From: Lorenzo Pegorari <lorenzo.pegorari2002@gmail.com>
Date: Tue, 07 Apr 2026 22:07:02 GMT
Subject: Re: [GSoC PATCH v3 2/5] pack-write: add helper to fill promisor file after repack
Message-ID: <adWABoOqHNkTTTyR@lorenzo-VM>
In-Reply-To: <xmqq8qazai3b.fsf@gitster.g>

On Mon, Apr 06, 2026 at 02:34:32PM -0700, Junio C Hamano wrote:
> LorenzoPegorari <lorenzo.pegorari2002@gmail.com> writes:
> 
> > A ".promisor" file may contain ref names (and their associated hashes)
> > that were fetched at the time the corresponding packfile was downloaded.
> > This information is used for debugging reasons. This information is
> > stored as lines structured like this: "<oid> <ref>".
> >
> > Create a `copy_promisor_content()` helper function that allows this
> > debugging info to not be lost after a `repack`, by coping it inside a new
> > ".promisor" file.
> 
> "coping" -> "copying"

Ack.

> > The function logic is the following:
> >  * Take all ".promisor" files contained inside the given `repo`.
> >  * Ignore those whose name is contained inside the given `strset
> >    not_repacked_names`, which basically acts as a "promisor ignorelist"
> >    (intended to be used for packfiles that have not been repacked).
> >  * Read each line of the remaining ".promisor" files, which can be:
> >     * "<oid> <ref>" if the ".promisor" file was never repacked. If so,
> >       add the time at which the ".promisor" file was last modified <time>
> >       to the line to create the string: "<oid> <ref> <time>".
> >     * "<oid> <ref> <time>" if the ".promisor" file was repacked. If so,
> >       don't modify it.
> >  * Ignore the line if its <oid> is not present inside the
> >    "<packtmp>-<dest_hex>.idx" file.
> >  * If the destination file "<packtmp>-<dest_hex>.promisor" does not
> >    already contain the line, append it to the file.
> >
> > The function assumes that the contents of all ".promisor" files are
> > correctly formed.
> >
> > The time of last data modification is used in place of the time of file
> > creation, because the former is much easier to obtain than the latter
> > one.
> 
> The time of file creation is not recorded anywhere if you are
> dealing with the usual UNIX filesystems (ctime is not creation
> time), so it is not the issue of "easier to obtain".

That's what I found out during my researches, but I wasn't sure. Thanks
for confirming it.

> The reason why this design chooses to add time is because in a
> never-repacked .promisor file, the modification time of the file
> itself can be used when you compare the entries in it with entries
> in another .promisor file that did get repacked.  By having
> timestamp, the debugger can tell at which time the refs at the
> remote repository pointed at what object---the same ref may appear
> twice in the same .promisor file and having timestamps would help
> understanding what happened over time.

Exactly. I'll improve the commit message to better explain the utility
of these timestamps.

Thanks,
Lorenzo