Re: Correct comment in RemoveNonParentXlogFiles()

On Thu, Aug 4, 2022 at 11:30 AM Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
wrote:

> At Wed, 3 Aug 2022 18:16:33 +0530, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>
> wrote in
> > Following comment in RemoveNonParentXlogFiles() says that we are trying
> to
> > remove any WAL file whose segment number is >= the segment number of the
> > first WAL file on the new timeline. However, looking at the code, I can
> say
> > that we are trying to remove the WAL files from the previous timeline
> whose
> > segment number is just greater than (not equal to) the segment number of
> > the first WAL file in the new timeline. I think we should improve this
> > comment, thoughts?
> >
> > /*
> > * Remove files that are on a timeline older than the new one
> we're
> > * switching to, but with a segment number >= the first segment
> on
> > the
> > * new timeline.
> > */
> > if (strncmp(xlde->d_name, switchseg, 8) < 0 &&
> > strcmp(xlde->d_name + 8, switchseg + 8) > 0)
>
> I'm not sure I'm fully getting your point. The current comment is
> correctly saying that it removes the segments "on a timeline older
> than the new one". I agree about segment comparison.
>

Yeah my complaint is about the comment on segment comparison for removal.

>
> So, if I changed that comment, I would finish with the following change.
>
> - * switching to, but with a segment number >= the first segment
> on
> + * switching to, but with a segment number greater than the
> first segment on
>

which looks correct to me and will inline it with the code.

>
> That disagreement started at the time the code was introduced by
> b2a5545bd6. Leaving the last segment in the old timeline is correct
> since it is renamed to .partial later. If timeline switch happened
> just at segment boundary, that segment would not not be created.
>

Yeah, that's why we keep the last segment (partially written) from the old
timeline, which means we're not deleting it here. So the comment should not
be saying that we are also removing the last wal segment from the old
timeline which is equal to the first segment from the new timeline.

--
With Regards,
Ashutosh Sharma.