Re: [PATCH v3 0/3] Remove special casing for PSEUDOREF updates - Han-Wen Nienhuys

From: Han-Wen Nienhuys <hanwen@google.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Han-Wen Nienhuys via GitGitGadget <gitgitgadget@gmail.com>,
	git <git@vger.kernel.org>, Han-Wen Nienhuys <hanwenn@gmail.com>
Subject: Re: [PATCH v3 0/3] Remove special casing for PSEUDOREF updates
Date: Mon, 3 Aug 2020 21:07:18 +0200	[thread overview]
Message-ID: <CAFQ2z_O1sRm-_SNP=-GvgNLqB+qgf6k9YVfbF1XCAmFWdeX6Ew@mail.gmail.com> (raw)
In-Reply-To: <xmqqpn8hgca7.fsf@gitster.c.googlers.com>

On Mon, Jul 27, 2020 at 6:20 PM Junio C Hamano <gitster@pobox.com> wrote:
> > On Fri, Jul 17, 2020 at 12:10 AM Junio C Hamano <gitster@pobox.com> wrote:
> >> I reviewed some codepaths that deal with FETCH_HEAD recently.
> >>
> >> As the file is quite different from all the other pseudo references
> >> in that it needs to store more than one object name and in that each
> >> ref in it needs more than just the object name, I doubt that it
> >> makes much sense to enhance the refs API so that its requirements
> >> can be covered.
> >
> > I agree. Do we ever pretend that FETCH_HEAD is a ref today?
>
> "git rev-parse FETCH_HEAD", "git show FETCH_HEAD" etc. all should keep
> working, so in that sense, it is treated as a ref.

I added this to the last version of the full reftable patch series
that I posted,  as patches
"Split off reading loose ref data in separate function" and "Read
FETCH_HEAD as loose ref".

Which other refs that aren't really refs should also be supported? The
JGit source code suggests that MERGE_HEAD should also be special
cased?

It's not pretty, though. The entry point is read_raw_ref(), which gets
a ref_store as argument. ref_store doesn't have generic API to get at
the repository directory, so the implementation of reading the
FETCH_HEAD file has to be pushed down to the ref backend (which does
know the directory).

> It does not
> protect the history leading to the objects listed in it from being
> collected, though.

Is there documented agreement of how pseudo refs and GC should interact?

> "git merge FETCH_HEAD" is an interesting case---I haven't thought it
> through.
>
> What should happen after "git pull origin foo bar" attempts to grab
> two branches and make an octopus merge into the branch currently
> checked out, and then "git reset --hard && git merge FETCH_HEAD" is
> given?

I don't understand this question.

-- 
Han-Wen Nienhuys - Google Munich
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