Correct comment in RemoveNonParentXlogFiles()

At Wed, 3 Aug 2022 18:16:33 +0530, Ashutosh Sharma <ashu.coek88@gmail.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.

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

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.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center

At Thu, 04 Aug 2022 15:00:06 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in

At Wed, 3 Aug 2022 18:16:33 +0530, Ashutosh Sharma <ashu.coek88@gmail.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.

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

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.

"the last segment in the old timeline" here means "the segment in the
old timeline, with the segment number == the first segmetn on the new
timeline".

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center

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

At Wed, 3 Aug 2022 18:16:33 +0530, Ashutosh Sharma <ashu.coek88@gmail.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.