FTWRL behavior on same connection

FTWRL with mysql is no-op after first one in same connection.

================================================
MySQL [(none)]> flush tables with read lock;
Query OK, 0 rows affected (0.00 sec)

MySQL [(none)]> flush tables with read lock;
Query OK, 0 rows affected (0.00 sec)

MySQL [(none)]> flush tables with read lock;
Query OK, 0 rows affected (0.00 sec)

MySQL [(none)]> flush tables with read lock;
Query OK, 0 rows affected (0.00 sec)
=========================================================

With WSREP it tries to resume the provider and then again pause
it

============================================

2014-01-14 22:41:19 15515 [Note] WSREP: Provider paused at 527ab192-366c-11e3-925b-0f08c7cfb132:106493
2014-01-14 22:41:20 15515 [Note] WSREP: GRL was in block commit mode when entering make_global_read_lock_block_commit
2014-01-14 22:41:20 15515 [Note] WSREP: Provider resumed.
2014-01-14 22:41:20 15515 [Note] WSREP: Provider paused at 527ab192-366c-11e3-925b-0f08c7cfb132:106493

=======================================================

The fix for https://bugs.launchpad.net/percona-xtradb-cluster/+bug/1170821 is not used due to this.

This happens because in Global_read_lock::make_global_read_lock_block_commit:

--------------------------------------

  /*
    If we didn't succeed lock_global_read_lock(), or if we already suceeded
    make_global_read_lock_block_commit(), do nothing.
  */

#ifdef WITH_WSREP
  if (m_mdl_blocks_commits_lock)
  {
    WSREP_DEBUG("GRL was in block commit mode when entering "
  "make_global_read_lock_block_commit");
    thd->mdl_context.release_lock(m_mdl_blocks_commits_lock);
    m_mdl_blocks_commits_lock= NULL;
    wsrep_locked_seqno= WSREP_SEQNO_UNDEFINED;
    wsrep->resume(wsrep);
    m_state= GRL_ACQUIRED;
  }
#endif /* WITH_WSREP */

  if (m_state != GRL_ACQUIRED)
    DBUG_RETURN(0);

------------------------------------------------

I think the check

  if (m_state != GRL_ACQUIRED)
    DBUG_RETURN(0);

should go earlier than the WSREP branch.

This is in turn because

 in Global_read_lock::lock_global_read_lock:

 --------------------------

  if (!m_state)
  {
    MDL_request mdl_request;

    DBUG_ASSERT(! thd->mdl_context.is_lock_owner(MDL_key::GLOBAL, "", "",
                                                 MDL_SHARED));
    mdl_request.init(MDL_key::GLOBAL, "", "", MDL_SHARED, MDL_EXPLICIT);

    if (thd->mdl_context.acquire_lock(&mdl_request,
                                      thd->variables.lock_wait_timeout))
      DBUG_RETURN(1);

    m_mdl_global_shared_lock= mdl_request.ticket;
    m_state= GRL_ACQUIRED;
  }
  /*
    We DON'T set global_read_lock_blocks_commit now, it will be set after
    tables are flushed (as the present function serves for FLUSH TABLES WITH
    READ LOCK only). Doing things in this order is necessary to avoid
    deadlocks (we must allow COMMIT until all tables are closed; we should not
    forbid it before, or we can have a 3-thread deadlock if 2 do SELECT FOR
    UPDATE and one does FLUSH TABLES WITH READ LOCK).
  */
  DBUG_RETURN(0);

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GRL is not acquired if it has already been acquired.