Re: Distinguishing FF vs non-FF updates in the reflog?

On Mon, Mar 22 2021, Han-Wen Nienhuys wrote:

On Mon, Mar 22, 2021 at 2:26 PM Ævar Arnfjörð Bjarmason
[off-list ref] wrote:

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I'm working on some extensions to Gerrit for which it would be very
beneficial if we could tell from the reflog if an update is a
fast-forward or not: if we find a SHA1 in the reflog, and see there
were only FF updates since, we can be sure that the SHA1 is reachable
from the branch, without having to open packfiles and decode commits.

For the reftable format, I think we could store this easily by
introducing more record types. [snip].

Aside from what others have mentioned here, you're talking about the
log_type field are you not? I.e.:
https://googlers.googlesource.com/sop/jgit/+/reftable/Documentation/technical/reftable.md#log-block-format

Correct.

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Has that "log_type = 0x0" tombstone proven to be a worthwhile
optimization past the stash case mention there (which is presumably not
relevant to the vast majority of Google's use-cases).

I've never really understood the log_type=0x0 use case. I think it was
added solely to cater for a use case in CGit's stash command.

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I.e. it's redundant to looking at the record and seeing if new_id =
ZERO_OID.

Similarly can't ff v.s. non-ff be deduced unambiguously by looking ahead
to the next record, and seeing if the current record's "old_id" matches
that of the last record's "new_id". If it does it's a FF, if not it's a
non-FF (or a create/delete).

I don't see how that will tell you FF vs non-FF-ness.  Both an FF
update and a non-FF  update look like 'new_oid = 20-random-bytes'.
Barring further info, you have to lookup the commit object for those
bytes, and then walk back to see if you pass old_oid.

Because both the next reflog format and reftable's have the update-ref
<newvalue> <oldvalue> for each update. So:

    $ cut -d ' ' -f1-2 .git/logs/refs/remotes/origin/master | head -n 2
    1c52ecf4ba0f4f7af72775695fee653f50737c71 ba2aa15129e59f248d8cdd30404bc78b5178f61d
    ba2aa15129e59f248d8cdd30404bc78b5178f61d 6d3ef5b467eccd2769f1aa1c555d317d3c8dc707

We can know with !strcmp(rows[0][1], rows[1][0]) whether the latest
update is a ff or non-ff (it is). As opposed to:

    $ cut -d ' ' -f1-2 .git/logs/refs/remotes/origin/seen | head -n 2
    8b26a41f4bf7e7c0f097cb91012d08fe8ae30e7f 0f3a981cbd5be5f97e9504ab770cd88f988fe820
    0f3a981cbd5be5f97e9504ab770cd88f988fe820 fdd019edfe6bc60d0100d5751c41e4f6ad28a2ef

Where the rows[0][1] value is not the same as rows[1][0].

What you can't do is find whether any given commit(s) in those ranges
are part of the FF or not, since you just have the start/end points.

But I'm vaguely paranoid that we're talking past one another here and
I've misunderstood what you want...

AFAICT, a correct sequence of ref updates (FF or not) always has
prev.new_oid = current.old_oid.

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[Ævar: snipped from earlier] Today we have 0 = deletion, 1 = update,
and we could add 2 = FF update, 3 = non-FF update.

I've written log table implementations (a site table in a RDBMS) for git
(one table for refs) which had:

    create, ff, non-ff, delete

I wonder if that quad-state would be useful for reftable too, with this
proposed change you'd still need to unpack the record and see if the
old_id is ZERO_OID to check if it's a creation, would you not?

Delete & create are handled with ZERO_OID.

The reftable format makes it so that you have to decode a record in
order to read past it (there is no size framing the table entry
level), so there is no big performance advantage in encoding this
information in the log_type. You merely use a log_type bit rather than
a 20 byte raw ID. Since log records are zlib compressed anyway, it
probably also makes no space difference.

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I also wonder if it couldn't be:

    0 = deletion, 1 = non-ff-update, 2 = ff-update, 4 = creation

So the format wouldn't forever carry the historical wart of this not
having been considered from the beginning.

If you do it like this, you will force that all implementations to
have to compute whether a (forced) update is a FF or not. I don't know
if that is a problem. A 'maybe non-FF' value would be useful. Perhaps
we could even do simply

     0 = deletion, 1 = maybe-ff-update, 2 = guaranteed-ff-update

The point is that nobody relies on this now, but also that you'd of
course want 1 = i-know-for-sure-non-ff-update, 2 =
i-know-for-sure-ff-update.

But since logs are transitory (aren't they also expired in reftable,
can't see that from skimming hte spec) and this is just a helpful
side-index I think a one-time migration for the tiny minority of
reftable users who'd care would be preferrable to the vast majority of
future reftable users (if it ever lands in git.git) having to deal with
this (albeit small) special-case forever.

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It would mean that the few current reftable users (just Google?) would
have to look at the record to see if it's *really* a non-ff-update, but
presumably they need to do so now for ff v.s. non-ff, so they're no
worse off than they are now.

At Google, we currently don't record log records in reftable yet. From
our perspective, we could probably change the standard 'in place'.
JGit has supported reftable since Nov 2019, but I'm unaware of users;
I did hear about GerritForge wanting to try it out in production this
year.

Ah, so not even Google's using it, the backcompat is just for good
measure...

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Then when those users know they're on a version that distinguishes these
they can hard rely on 1 not being a "ff for sure", not a "maybe" status
for new updates. Presumably they either don't care about ancient reflog
records, or a one-off migration of rewriting the records for older
entries could be done.

Also between my [1] and this proposal we have at least a reftable v1.01
in the wild (the filename locking behavior change discussed in [1]), and
this would make it v1.02, but the only up-to-date spec is for v1.00 (and
maybe JGit has other changes I haven't tracked).

The file locking update has been added to the standard,
https://github.com/git/git/pull/951.

Ah yes. Now I remember. I managed to miss/not recall that. Thanks.