Re: [PATCH/RFC] get_oid: new extended SHA-1 syntax to control resolution process

[Duy Nguyen <pclouds@gmail.com>]

On Sun, Jun 30, 2019 at 4:30 PM Jakub Narebski <jnareb@gmail.com> wrote:
>
> Bikeshed painting ahead.
>
> Nguyễn Thái Ngọc Duy <pclouds@gmail.com> writes:
> [...]
> > The problem is we try every possible way to resolve a rev. Let's have
> > some annotation to express that we only want to resolve a rev in a
> > certain way:
> >
> > - <hash>@{hash} only accepts a full hash or a short hash. If it's a
> >   short hash, it cannot be ambiguous.
> >
> > - <ref>@{literal} only accepts full ref. No turning "master" into
> >   "refs/heads/master".
> >
> > - <output>@{describe} interprets <output> as git-describe output
> >   only, not an object name or a reference.
> >
> > This gives scripts much better control over get_oid(), which
> > translates to rev-parse and a bunch other commands.
> >
> > PS. The new syntax can stack with existing ones. E.g. you could write
> > refs/heads/master@{literal}@{yesterday} or <hash>@{hash}^{tree}.
> > Perhaps I should allow these tags at the end too, so you can enforce a
> > variable like "$REV"@{literal} where $REV could be even HEAD~123
>
> I think it would be better to use <hash>^{hash} instead of
> <hash>@{hash}.
>
> The <ref-ish>@{<something>} is used currently for information that is
> outside the DAG, like @{<date>}, @{<n>}, @{-<n>} uses information from
> reflog, and @{upstream} and @{push} uses information from the config.
>
> On the other hand ^{<type>}, ^{/<search text>}, and the future
> ^{<hashalgo>} all use DAG-only information.

I wasn't aware of ^{<hashalgo>}. hash-function-transition.txt is a bit
light on the exact behavior, but maybe we can just define it like
<hash>@{hash} in this thread (then we don't even need to introduce
@{hash})?

That is, if <hash>^{<hashalgo>} fails to find a matching object, the
get_oid() result is "not found", there will be no further attempt to
interpret the given revision in a different way (e.g. try to see if
the same ref exists...). It also means <hash>^{<hashalgo>} will remain
supported even after the sha-512 transition period.

@{describe} might go  the same way as @{hash} (that is, becoming
^{describe}). I think it's just a slightly different way to write
hashes.

> Though one could argue that refs information _is_ outside the DAG...

Refs info is outside DAG. But that's mostly for <ref>@{literal} and it
allows something like <ref>@{literal}@{upstream}. But hash (or object
name) is already used inside DAG (to link between commits, trees and
blobs). Though one might still argue that's only true for full hash,
not short ones.
-- 
Duy