[scan] Fix missing rescan if a file is modified quickly multiple times

If db_timestamp == file_mtime we didn't rescan, just pinged the file in the db

Fixes #1782

src/db.c

@@ -7055,7 +7055,10 @@ db_statements_prepare_ping(const char *table)
   sqlite3_stmt *stmt;
   int ret;
 
-  CHECK_NULL(L_DB, query = db_mprintf("UPDATE %s SET db_timestamp = ?, disabled = 0 WHERE path = ? AND db_timestamp >= ?;", table));
+  // The last param will be the file mtime. We must not update if the mtime is
+  // newer or equal than the current db_timestamp, since the file may have been
+  // modified and must be rescanned.
+  CHECK_NULL(L_DB, query = db_mprintf("UPDATE %s SET db_timestamp = ?, disabled = 0 WHERE path = ? AND db_timestamp > ?;", table));
 
   ret = db_blocking_prepare_v2(query, -1, &stmt, NULL);
   if (ret != SQLITE_OK)

src/library/filescanner.c

@@ -572,10 +572,12 @@ process_regular_file(const char *file, struct stat *sb, int type, int flags, int
   char virtual_path[PATH_MAX];
   int ret;
 
-  // 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
   if (!(flags & F_SCAN_METARESCAN))
     {
+      // Will return 0 if file is not in library or if file mtime is not older
+      // than the library timestamp. If mtime is equal we must rescan, since a
+      // fast update may have been made, see issue #1782. If mtime is 0 then we
+      // always scan.
       ret = db_file_ping_bypath(file, sb->st_mtime);
       if ((sb->st_mtime != 0) && (ret != 0))
         return;