old_contents: from trac.db import Table, Column, Index, DatabaseManager
  from trac.versioncontrol.cache import CACHE_YOUNGEST_REV
  
  def do_upgrade(env, ver, cursor):
      """Modify the repository cache scheme.
  
      Now we use the 'youngest_rev' entry in the system table
      to explicitly store the youngest rev in the cache.
      """
      db = env.get_db_cnx()
      repos = env.get_repository(None, sync=False)
      youngest = repos.get_youngest_rev_in_cache(db) or ''
      # deleting first, for the 0.11dev and 0.10.4dev users
      cursor.execute("DELETE FROM system WHERE name=%s", (CACHE_YOUNGEST_REV,))
      cursor.execute("INSERT INTO system (name, value) VALUES (%s, %s)",
                     (CACHE_YOUNGEST_REV, youngest))
  
new_contents: from trac.db import Table, Column, Index, DatabaseManager
  from trac.core import TracError
  from trac.versioncontrol.cache import CACHE_YOUNGEST_REV
  
  def do_upgrade(env, ver, cursor):
      """Modify the repository cache scheme (if needed)
  
      Now we use the 'youngest_rev' entry in the system table
      to explicitly store the youngest rev in the cache.
      """
      db = env.get_db_cnx()
      try:
          repos = env.get_repository(None, sync=False)
          youngest = repos.get_youngest_rev_in_cache(db) or ''
          # deleting first, for the 0.11dev and 0.10.4dev users
          cursor.execute("DELETE FROM system WHERE name=%s",
                         (CACHE_YOUNGEST_REV,))
          cursor.execute("INSERT INTO system (name, value) VALUES (%s, %s)",
                         (CACHE_YOUNGEST_REV, youngest))
      except TracError: # no repository available
          pass
          
  
subject: Upgrade database version 20 should succeed even for environment not using the versioncontrol subsystem. Closes #5015.