session: fix ineffectual code

* Rewrite directory index suggestion to prevent ineffectual code.

session.go

@@ -213,6 +213,11 @@ func (fs *FileSystemRouter) Router() Middleware {
 				fastcgiScriptName, fastcgiPathInfo = matches[1], matches[2]
 			}
 
+			// If accessing a directory, try accessing document index file
+			if strings.HasSuffix(fastcgiScriptName, "/") {
+				fastcgiScriptName = path.Join(fastcgiScriptName, "index.php")
+			}
+
 			req.Params["PATH_INFO"] = fastcgiPathInfo
 			req.Params["PATH_TRANSLATED"] = filepath.Join(docroot, fastcgiPathInfo)
 			req.Params["SCRIPT_NAME"] = fastcgiScriptName
@@ -228,11 +233,6 @@ func (fs *FileSystemRouter) Router() Middleware {
 			}
 
 			// handle directory index
-			urlPath := r.URL.Path
-			if strings.HasSuffix(urlPath, "/") {
-				urlPath = path.Join(urlPath, "index.php")
-			}
-			req.Params["SCRIPT_FILENAME"] = path.Join(fs.DocRoot, urlPath)
 
 			return inner(client, req)
 		}

session_test.go

@@ -180,7 +180,7 @@ func TestMapFilterRequest(t *testing.T) {
 	}
 }
 
-func TestFileSystemRouter(t *testing.T) {
+func TestFileSystemRouter_PathTraversal(t *testing.T) {
 	fs := &gofast.FileSystemRouter{
 		DocRoot:  "/non-exists/folder/structure",
 		Exts:     []string{"php"},
@@ -211,3 +211,35 @@ func TestFileSystemRouter(t *testing.T) {
 		t.Errorf("expected \"%s\", got \"%s\"", want, have)
 	}
 }
+
+func TestFileSystemRouter_DirectoryPath(t *testing.T) {
+	fs := &gofast.FileSystemRouter{
+		DocRoot:  "/non-exists/folder/structure",
+		Exts:     []string{"php"},
+		DirIndex: []string{"index.php"},
+	}
+
+	h := gofast.Chain(
+		gofast.BasicParamsMap,
+		fs.Router(),
+	)(func(client gofast.Client, req *gofast.Request) (resp *gofast.ResponsePipe, err error) {
+		err = fmt.Errorf("SCRIPT_FILENAME=%s", req.Params["SCRIPT_FILENAME"])
+		return
+	})
+
+	r, err := http.NewRequest("GET", "http://foobar.com/", nil)
+	if err != nil {
+		t.Errorf("unexpected error: %s", err)
+	}
+	r.URL.Path = "/hello/"
+
+	_, err = h(nil, gofast.NewRequest(r))
+	if err == nil {
+		t.Errorf("expected error, got nil")
+		return
+	}
+
+	if want, have := "SCRIPT_FILENAME=/non-exists/folder/structure/hello/index.php", err.Error(); want != have {
+		t.Errorf("expected \"%s\", got \"%s\"", want, have)
+	}
+}