net/http: use the correct error variable in serveFile

It was generating the wrong error message, always defaulting to "500
Internal Server Error", since the err variable used was always nil.

Fixes #12991

Change-Id: I94b0e516409c131ff3b878bcb91e65f0259ff077
Reviewed-on: https://go-review.googlesource.com/16060Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/export_test.go

@@ -130,3 +130,5 @@ var ExportServerNewConn = (*Server).newConn
 var ExportCloseWriteAndWait = (*conn).closeWriteAndWait
 
 var ExportErrRequestCanceled = errRequestCanceled
+
+var ExportServeFile = serveFile

src/net/http/fs.go

@@ -369,8 +369,8 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 	}
 	defer f.Close()
 
-	d, err1 := f.Stat()
-	if err1 != nil {
+	d, err := f.Stat()
+	if err != nil {
 		msg, code := toHTTPError(err)
 		Error(w, msg, code)
 		return

src/net/http/fs_test.go

@@ -850,6 +850,28 @@ func TestServeContent(t *testing.T) {
 	}
 }
 
+// Issue 12991
+func TestServerFileStatError(t *testing.T) {
+	rec := httptest.NewRecorder()
+	r, _ := NewRequest("GET", "http://foo/", nil)
+	redirect := false
+	name := "file.txt"
+	fs := issue12991FS{}
+	ExportServeFile(rec, r, fs, name, redirect)
+	if body := rec.Body.String(); !strings.Contains(body, "403") || !strings.Contains(body, "Forbidden") {
+		t.Errorf("wanted 403 forbidden message; got: %s", body)
+	}
+}
+
+type issue12991FS struct{}
+
+func (issue12991FS) Open(string) (File, error) { return issue12991File{}, nil }
+
+type issue12991File struct{ File }
+
+func (issue12991File) Stat() (os.FileInfo, error) { return nil, os.ErrPermission }
+func (issue12991File) Close() error               { return nil }
+
 func TestServeContentErrorMessages(t *testing.T) {
 	defer afterTest(t)
 	fs := fakeFS{