Re: Rework the way multixact truncations work

On Wed, Dec 9, 2015 at 8:23 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> On Wed, Dec 09, 2015 at 11:08:32AM -0500, Robert Haas wrote:
>> On Wed, Dec 9, 2015 at 9:43 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>> > I hope those who have not already read commit 4f627f8 will not waste time
>> > reading it. They should instead ignore multixact changes from commit 4f627f8
>> > through its revert. The 2015-09-26 commits have not appeared in a supported
>> > release, and no other work has built upon them. They have no tenure. (I am
>> > glad you talked the author out of back-patching; otherwise, 9.4.5 and 9.3.10
>> > would have introduced a data loss bug.) Nobody reported a single defect
>> > before my review overturned half the patch. A revert will indeed impose on
>> > those who invested time to understand commit 4f627f8, but the silence about
>> > its defects suggests the people understanding it number either zero or one.
>> > Even as its author and reviewers, you would do better to set aside what you
>> > thought you knew about this code.
>>
>> I just don't find this a realistic model of how people use the git
>> log. Maybe you use it this way; I don't. I don't *want* git blame to
>> make it seem as if 4f627f8 is not part of the history. For better or
>> worse, it is.
>
> I would like to understand how you use git, then. What's one of your models
> of using "git log" and/or "git blame" in which you foresee the revert making
> history less clear, not more clear?

Well, suppose I wanted to know what bugs were fixed between 9.5beta
and 9.5.0, for example. I mean, I'm going to run git log
src/backend/access/transam/multixact.c ... and the existing commits
are going to be there.

> By the way, it occurs to me that I should also make pg_upgrade blacklist the
> range of catversions that might have data loss. No sense in putting ourselves
> in the position of asking whether data files of a 9.9.3 cluster spent time in
> a 9.5beta2 cluster.

Maybe. But I think we could use a little more vigorous discussion of
that issue, since Andres doesn't seem to be very convinced by your
analysis, and I don't currently understand what you've fixed because I
can't, as mentioned several times, follow your patch stack.

>> Ripping it out and replacing it monolithically will not
>> change that; it will only make the detailed history harder to
>> reconstruct, and I *will* want to reconstruct it.
>
> What's something that might happen six months from now and lead you to inspect
> master or 9.5 multixact.c between 4f627f8 and its revert?

I don't know have anything to add to what others have said in response
to this point, except this: the whole point of using a source code
management system is to tell you what changed and when. What you are
proposing to do makes it unusable for that purpose.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company