[WIP] Support internal redirects

TODO:
- Move proxyRequest elsewhere, and attach the request state to req
- Investigate the implications of calling req.emit('end');
  (It's called in order to trigger reverseProxy.end(), needed to start
  the redirect)

lib/cors-anywhere.js

@@ -86,6 +86,8 @@ function isForbidden(host) {
  * @param proxy {HttpProxy}
  * @param location {object} See parseURL
  * @param requestState.proxyBaseUrl {string} Base URL of the CORS API endpoint.
+ * @param requestState.maxRedirects {number} Maximum number of redirects
+ * @param requestState.redirectCount_ {number} Internally used to count redirects
  */
 function proxyRequest(req, res, proxy, location, requestState) {
   if (isForbidden(location.hostname)) {
@@ -107,6 +109,20 @@ function proxyRequest(req, res, proxy, location, requestState) {
       var locationHeader = response.headers['location'];
       if (locationHeader) {
         locationHeader = url.resolve(location.full_url, locationHeader);
+
+        if (statusCode === 301 || statusCode === 302 || statusCode === 303) {
+          // Exclude 307 & 308, because they are rare, and require preserving the method + request body
+          requestState.redirectCount_ = requestState.requestState_ + 1 || 1;
+          if (requestState.redirectCount_ <= requestState.maxRedirects) {
+            req.method = 'GET';
+            proxyRequest(req, res, proxy, parseURL(locationHeader), requestState);
+
+            response.end();
+            // The proxyResponse event is wrapped in a try-catch, throwing an error
+            // prevents the response from being passed to the client.
+            throw new Error('Prevent current response from being passed through.');
+          }
+        }
         response.headers['location'] = requestState.proxyBaseUrl + '/' + locationHeader;
       }
     }
@@ -127,6 +143,9 @@ function proxyRequest(req, res, proxy, location, requestState) {
       https: location.isHttps
     }
   });
+  if (requestState.redirectCount_) {
+    req.emit('end');
+  }
 }
 
 
@@ -164,6 +183,7 @@ function parseURL(req_url) {
 // Request handler factory
 var getHandler = exports.getHandler = function(options) {
   var corsAnywhere = {
+    maxRedirects: 5,          // Maximum number of redirects to be followed.
     requireHeader: null,      // Require a header to be set?
     removeHeaders: []         // Strip these request headers
   };
@@ -246,6 +266,7 @@ var getHandler = exports.getHandler = function(options) {
     });
 
     proxyRequest(req, res, proxy, location, {
+      maxRedirects: corsAnywhere.maxRedirects,
       proxyBaseUrl: proxyBaseUrl
     });
   };