Changes 5.5.19:
* Performance of metadata locking operations on Windows XP systems was improved
  by instituting a cache for metadata lock objects. This permits the server to
  avoid expensive operations for creation and destruction of synchronization
  objects on XP. A new system variable, metadata_locks_cache_size, permits
  control over the size of the cache. The default size is 1024.
* Replication: Previously, replication slaves could connect to the master
  server through master accounts that use nonnative authentication, except
  Windows native authentication. This is now also true for Windows native
  authentication.

Bugs Fixed:
* InnoDB Storage Engine: An internal deadlock could occur within InnoDB, on
  a server doing a substantial amount of change buffering for DML operations,
  particularly DELETE statements.
* Rounding DBL_MAX returned DBL_MAX, not 'inf'.
* mysql_upgrade did not upgrade the system tables or create the
  mysql_upgrade_info file when run with the --write-binlog or
  --skip-write-binlog option.
* If a plugin was uninstalled, thread local variables for plugin variables of
  string type with wth PLUGIN_VAR_MEMALLOC flag were not freed.
* Deadlock could occur when these four things happened at the same time: 1) An
  old dump thread was waiting for the binary log to grow. 2) The slave server
  that replicates from the old dump thread tried to reconnect. During
  reconnection, the new dump thread tried to kill the old dump thread.
  3) A KILL statement tried to kill the old dump thread. 4) An INSERT statement
  caused a binary log rotation.


(adam)