templates: Properly propagate response status code (fixes #1841)

Benchmarks with wrk showed no noticeable performance impact

caddyhttp/httpserver/recorder.go

@@ -195,6 +195,28 @@ func (rb *ResponseBuffer) ReadFrom(src io.Reader) (int64, error) {
 	return rb.Buffer.ReadFrom(src)
 }
 
+// StatusCodeWriter returns an http.ResponseWriter that always
+// writes the status code stored in rb from when a response
+// was buffered to it.
+func (rb *ResponseBuffer) StatusCodeWriter(w http.ResponseWriter) http.ResponseWriter {
+	return forcedStatusCodeWriter{w, rb}
+}
+
+// forcedStatusCodeWriter is used to force a status code when
+// writing the header. It uses the status code saved on rb.
+// This is useful if passing a http.ResponseWriter into
+// http.ServeContent because ServeContent hard-codes 2xx status
+// codes. If we buffered the response, we force that status code
+// instead.
+type forcedStatusCodeWriter struct {
+	http.ResponseWriter
+	rb *ResponseBuffer
+}
+
+func (fscw forcedStatusCodeWriter) WriteHeader(int) {
+	fscw.ResponseWriter.WriteHeader(fscw.rb.status)
+}
+
 // respBufPool is used for io.CopyBuffer when ResponseBuffer
 // is configured to stream a response.
 var respBufPool = &sync.Pool{

caddyhttp/templates/templates.go

@@ -40,7 +40,7 @@ func (t Templates) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error
 				if reqExt == "" {
 					// request has no extension, so check response Content-Type
 					ct := mime.TypeByExtension(ext)
-					if strings.Contains(header.Get("Content-Type"), ct) {
+					if ct != "" && strings.Contains(header.Get("Content-Type"), ct) {
 						return true
 					}
 				} else if reqExt == ext {
@@ -96,13 +96,14 @@ func (t Templates) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error
 		// set the actual content length now that the template was executed
 		w.Header().Set("Content-Length", strconv.Itoa(buf.Len()))
 
-		// get the modification time in preparation to ServeContent
+		// get the modification time in preparation for http.ServeContent
 		modTime, _ := time.Parse(http.TimeFormat, w.Header().Get("Last-Modified"))
 
-		// at last, write the rendered template to the response
-		http.ServeContent(w, r, templateName, modTime, bytes.NewReader(buf.Bytes()))
+		// at last, write the rendered template to the response; make sure to use
+		// use the proper status code, since ServeContent hard-codes 2xx codes...
+		http.ServeContent(rb.StatusCodeWriter(w), r, templateName, modTime, bytes.NewReader(buf.Bytes()))
 
-		return http.StatusOK, nil
+		return 0, nil
 	}
 
 	return t.Next.ServeHTTP(w, r)