Re: format-patch: "magic" mbox timestamp

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Nov 10, 2020, 23:12 UTC
Message-ID: <nycvar.QRO.7.76.6.2011102347320.18437@tvgsbejvaqbjf.bet>
In-Reply-To: <CAETBDP6d8UwiJEF_pX0p=xLG79pwHeEtectmOnjPiUpjUCPaqw@mail.gmail.com>

Hi Brandon,

On Tue, 10 Nov 2020, Brandon Richardson wrote:

Show 31 quoted lines

> After submitting a patch internally within our organization today, I
> was looking through the `format-patch` output and was curious to see
> the strange timestamp on the "From" line. At first glance I thought
> the parent commit timestamp might have been off, but that wasn't the
> case. I thought it might be a bug but quickly noticed the fixed
> timestamp string in `log-tree`.
>
> Reading through the various revisions of `log-tree.c` didn't answer
> many questions either, until I turned to the docs and read:
>
> > The patch produced by git format-patch is in UNIX mailbox format, with a fixed "magic" time stamp to indicate that the file is output from format-patch rather than a real mailbox [...]
>
> I find this pretty interesting, and would like to hear more from those
> that introduced change. It looks like this was first introduced in
> 3eefc18917 (Tentative built-in format-patch., 2006-04-18), albeit with
> a different "magic" timestamp, and then changed to its current
> timestamp value in 698ce6f87e (fmt-patch: Support --attach,
> 2006-05-20).
>
> Please correct me if I'm wrong, but I'm assuming the "UNIX mailbox
> format" referenced in the docs refers to the mbox database format
> described in appendix A of RFC-4155. If so, since we use a commit id
> in place of the sender email address, would that itself be sufficient
> to indicate that the output isn't from a real mailbox? A commit id
> will never match the addr-spec in RFC-2822, so I figure that anyone
> looking at `format-patch` output could safely assume that it did not
> originate from a mailbox.
>
> I could see this as a good opportunity to use a more relevant
> timestamp, perhaps the commit timestamp of the first patch in the
> series.

An interesting hike down memory lane. As to the explanation for the latter change ("fmt-patch: Support --attach"), here is your answer: https://lore.kernel.org/git/Pine.LNX.4.63.0605201904320.31887@wbgn013.biozentrum.uni-wuerzburg.de/

	On Sat, 20 May 2006, Jakub Narebski wrote:

	> Johannes Schindelin wrote:
	>
	> > -             printf("From %s  Thu Apr 7 15:13:13 2005\n",
	> > -                    sha1_to_hex(commit->object.sha1));
	> > +             printf("From %s Mon Sep 17 00:00:00 2001\n", sha1);
	>
	> What's with the date?

	Does not matter. This is not part of the mail header (the date is
	displayed as a "Date:" line).

	I only changed the "From " line (note the missing ":") to match the output
	from git-format-patch.sh.

As to "Tentative built-in format-patch", I can only _guess_ that the timestamp was chosen to be identical to the first ever Git commit's timestamp: e83c5163316 (Initial revision of "git", the information manager from hell, 2005-04-07).

The original magic timestamp, however, was the one I reintroduced, and as you can see from the part I quoted above, I only imitated the original script. I do remember, even if it has been a lifetime ago, that I wondered what kind of special date September 17th could have been for Junio, but I felt it might have been too personal a question. That timestamp was introduced into Git's source code in 5c2c972f474 ([PATCH] format-patch: --mbox and --check., 2005-07-22), but the commit message is also quite mum about the motivation between that particular timestamp.

Ciao, Johannes