Allow nginx to start even if resolution failed

If nginx can't access the resolver during startup for any reason, do not
prevent nginx from starting and let it recover later.

Also fixes a crash in case the resolution would suddently return no
entities.

ngx_http_upstream_jdomain.c

@@ -221,9 +221,19 @@ ngx_http_upstream_jdomain_get_peer(ngx_peer_connection_t *pc, void *data)
 	if(ngx_resolve_name(ctx) != NGX_OK) {
 		ngx_log_error(NGX_LOG_ALERT, pc->log, 0,
 			"upstream_jdomain: resolve name \"%V\" fail", &ctx->name);
+		urcf->resolved_access = ngx_time();
+		urcf->resolved_status = NGX_JDOMAIN_STATUS_DONE;
 	}
 
 assign:
+	/* If the resolution failed during startup or if resolution returned no entries,
+	   fail all requests until it recovers */
+	if (urcf->resolved_num == 0) {
+		ngx_log_error(NGX_LOG_ALERT, pc->log, 0,
+			"upstream_jdomain: no resolved entry for \"%V\" fail", &urcf->resolved_domain);
+		return NGX_ERROR;
+	}
+
 	ngx_log_debug1(NGX_LOG_DEBUG_HTTP, pc->log, 0,
 		"upstream_jdomain: resolved_num=%ud", urcf->resolved_num); 
 
@@ -370,6 +380,10 @@ ngx_http_upstream_jdomain(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
 	urcf->resolved_max_ips = max_ips;
 	urcf->upstream_retry = retry;
 
+	urcf->resolved_num = 0;
+	/*urcf->resolved_index = 0;*/
+	urcf->resolved_access = ngx_time();
+
 	ngx_memzero(&u, sizeof(ngx_url_t));
 	u.url = value[1];
 	u.default_port = (in_port_t) urcf->default_port;
@@ -379,11 +393,18 @@ ngx_http_upstream_jdomain(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
 			ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
 				"%s in upstream \"%V\"", u.err, &u.url);
 		}
-
+
+		/* In case the error is a resolution issue, do not prevent nginx from starting
+			and let the resolution happen again later */
+		const char *rslverr = "host not found";
+		if (strlen(u.err) == strlen(rslverr)
+			&& ngx_strncmp(u.err, rslverr, strlen(rslverr)) == 0) {
+			return NGX_CONF_OK;
+		}
+
 		return NGX_CONF_ERROR;
 	}
 
-	urcf->resolved_num = 0;
 	for(i = 0; i < u.naddrs ;i++){
 		paddr = &urcf->peers[urcf->resolved_num];
 		paddr->sockaddr = *(struct sockaddr*)u.addrs[i].sockaddr;
@@ -396,8 +417,6 @@ ngx_http_upstream_jdomain(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
 		if( urcf->resolved_num >= urcf->resolved_max_ips)
 			break;
 	}
-	/*urcf->resolved_index = 0;*/
-	urcf->resolved_access = ngx_time();
 
 	return NGX_CONF_OK;