[jsonapi] Prevent browsers to cache playlist tracks

The tracks of a smart playlist might change between library rescans.
Allowing them to be cached based on the last rescan timestamp
("Last-Modified" header in the response) leads to potentially showing
incorrect track listing if a cached version is used. Thus the response
for playlist tracks should never be cached by the browser (this is
achieved with setting "Cache-Control" header to "no-store").

src/httpd.c

@@ -344,6 +344,22 @@ httpd_request_not_modified_since(struct evhttp_request *req, time_t mtime)
   return false;
 }
 
+void
+httpd_response_not_cachable(struct evhttp_request *req)
+{
+  struct evkeyvalq *output_headers;
+
+  output_headers = evhttp_request_get_output_headers(req);
+
+  // Remove potentially set cache control headers
+  evhttp_remove_header(output_headers, "Cache-Control");
+  evhttp_remove_header(output_headers, "Last-Modified");
+  evhttp_remove_header(output_headers, "ETag");
+
+  // Tell clients that they are not allowed to cache this response
+  evhttp_add_header(output_headers, "Cache-Control", "no-store");
+}
+
 static void
 serve_file(struct evhttp_request *req, const char *uri)
 {

src/httpd.h

@@ -107,6 +107,9 @@ httpd_request_not_modified_since(struct evhttp_request *req, time_t mtime);
 bool
 httpd_request_etag_matches(struct evhttp_request *req, const char *etag);
 
+void
+httpd_response_not_cachable(struct evhttp_request *req);
+
 /*
  * Gzips an evbuffer
  *

src/httpd_jsonapi.c

@@ -3516,8 +3516,8 @@ jsonapi_reply_library_playlist_tracks(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
-  if (!is_modified(hreq->req, DB_ADMIN_DB_MODIFIED))
-    return HTTP_NOTMODIFIED;
+  // Due to smart playlists possibly changing their tracks between rescans, disable caching in clients
+  httpd_response_not_cachable(hreq->req);
 
   ret = safe_atoi32(hreq->uri_parsed->path_parts[3], &playlist_id);
   if (ret < 0)