fix: preserve a req.url rewritten inside a mounted layer

README.md

@@ -65,6 +65,10 @@ consider it one for handling `OPTIONS` requests.
 
 * Note: If a `path` is specified, that `path` is stripped from the start of
   `req.url`.
+* Note: The stripped `path` is added back to `req.url` when the middleware calls
+  `next()`. A `req.url` rewritten by the middleware is treated as relative to the
+  mount `path`, so rewriting `req.url` to `/index.html` inside a router mounted
+  on `/app` continues with `/app/index.html`.
 
 <!-- eslint-disable no-undef -->
 

index.js

@@ -158,7 +158,7 @@ Router.prototype.handle = function handle (req, res, callback) {
   const protohost = getProtohost(req.url) || ''
   let removed = ''
   const self = this
-  let slashAdded = false
+  let slashAddedUrl = null
   let sync = 0
   const paramcalled = {}
 
@@ -190,10 +190,10 @@ Router.prototype.handle = function handle (req, res, callback) {
       ? null
       : err
 
-    // remove added slash
-    if (slashAdded) {
-      req.url = req.url.slice(1)
-      slashAdded = false
+    // remove added slash unless the layer rewrote req.url
+    if (slashAddedUrl !== null) {
+      if (req.url === slashAddedUrl) req.url = req.url.slice(1)
+      slashAddedUrl = null
     }
 
     // restore altered req.url
@@ -325,7 +325,7 @@ Router.prototype.handle = function handle (req, res, callback) {
       // Ensure leading slash
       if (!protohost && req.url[0] !== '/') {
         req.url = '/' + req.url
-        slashAdded = true
+        slashAddedUrl = req.url
       }
 
       // Setup base URL (no trailing slash)

test/router.js

@@ -1393,6 +1393,36 @@ describe('Router', function () {
           .expect('x-saw-1', 'GET /')
           .expect(200, 'saw GET /foo/', done)
       })
+
+      it('should restore a req.url rewritten inside the layer', function (done) {
+        const router = new Router()
+        const server = createServer(router)
+
+        router.use('/foo', function (req, res, next) {
+          req.url = '/bar'
+          next()
+        })
+        router.use(saw)
+
+        request(server)
+          .get('/foo')
+          .expect(200, 'saw GET /foo/bar', done)
+      })
+
+      it('should restore a req.url rewritten inside the layer when a query string was present', function (done) {
+        const router = new Router()
+        const server = createServer(router)
+
+        router.use('/foo', function (req, res, next) {
+          req.url = '/bar'
+          next()
+        })
+        router.use(saw)
+
+        request(server)
+          .get('/foo?fizz=buzz')
+          .expect(200, 'saw GET /foo/bar', done)
+      })
     })
   })