+					LM_DBG("%s slot: [%d] endpoint: [%.*s]"
+						" port: [%d]\n",
+						is_ask ? "ASK" : "MOVED",
+						moved_info->slot,
+						moved_info->endpoint.len,
+						moved_info->endpoint.s,
+						moved_info->port);
+					node = get_redis_connection_by_endpoint(
+						con, moved_info);
+
+					pkg_free(moved_info);
+					freeReplyObject(reply);
+					reply = NULL;
+
+					if (node == NULL) {
+						LM_ERR("Unable to locate connection"
+							" by endpoint\n");
+						last_err = -10;
+						goto try_next_con;
+					}
+
+					if (node->context == NULL) {
+						if (redis_reconnect_node(con,node) < 0) {
+							LM_ERR("Unable to reconnect to"
+								" node %s:%d\n",
+								node->ip, node->port);
+							last_err = -1;
 							goto try_next_con;
 						}
+					}
 
-						if (node->context == NULL) {
-							if (redis_reconnect_node(con,node) < 0) {
-							  LM_ERR("Unable to reconnect to node %p endpoint: %s:%d\n", node, node->ip, node->port);
-								last_err = -1;
-								goto try_next_con;
-							}
+					if (is_ask) {
+						/* ASK requires sending ASKING before
+						 * retrying the command on the new node */
+						redisReply *asking_reply;
+						asking_reply = redisCommand(
+							node->context, "ASKING");
+						if (!asking_reply ||
+								asking_reply->type ==
+								REDIS_REPLY_ERROR) {
+							LM_ERR("ASKING command failed"
+								" on %s:%d\n",
+								node->ip, node->port);
+							if (asking_reply)
+								freeReplyObject(
+									asking_reply);
+							last_err = -1;
+							goto try_next_con;
 						}
-
-						i = QUERY_ATTEMPTS; // New node that is the target being MOVED to, should have the attempts reset
-						continue;
+						freeReplyObject(asking_reply);
 					}
+
+					i = QUERY_ATTEMPTS;
+					continue;
 				}
 
 				freeReplyObject(reply);

modules/cachedb_redis/cachedb_redis_utils.c

@@ -81,7 +81,7 @@ uint16_t crc16(const char *buf, int len)
 
 unsigned int redisHash(redis_con *con, str* key)
 {
-	return crc16(key->s,key->len) & con->slots_assigned;
+	return crc16(key->s,key->len) % REDIS_CLUSTER_SLOTS;
 }
 
 cluster_node *get_redis_connection(redis_con *con,str *key)
@@ -367,35 +367,35 @@ int build_cluster_nodes(redis_con *con,char *info,int size)
  } redis_moved;
 
 */
-int parse_moved_reply(redisReply *reply, redis_moved *out) {
-	int i;
+int parse_redirect_reply(redisReply *reply, redis_moved *out,
+		const char *prefix, size_t prefix_len) {
+	size_t i;
 	int slot = 0;
 	const char *p;
 	const char *end;
 	const char *host_start;
 	const char *colon = NULL;
 	const char *port_start;
-	int port = REDIS_DF_PORT; // Default to Redis standard port
+	int port = REDIS_DF_PORT;
 
-	if (!reply || !reply->str || reply->len < MOVED_PREFIX_LEN || !out)
+	if (!reply || !reply->str || (size_t)reply->len < prefix_len || !out)
 		return ERR_INVALID_REPLY;
 
 	p = reply->str;
 	end = reply->str + reply->len;
 
-	for (i = 0; i < MOVED_PREFIX_LEN; ++i) {
-		if (p[i] != MOVED_PREFIX[i]) {
-		return ERR_INVALID_REPLY;
-		}
+	for (i = 0; i < prefix_len; ++i) {
+		if (p[i] != prefix[i])
+			return ERR_INVALID_REPLY;
 	}
-	p += MOVED_PREFIX_LEN;
+	p += prefix_len;
 
 	// Parse slot number
 	while (p < end && *p >= '0' && *p <= '9') {
 		slot = slot * 10 + (*p - '0');
 		p++;
 	}
-	if (slot == 0 && (p == reply->str + MOVED_PREFIX_LEN || *(p - 1) < '0' || *(p - 1) > '9'))
+	if (slot == 0 && (p == reply->str + prefix_len || *(p - 1) < '0' || *(p - 1) > '9'))
 		return ERR_INVALID_SLOT;
 
 	// Skip spaces

modules/cachedb_redis/cachedb_redis_utils.h

@@ -27,10 +27,14 @@
 #define CACHEDB_REDIS_UTILSH
 
 #define REDIS_DF_PORT  6379
+#define REDIS_CLUSTER_SLOTS 16384
 
 #define MOVED_PREFIX "MOVED "
 #define MOVED_PREFIX_LEN (sizeof(MOVED_PREFIX) - 1)
 
+#define ASK_PREFIX "ASK "
+#define ASK_PREFIX_LEN (sizeof(ASK_PREFIX) - 1)
+
 #define ERR_INVALID_REPLY -1
 #define ERR_INVALID_SLOT -2
 #define ERR_INVALID_PORT -3
@@ -41,7 +45,16 @@ int build_cluster_nodes(redis_con *con,char *info,int size);
 cluster_node *get_redis_connection(redis_con *con,str *key);
 cluster_node *get_redis_connection_by_endpoint(redis_con *con, redis_moved *redis_info);
 void destroy_cluster_nodes(redis_con *con);
-int parse_moved_reply(redisReply *reply, redis_moved *out);
+int parse_redirect_reply(redisReply *reply, redis_moved *out,
+		const char *prefix, size_t prefix_len);
+
+static inline int parse_moved_reply(redisReply *reply, redis_moved *out) {
+	return parse_redirect_reply(reply, out, MOVED_PREFIX, MOVED_PREFIX_LEN);
+}
+
+static inline int parse_ask_reply(redisReply *reply, redis_moved *out) {
+	return parse_redirect_reply(reply, out, ASK_PREFIX, ASK_PREFIX_LEN);
+}
 
 static inline int match_prefix(const char *buf, size_t len, const char *prefix, size_t prefix_len) {
 	size_t i;