PROBLEMS
========
* Dump threads acquired LOCK_log when reading each event. In fact, it
  is not necessary in most of the cases.

* The code was too complex and mysql_binlog_send() was too huge. It was
  very difficult to maintain.

* There was some code never used.

* Deadlock on semi-synchronous replication
  WL#6355 introduces a feature that semi-synchronous waits for ACK from
  slave when the transaction's events is written to disc but before the
  transaction is committed.

  It introduced a three way deadlock.

    SESSION 1(Rotate)              SESSION 2(Trx Commit) DUMP THREAD
    ---------                      ---------             -----------
  1                                Flush Stage
                                   Signal update
  2 Rotate(hold LOCK_log)
    WAITING for Session2 to commit
  3                                Sync Stage            WAITING for Session1
                                                         to release LOCK_log
  4                                semisync
                                   WAITING for ACK

  Session1 had to hold LOCK_log to block other sessions appending new events.
  It also blocked dump threads reading events.

DESIGN AND IMPLEMENTATION
=========================
Binary events can only be appended at the end of the active file. So it is safe
to read both inactive and active binlog files without lock. The only problem
is that dump thread may read an incomplete event at the end of active log file.

To solve the problem, MYSQL_BIN_LOG needs a variable which records the end
position of the last event in active file.

* MYSQL_BIN_LOG::binlog_end_pos
  The end position of active file. It is safe to read all events before it
  without any lock. It should be updated after appending events but before
  broadcasting update signal.

* MYSQL_BIN_LOG::LOCK_binlog_end_pos
  It is used to protect binlog_end_pos. The lock is required when both writing
  and reading binlog_end_pos.

* Binlog_sender class
  The class is designed to make the code of mysql_binlog_send() has better
  modularity and readability. All code of original mysql_binlog_send is
  moved to the class. Now mysql_binlog_send just need to call Binlog_sender.