If a websocket SUBSCRIBEs, use new redis context.

This patch emulates functionality already in cmd_run, in the context of
websockets. A client running "[P]SUBSCRIBE" will restrict the commands
allowed to be issued in the context used. Therefore, websocket clients
making subscriptions get their own context, so as to not pollute the
workers' own Redis contexts.

Resource/context deallocation should occur upon http client disconnect,
just as it is for client-specific contexts from cmd_run.

websocket.c

@@ -192,8 +192,21 @@ ws_execute(struct http_client *c, const char *frame, size_t frame_len) {
 			cmd_setup(cmd, c);
 			cmd->is_websocket = 1;
 
-			/* get Redis connection from pool */
-			cmd->ac = (redisAsyncContext*)pool_get_context(c->w->pool);
+			if (c->pub_sub != NULL) {
+				/* This client already has its own connection
+				 * to Redis due to a subscription; use it from
+				 * now on. */
+				cmd->ac = c->pub_sub->ac;
+			} else if (cmd_is_subscribe(cmd)) {
+				/* New subscribe command; make new Redis context
+				 * for this client */
+				cmd->ac = pool_connect(c->w->pool, 0);
+				c->pub_sub = cmd;
+				cmd->pub_sub_client = c;
+			} else {
+				/* get Redis connection from pool */
+				cmd->ac = (redisAsyncContext*)pool_get_context(c->w->pool);
+			}
 
 			/* send it off */
 			cmd_send(cmd, fun_reply);