[httpd/jsonapi] Add cache control headers to some json api endpoints

Adds utility functions to httpd.c for checking the request headers for
either an "If-None-Match" or an "If-Not-Modified-Since" headers. If the
header value is found and it matches the current value for the requested
resource, we return early with a http response code 403 (Not Modified).
If the request header value is not present or does not match we add the
current ETag/Last-Modified values to the response headers and process
the request normally.

src/httpd.c

@@ -268,6 +268,75 @@ httpd_redirect_to_admin(struct evhttp_request *req)
   httpd_send_reply(req, HTTP_MOVETEMP, "Moved", NULL, HTTPD_SEND_NO_GZIP);
 }
 
+/*
+ * Checks if the given ETag matches the "If-None-Match" request header
+ *
+ * If the request does not contains a "If-None-Match" header value or the value
+ * does not match the given ETag, it returns false (modified) and adds the
+ * "Cache-Control" and "ETag" headers to the response header.
+ *
+ * @param req The request with request and response headers
+ * @param etag The valid ETag for the requested resource
+ * @return True if the given ETag matches the request-header-value "If-None-Match", otherwise false
+ */
+bool
+httpd_request_etag_matches(struct evhttp_request *req, const char *etag)
+{
+  struct evkeyvalq *input_headers;
+  struct evkeyvalq *output_headers;
+  const char *none_match;
+
+  input_headers = evhttp_request_get_input_headers(req);
+  none_match = evhttp_find_header(input_headers, "If-None-Match");
+
+  // Return not modified, if given timestamp matches "If-Modified-Since" request header
+  if (none_match && (strcasecmp(etag, none_match) == 0))
+    return true;
+
+  // Add cache headers to allow client side caching
+  output_headers = evhttp_request_get_output_headers(req);
+  evhttp_add_header(output_headers, "Cache-Control", "private");
+  evhttp_add_header(output_headers, "ETag", etag);
+
+  return false;
+}
+
+/*
+ * Checks if the given timestamp matches the "If-Modified-Since" request header
+ *
+ * If the request does not contains a "If-Modified-Since" header value or the value
+ * does not match the given timestamp, it returns false (modified) and adds the
+ * "Cache-Control" and "Last-Modified" headers to the response header.
+ *
+ * @param req The request with request and response headers
+ * @param mtime The last modified timestamp for the requested resource
+ * @return True if the given timestamp matches the request-header-value "If-Modified-Since", otherwise false
+ */
+bool
+httpd_request_not_modified_since(struct evhttp_request *req, const time_t *mtime)
+{
+  struct evkeyvalq *input_headers;
+  struct evkeyvalq *output_headers;
+  char last_modified[1000];
+  const char *modified_since;
+
+  input_headers = evhttp_request_get_input_headers(req);
+  modified_since = evhttp_find_header(input_headers, "If-Modified-Since");
+
+  strftime(last_modified, sizeof(last_modified), "%a, %d %b %Y %H:%M:%S %Z", gmtime(mtime));
+
+  // Return not modified, if given timestamp matches "If-Modified-Since" request header
+  if (modified_since && (strcasecmp(last_modified, modified_since) == 0))
+    return true;
+
+  // Add cache headers to allow client side caching
+  output_headers = evhttp_request_get_output_headers(req);
+  evhttp_add_header(output_headers, "Cache-Control", "private");
+  evhttp_add_header(output_headers, "Last-Modified", last_modified);
+
+  return false;
+}
+
 static void
 serve_file(struct evhttp_request *req, const char *uri)
 {
@@ -276,15 +345,11 @@ serve_file(struct evhttp_request *req, const char *uri)
   char deref[PATH_MAX];
   char *ctype;
   struct evbuffer *evbuf;
-  struct evkeyvalq *input_headers;
   struct evkeyvalq *output_headers;
   struct stat sb;
   int fd;
   int i;
   uint8_t buf[4096];
-  const char *modified_since;
-  char last_modified[1000];
-  struct tm *tm_modified;
   bool slashed;
   int ret;
 
@@ -384,13 +449,7 @@ serve_file(struct evhttp_request *req, const char *uri)
       return;
     }
 
-  tm_modified = gmtime(&sb.st_mtime);
+  if (httpd_request_not_modified_since(req, &sb.st_mtime))
-  strftime(last_modified, sizeof(last_modified), "%a, %d %b %Y %H:%M:%S %Z", tm_modified);
-
-  input_headers = evhttp_request_get_input_headers(req);
-  modified_since = evhttp_find_header(input_headers, "If-Modified-Since");
-
-  if (modified_since && strcasecmp(last_modified, modified_since) == 0)
     {
       httpd_send_reply(req, HTTP_NOTMODIFIED, NULL, NULL, HTTPD_SEND_NO_GZIP);
       return;
@@ -448,10 +507,6 @@ serve_file(struct evhttp_request *req, const char *uri)
   output_headers = evhttp_request_get_output_headers(req);
   evhttp_add_header(output_headers, "Content-Type", ctype);
 
-  // Allow browsers to cache the file
-  evhttp_add_header(output_headers, "Cache-Control", "private");
-  evhttp_add_header(output_headers, "Last-Modified", last_modified);
-
   httpd_send_reply(req, HTTP_OK, "OK", evbuf, HTTPD_SEND_NO_GZIP);
 
   evbuffer_free(evbuf);

src/httpd.h

@@ -4,6 +4,7 @@
 
 #include <stdbool.h>
 #include <regex.h>
+#include <time.h>
 #include <event2/http.h>
 #include <event2/buffer.h>
 #include <event2/keyvalq_struct.h>
@@ -100,6 +101,12 @@ httpd_request_parse(struct evhttp_request *req, struct httpd_uri_parsed *uri_par
 void
 httpd_stream_file(struct evhttp_request *req, int id);
 
+bool
+httpd_request_not_modified_since(struct evhttp_request *req, const time_t *mtime);
+
+bool
+httpd_request_etag_matches(struct evhttp_request *req, const char *etag);
+
 /*
  * Gzips an evbuffer
  *

src/httpd_jsonapi.c

@@ -1706,6 +1706,7 @@ jsonapi_reply_queue(struct httpd_request *hreq)
   int start_pos, end_pos;
   int version;
   int count;
+  char etag[21];
   struct player_status status;
   struct db_queue_item queue_item;
   json_object *reply;
@@ -1713,12 +1714,16 @@ jsonapi_reply_queue(struct httpd_request *hreq)
   json_object *item;
   int ret = 0;
 
-  memset(&query_params, 0, sizeof(struct query_params));
-  reply = json_object_new_object();
-
   version = db_admin_getint(DB_ADMIN_QUEUE_VERSION);
   count = db_queue_get_count();
 
+  snprintf(etag, sizeof(etag), "%d", version);
+  if (httpd_request_etag_matches(hreq->req, etag))
+    return HTTP_NOTMODIFIED;
+
+  memset(&query_params, 0, sizeof(struct query_params));
+  reply = json_object_new_object();
+
   json_object_object_add(reply, "version", json_object_new_int(version));
   json_object_object_add(reply, "count", json_object_new_int(count));
 
@@ -1880,6 +1885,7 @@ jsonapi_reply_player_volume(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_artists(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   const char *param;
   enum media_kind media_kind;
@@ -1888,6 +1894,11 @@ jsonapi_reply_library_artists(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   media_kind = 0;
   param = evhttp_find_header(hreq->query, "media_kind");
   if (param)
@@ -1940,10 +1951,16 @@ jsonapi_reply_library_artists(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_artist(struct httpd_request *hreq)
 {
+  time_t db_update;
   const char *artist_id;
   json_object *reply;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   artist_id = hreq->uri_parsed->path_parts[3];
 
   reply = fetch_artist(artist_id);
@@ -1969,6 +1986,7 @@ jsonapi_reply_library_artist(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_artist_albums(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   const char *artist_id;
   json_object *reply;
@@ -1976,6 +1994,11 @@ jsonapi_reply_library_artist_albums(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   artist_id = hreq->uri_parsed->path_parts[3];
 
   reply = json_object_new_object();
@@ -2018,6 +2041,7 @@ jsonapi_reply_library_artist_albums(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_albums(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   const char *param;
   enum media_kind media_kind;
@@ -2026,6 +2050,11 @@ jsonapi_reply_library_albums(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   media_kind = 0;
   param = evhttp_find_header(hreq->query, "media_kind");
   if (param)
@@ -2078,10 +2107,16 @@ jsonapi_reply_library_albums(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_album(struct httpd_request *hreq)
 {
+  time_t db_update;
   const char *album_id;
   json_object *reply;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   album_id = hreq->uri_parsed->path_parts[3];
 
   reply = fetch_album(album_id);
@@ -2107,6 +2142,7 @@ jsonapi_reply_library_album(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_album_tracks(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   const char *album_id;
   json_object *reply;
@@ -2114,6 +2150,11 @@ jsonapi_reply_library_album_tracks(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   album_id = hreq->uri_parsed->path_parts[3];
 
   reply = json_object_new_object();
@@ -2156,12 +2197,18 @@ jsonapi_reply_library_album_tracks(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_playlists(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   json_object *reply;
   json_object *items;
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   reply = json_object_new_object();
   items = json_object_new_array();
   json_object_object_add(reply, "items", items);
@@ -2202,10 +2249,16 @@ jsonapi_reply_library_playlists(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_playlist(struct httpd_request *hreq)
 {
+  time_t db_update;
   const char *playlist_id;
   json_object *reply;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   playlist_id = hreq->uri_parsed->path_parts[3];
 
   reply = fetch_playlist(playlist_id);
@@ -2231,6 +2284,7 @@ jsonapi_reply_library_playlist(struct httpd_request *hreq)
 static int
 jsonapi_reply_library_playlist_tracks(struct httpd_request *hreq)
 {
+  time_t db_update;
   struct query_params query_params;
   json_object *reply;
   json_object *items;
@@ -2238,6 +2292,11 @@ jsonapi_reply_library_playlist_tracks(struct httpd_request *hreq)
   int total;
   int ret = 0;
 
+  db_update = (time_t) db_admin_getint64(DB_ADMIN_START_TIME);
+  if (db_update && httpd_request_not_modified_since(hreq->req, &db_update))
+    return HTTP_NOTMODIFIED;
+
+
   ret = safe_atoi32(hreq->uri_parsed->path_parts[3], &playlist_id);
   if (ret < 0)
     {
@@ -2666,6 +2725,9 @@ jsonapi_request(struct evhttp_request *req, struct httpd_uri_parsed *uri_parsed)
       case HTTP_NOCONTENT:           /* 204 No Content */
 	httpd_send_reply(req, status_code, "No Content", hreq->reply, HTTPD_SEND_NO_GZIP);
 	break;
+      case HTTP_NOTMODIFIED:         /* 304 Not Modified */
+	httpd_send_reply(req, HTTP_NOTMODIFIED, NULL, NULL, HTTPD_SEND_NO_GZIP);
+	break;
 
       case HTTP_BADREQUEST:          /* 400 Bad Request */
 	httpd_send_error(req, status_code, "Bad Request");