Adds support for protocol relative URLs in location header

`url.parse` doesn't parse protocol relative URLs, but they are valid
in location headers according to RFC7231
(https://tools.ietf.org/html/rfc7231#section-7.1.2, and specified in
RFC3986 https://tools.ietf.org/html/rfc3986#section-4.2)

This commit fixes that by temporarily adding an `http:` to a URL that
begins with `//` before parsing and removing the protocol before
formatting.

lib/http-proxy/passes/web-outgoing.js

@@ -51,8 +51,15 @@ module.exports = { // <--
     if ((options.hostRewrite || options.autoRewrite || options.protocolRewrite)
         && proxyRes.headers['location']
         && redirectRegex.test(proxyRes.statusCode)) {
+      var protocolPrefix = '';
+      if (proxyRes.headers['location'].substr(0, 2) === '//') {
+        // Make protocol relative URLs simply http
+        // This just enables correct parsing in url.parse,
+        // we will strip the protocol on the end result anyway
+        protocolPrefix = 'http:';
+      }
       var target = url.parse(options.target);
-      var u = url.parse(proxyRes.headers['location']);
+      var u = url.parse(protocolPrefix + proxyRes.headers['location']);
 
       // make sure the redirected host matches the target host before rewriting
       if (target.host != u.host) {
@@ -68,6 +75,13 @@ module.exports = { // <--
         u.protocol = options.protocolRewrite;
       }
 
+      // If we had to prefix the URL with a protocol,
+      // remove it so we retain the protocol relative
+      // of the response URL
+      if (protocolPrefix) {
+        u.protocol = '';
+      }
+
       proxyRes.headers['location'] = u.format();
     }
   },

test/lib-http-proxy-passes-web-outgoing-test.js

@@ -63,6 +63,12 @@ describe('lib/http-proxy/passes/web-outgoing.js', function () {
         httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
         expect(this.proxyRes.headers.location).to.eql('http://backend.com:8080/');
       });
+
+      it('handles protocol relative URLs', function() {
+        this.proxyRes.headers.location = '//backend.com';
+        httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
+        expect(this.proxyRes.headers.location).to.eql('//ext-manual.com/');
+      });
     });
 
     context('rewrites location host with autoRewrite', function() {
@@ -128,17 +134,37 @@ describe('lib/http-proxy/passes/web-outgoing.js', function () {
         expect(this.proxyRes.headers.location).to.eql('http://backend.com/');
       });
 
+      it('not when protocol relative URL is used', function() {
+        this.proxyRes.headers.location = '//backend.com';
+        httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
+        expect(this.proxyRes.headers.location).to.eql('//backend.com/');
+      });
+
       it('works together with hostRewrite', function() {
         this.options.hostRewrite = 'ext-manual.com';
         httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
         expect(this.proxyRes.headers.location).to.eql('https://ext-manual.com/');
       });
 
+      it('not with hostRewrite when a protocol relative URL is used', function() {
+        this.options.hostRewrite = 'ext-manual.com';
+        this.proxyRes.headers.location = '//backend.com';
+        httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
+        expect(this.proxyRes.headers.location).to.eql('//ext-manual.com/');
+      });
+
       it('works together with autoRewrite', function() {
         this.options.autoRewrite = true;
         httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
         expect(this.proxyRes.headers.location).to.eql('https://ext-auto.com/');
       });
+
+      it('not with autoRewrite when a protocol relative URL is used', function() {
+        this.options.autoRewrite = true;
+        this.proxyRes.headers.location = '//backend.com/';
+        httpProxy.setRedirectHostRewrite(this.req, {}, this.proxyRes, this.options);
+        expect(this.proxyRes.headers.location).to.eql('//ext-auto.com/');
+      });
     });
   });