[BUG] Destructive access to an "objects/info/alternates" repository

From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Dec 28, 2020, 18:50 UTC
Message-ID: <jwvpn2tdb0r.fsf-monnier+gmane.comp.version-control.git@gnu.org>

[ Resending with a subject that clarifies that it's a bug report.  ]

> Thank you for filling out a Git bug report!
> Please answer the following questions to help us understand your issue.

> What did you do before the bug happened? (Steps to reproduce your
> issue)

I did basically:

    git clone --reference ~otheruser/nongnu
    [ Days passed and I did various things.  ]
    cd nongnu; git gc

Note that `otheruser` is [..wait for it..] another user, so I only have read access to the referenced repository.

> What did you expect to happen? (Expected behavior)

That it would GC the local part of the repository and that it wouldn't try to modify the referenced repository.

> What happened instead? (Actual behavior)

I got warnings indicating that Git was trying to delete files from the referenced repository:

    Enumerating objects: 35095, done.
    Nothing new to pack.
    warning: unable to unlink
'/home/otheruser/nongnu/.git/objects/pack/pack-0d85e74ac2f7e51ce26f281e64eb738e8182fa95.idx':
Permission denied
    warning: unable to unlink
'/home/otheruser/nongnu/.git/objects/pack/pack-5fe14feff49ccdee5469af9dc94f6784e8464a6b.idx':
Permission denied

> What's different between what you expected and what actually happened?

The warnings.

> Anything else you want to add:

The warnings make me wonder if I should worry about other possible misbehaviors when using such a setup.

> Please review the rest of the bug report below.
> You can delete any lines you don't wish to share.

This is on a Debian stable system but with Git from Debian testing (because the version from Debian stable crashed in a `git log` command I was using in my scripts).

[System Info] git version: git version 2.29.2 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2 (2020-04-29) x86_64 compiler info: gnuc: 10.2 libc info: glibc: 2.28 $SHELL (typically, interactive shell): /bin/bash

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