Reduce OPTIONS preflight requests in browsers.

Each request with an Accept-Datetime header requires
an OPTIONS preflight request in the browser.
To avoid this, only the first request to a "root" resource
(e.g., http://example.org/fragments) gets the header;
subsequent requests to "related" resources
(e.g., http://example.org/fragments?subject=bar)
will not carry this header anymore.

lib/util/Request-browser.js

@@ -3,19 +3,29 @@
 
 var EventEmitter = require('events').EventEmitter,
     SingleIterator = require('../../lib/iterators/Iterator').SingleIterator,
+    parseLink = require('parse-link-header'),
     _ = require('lodash');
 
 require('setimmediate');
 
 // Headers we cannot send (see https://www.w3.org/TR/XMLHttpRequest/#the-setrequestheader()-method)
 var UNSAFE_REQUEST_HEADERS = ['accept-encoding', 'user-agent', 'referer'];
 // Headers we need to obtain
-var RESPONSE_HEADERS = ['content-type', 'link', 'memento-datetime'];
+var RESPONSE_HEADERS = ['content-type', 'content-location', 'link', 'memento-datetime'];
+
+// Resources that were already time-negotiated
+var negotiatedResources = Object.create(null);
 
 // Creates an HTTP request with the given settings
 function createRequest(settings) {
   var request = new EventEmitter();
 
+  // PERFORMANCE HACK:
+  // Reduce OPTIONS preflight requests by removing the Accept-Datetime header
+  // on requests for resources that are presumed to have been time-negotiated
+  if (negotiatedResources[removeQuery(settings.url)])
+    delete settings.headers['accept-datetime'];
+
   // Delegate the request to jQuery's AJAX module
   var jqXHR = jQuery.ajax({
     url: settings.url,
@@ -29,6 +39,18 @@ function createRequest(settings) {
     response.statusCode = jqXHR.status;
     response.headers = _.object(RESPONSE_HEADERS, RESPONSE_HEADERS.map(jqXHR.getResponseHeader));
     request.emit('response', response);
+
+    // If the resource was time-negotiated, store its "base" URI (= no query string)
+    if (settings.headers['accept-datetime'] && response.headers['memento-datetime']) {
+      var resource = removeQuery(response.headers['content-location'] || settings.url);
+      if (!negotiatedResources[resource]) {
+        // Ensure the resource is not a timegate
+        var links = response.headers.link && parseLink(response.headers.link),
+            timegate = removeQuery(links && links.timegate && links.timegate.url);
+        if (resource !== timegate)
+          negotiatedResources[resource] = true;
+      }
+    }
   })
   // Emit an error if the request fails
   .fail(function () {
@@ -40,4 +62,9 @@ function createRequest(settings) {
   return request;
 }
 
+// Removes the query string from a URL
+function removeQuery(url) {
+  return url ? url.replace(/\?.*$/, '') : '';
+}
+
 module.exports = createRequest;