Archiving control (a part of synch rep patches)

Hi,

Some people want to disable xlog archiving only during synchronous
replication. So, first, I created and attached the self-contained patch
to control xlog archiving dynamically. Since it would still take time to
fix whole patch of synch rep, and opening it at a time would burden
reviewers, I opened this patch this time.
http://archives.postgresql.org/pgsql-hackers/2008-12/msg00718.php

This patch provides three abilities:

(1)
Turn off xlog archiving from the specified file. For example, if the
xlog file including replication starting position is specified, we can
stop archiving the xlog files which are generated during replication.

Specifically, we only skip executing archive command after turning
off, that is, archiver is still in progress and .done file is created.
This reduces the burden of resuming archiving.

(2)
Turn back on xlog archiving from the specified file.

(3)
Revert the arhive status file of the specified file from .done to .ready.
This would try to archive the corresponding xlog file again. The archive
status of only skipped xlog file can be reverted.

We resume xlog archiving by using (2) and (3), when replication
ends.

(1) and (2) are provided as PgArchControl() in pgarch.c.
(3) is provided as XLogArchiveRevert() in xlog.c.

Please feel free to comment!

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center