Re: Verified fix for Bug 4137

Simon Riggs wrote:
> We were already assuming archive files were "OK to delete, if before".
> The whole of recovery already relies heavily on the alphabetic sorting
> property of WAL and associated filenames. Those filenames have been
> specifically documented as maintaining that sorted order for that
> reason. If somebody wanted to recover files in non-sorted order, then
> yes I would expect a few things to break - this aspect wouldn't be the
> most critical thing though.

I didn't suggest that alphabetical sorting property is a new assumption;
it sure isn't. The new assumption is that you never call ReadRecord()
for record 0002 before you call it for record 0001 (before initializing
the checkPointCopy field from the checkpoint record, anyway).

I can imagine a future patch to do xlog file prefetching, for example,
that breaks that assumption. Or falling back to the previous checkpoint
as discussed. Or maybe you screwed up your recovery.conf, and try to use
WAL files that belong to a different installation. The database won't
start up, of course, because the checkpoint record isn't in the right
place, but the damage has already been done because the recovery command
deleted some files it shouldn't have.

Granted, none of those are particularly likely, but we should be extra
careful when deleting files.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com