net/http: clean the path of the stripped URL by StripPrefix

The path of the new stripped URL should also be cleaned. Since an empty path
may cause unexpected errors in some HTTP handlers, e.g. http.ServeFile.

Fixes #30165

Change-Id: Ib44fdce6388b5d62ffbcab5266925ef8f13f26e2
Reviewed-on: https://go-review.googlesource.com/c/161738Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -2900,6 +2900,15 @@ func TestStripPrefix(t *testing.T) {
 		t.Errorf("test 2: got status %v, want %v", g, e)
 	}
 	res.Body.Close()
+
+	res, err = c.Get(ts.URL + "/foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, e := res.Header.Get("X-Path"), "/"; g != e {
+		t.Errorf("test 3: got %s, want %s", g, e)
+	}
+	res.Body.Close()
 }
 
 // https://golang.org/issue/18952.

src/net/http/server.go

@@ -2030,7 +2030,7 @@ func StripPrefix(prefix string, h Handler) Handler {
 			*r2 = *r
 			r2.URL = new(url.URL)
 			*r2.URL = *r.URL
-			r2.URL.Path = p
+			r2.URL.Path = cleanPath(p)
 			h.ServeHTTP(w, r2)
 		} else {
 			NotFound(w, r)