[library] support forced metadata scan of library, via '.meta-rescan' file

README.md

@@ -417,6 +417,9 @@ Now you can make a cron job that runs this command:
 When forked-daapd detects a file with filename ending .init-rescan it will
 perform a bulk scan similar to the startup scan.
 
+Alternatively, you can force a metadata scan of the library even if the
+files have not changed by creating a filename ending `.meta-rescan`.
+
 
 ### Troubleshooting library issues
 

src/library.c

@@ -294,6 +294,49 @@ rescan(void *arg, int *ret)
   return COMMAND_END;
 }
 
+static enum command_state
+metarescan(void *arg, int *ret)
+{
+  time_t starttime;
+  time_t endtime;
+  int i;
+
+  DPRINTF(E_LOG, L_LIB, "Library meta rescan triggered\n");
+  listener_notify(LISTENER_UPDATE);
+  starttime = time(NULL);
+
+  for (i = 0; sources[i]; i++)
+    {
+      if (!sources[i]->disabled && sources[i]->metarescan)
+	{
+	  DPRINTF(E_INFO, L_LIB, "Meta rescan library source '%s'\n", sources[i]->name);
+	  sources[i]->metarescan();
+	}
+      else
+	{
+	  DPRINTF(E_INFO, L_LIB, "Library source '%s' is disabled\n", sources[i]->name);
+	}
+    }
+
+  purge_cruft(starttime);
+
+  DPRINTF(E_DBG, L_LIB, "Running post library scan jobs\n");
+  db_hook_post_scan();
+
+  endtime = time(NULL);
+  DPRINTF(E_LOG, L_LIB, "Library meta rescan completed in %.f sec (%d changes)\n", difftime(endtime, starttime), deferred_update_notifications);
+  scanning = false;
+
+  if (handle_deferred_update_notifications())
+    listener_notify(LISTENER_UPDATE | LISTENER_DATABASE);
+  else
+    listener_notify(LISTENER_UPDATE);
+
+  *ret = 0;
+  return COMMAND_END;
+}
+
+
 static enum command_state
 fullrescan(void *arg, int *ret)
 {
@@ -382,6 +425,18 @@ library_rescan()
   commands_exec_async(cmdbase, rescan, NULL);
 }
 
+void
+library_metarescan()
+{
+  if (scanning)
+    {
+      DPRINTF(E_INFO, L_LIB, "Scan already running, ignoring request to trigger metadata scan\n");
+      return;
+    }
+
+  scanning = true; // TODO Guard "scanning" with a mutex
+  commands_exec_async(cmdbase, metarescan, NULL);
+}
 void
 library_fullrescan()
 {

src/library.h

@@ -61,6 +61,11 @@ struct library_source
    */
   int (*rescan)(void);
 
+  /*
+   * Run a metadata rescan of library even if files not changed (called from the library thread)
+   */
+  int (*metarescan)(void);
+
   /*
    * Run a full rescan (purge library entries and rescan) (called from the library thread)
    */
@@ -99,6 +104,9 @@ library_queue_add(const char *path, int position, int *count, int *new_item_id);
 void
 library_rescan();
 
+void
+library_metarescan();
+
 void
 library_fullrescan();
 

src/library/filescanner.c

@@ -76,6 +76,7 @@
 #define F_SCAN_RESCAN  (1 << 1)
 #define F_SCAN_FAST    (1 << 2)
 #define F_SCAN_MOVED   (1 << 3)
+#define F_SCAN_METARESCAN  (1 << 4)
 
 #define F_SCAN_TYPE_FILE         (1 << 0)
 #define F_SCAN_TYPE_PODCAST      (1 << 1)
@@ -96,6 +97,7 @@ enum file_type {
   FILE_CTRL_LASTFM,
   FILE_CTRL_SPOTIFY,
   FILE_CTRL_INITSCAN,
+  FILE_CTRL_METASCAN, // forced scan for meta, preserves existing db records
   FILE_CTRL_FULLSCAN,
 };
 
@@ -366,6 +368,9 @@ file_type_get(const char *path) {
   if (strcasecmp(ext, ".init-rescan") == 0)
     return FILE_CTRL_INITSCAN;
 
+  if (strcasecmp(ext, ".meta-rescan") == 0)
+    return FILE_CTRL_METASCAN;
+
   if (strcasecmp(ext, ".full-rescan") == 0)
     return FILE_CTRL_FULLSCAN;
 
@@ -477,9 +482,12 @@ process_regular_file(const char *file, struct stat *sb, int type, int flags, int
 
   // Will return 0 if file is not in library or if file mtime is newer than library timestamp
   // - note if mtime is 0 then we always scan the file
-  ret = db_file_ping_bypath(file, sb->st_mtime);
+  if (!(flags & F_SCAN_METARESCAN))
-  if ((sb->st_mtime != 0) && (ret != 0))
+    {
-    return;
+      ret = db_file_ping_bypath(file, sb->st_mtime);
+      if ((sb->st_mtime != 0) && (ret != 0))
+        return;
+    }
 
   // File is new or modified - (re)scan metadata and update file in library
   memset(&mfi, 0, sizeof(struct media_file_info));
@@ -624,6 +632,15 @@ process_file(char *file, struct stat *sb, int type, int flags, int dir_id)
 	library_rescan();
 	break;
 
+      case FILE_CTRL_METASCAN:
+	if (flags & F_SCAN_BULK)
+	  break;
+
+	DPRINTF(E_LOG, L_SCAN, "Meta rescan triggered, found meta-rescan file: %s\n", file);
+
+	library_metarescan();
+	break;
+
       case FILE_CTRL_FULLSCAN:
 	if (flags & F_SCAN_BULK)
 	  break;
@@ -1630,6 +1647,24 @@ filescanner_rescan()
   return 0;
 }
 
+static int
+filescanner_metarescan()
+{
+  DPRINTF(E_LOG, L_SCAN, "meta rescan triggered\n");
+
+  inofd_event_unset(); // Clears all inotify watches
+  db_watch_clear();
+  inofd_event_set();
+  bulk_scan(F_SCAN_BULK | F_SCAN_METARESCAN);
+
+  if (!library_is_exiting())
+    {
+      /* Enable inotify */
+      event_add(inoev, NULL);
+    }
+  return 0;
+}
+
 static int
 filescanner_fullrescan()
 {
@@ -2133,6 +2168,7 @@ struct library_source filescanner =
   .deinit = filescanner_deinit,
   .initscan = filescanner_initscan,
   .rescan = filescanner_rescan,
+  .metarescan = filescanner_metarescan,
   .fullrescan = filescanner_fullrescan,
   .playlist_add = playlist_add,
   .playlist_remove = playlist_remove,