bug#63775: git describe on current master says: v1.3.0-38775-g6192acf8b7

bug#63775: git describe on current master says: v1.3.0-38775-g6192acf8b7

[Giovanni Biscuolo]

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Hi Jonathan,

I'm CC'ing guix-devel because I suspect many users who cloned/updated
the Guix repo are having the same results... and concerns.

This is a git bug, not an issue with our repo, and for this reason (I
hope) I'm closing this bug; please see below.

Jonathan Brielmaier via Bug reports for GNU Guix <bug-guix@gnu.org>
writes:

> Hm, I'm hitting this bug while trying to work on the openSUSE package.
> They offer a way to build RPM packages from the most recent master
> commit, but it's get the wrong version (1.3.0 instead of 1.4.0) due to
> this `git describe` result.

As pointed out by Simon last June the result of "git describe" is not
what users should get given the "Search strategy" documented in the
command manual: https://git-scm.com/docs/git-describe#_search_strategy:

--8<---------------cut here---------------start------------->8---

If multiple tags were found during the walk then the tag which has the
fewest commits different from the input commit-ish will be selected and
output. Here fewest commits different is defined as the number of
commits which would be shown by git log tag..input will be the smallest
number of commits possible.

--8<---------------cut here---------------end--------------->8---

The upstream bug report (and a reproducer) is this one:
«Subject: [BUG] `git describe` doesn't traverse the graph in topological
order»
https://lore.kernel.org/git/ZNffWAgldUZdpQcr@farprobe/

Another user also reported the issue and a reproducer:
https://public-inbox.org/git/PH0PR08MB773203CE3206B8DEFB172B2F94839@PH0PR08MB7732.namprd08.prod.outlook.com/

The "executive summary" is that "git describe" gets the count of "fewest
commits different from the input commit-ish" wrong (see anso previous
messages in this thread for details).

Anyway, even if this bug was solved, I'd warmly suggest NOT to base the
check for the latest stable Guix commit (usually tagged as v[0-9]*) on
the result of "git describe".

Today, if "guix describe" had no bugs, the correct result would be:
"base-for-issue-62196"... AFAIU :-)

This is a reproducer:

--8<---------------cut here---------------start------------->8---

$ git describe $(git rev-list --tags --max-count=1)
base-for-issue-62196

--8<---------------cut here---------------end--------------->8---

To get the value corresponding to the latest tagged version, we should
testrict the list of tags to the ones matching the "v[0-9]*" regexp:

--8<---------------cut here---------------start------------->8---

$ git describe $(git rev-list --tags="v[0-9]*" --max-count=1)
v1.4.0

--8<---------------cut here---------------end--------------->8---

To browse all the tags there is the "git tag" command, for example to
have the list and description of every Guix released version:

--8<---------------cut here---------------start------------->8---

$ git tag -l "v[0-9]*" --sort=-creatordate -n
v1.4.0          GNU Guix 1.4.0.
v1.4.0rc2       GNU Guix 1.4.0rc2.
v1.4.0rc1       GNU Guix 1.4.0rc1.
v1.3.0          GNU Guix 1.3.0.
v1.3.0rc2       GNU Guix 1.3.0rc2.
v1.3.0rc1       GNU Guix 1.3.0rc1.
v1.2.0          GNU Guix 1.2.0.
v1.2.0rc2       GNU Guix 1.2.0rc2.
v1.2.0rc1       GNU Guix 1.2.0rc1.
v1.1.0          GNU Guix 1.1.0.
v1.1.0rc2       GNU Guix 1.1.0rc2.
v1.1.0rc1       GNU Guix 1.1.0rc1.
v1.0.1          GNU Guix 1.0.1.
v1.0.0          GNU Guix 1.0.0.
v0.16.0         GNU Guix 0.16.0.
v0.15.0         GNU Guix 0.15.0.
v0.14.0         GNU Guix 0.14.0.
v0.13.0         GNU Guix 0.13.0.
v0.12.0         GNU Guix 0.12.0
v0.11.0         GNU Guix 0.11.0.
v0.10.0         GNU Guix 0.10.0.
v0.9.0          GNU Guix 0.9.0.
v0.8.3          GNU Guix 0.8.3.
v0.8.2          GNU Guix 0.8.2.
v0.8.1          GNU Guix 0.8.1.
v0.8            GNU Guix 0.8.
v0.7            GNU Guix 0.7.
v0.6            GNU Guix 0.6.
v0.5            GNU Guix 0.5.
v0.4            GNU Guix 0.4.
v0.3            GNU Guix 0.3.
v0.2            GNU Guix 0.2.
v0.1            GNU Guix 0.1.
v0.0            Guix 0.0, initial announcement.

--8<---------------cut here---------------end--------------->8---

HTH!

Happy hacking, Gio'

-- 
Giovanni Biscuolo

Xelera IT Infrastructures

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consider "git describe"... harmful? (if misused)

[Giovanni Biscuolo]

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Hello developers,

Ipse dixit: a tag is a tag is a tag.

Sorry to stress on this but AFAIU "git describe" and it's variants is
(mis)used by some (many?) to obtain the last revision number of packages
got from a git tag on a repo, even in few upstream build config/scripts
(patched in Guix); here are just a few examples I've observed from
messages in this mailing list and our package definitions:

- https://yhetil.org/guix/7a759ffb-fca8-478d-a4aa-08e6b674da75@archlinux.org:
  `git describe --tags`, which is often used for --version output
  (especially in Go projects)

- https://yhetil.org/guix/87ediha5p0.wl-hako@ultrarare.space: I usually
  obtain the revision number from the output of 'git describe --tags', I
  think it's fine to use it when available.

- https://yhetil.org/guix/c93c18e5-8e01-45a0-b79f-05d72f6f8230@archlinux.org
  The output of `git describe --always --tags --dirty` was also embedded.

Some code/comments I got running "find . -type f -exec grep --color=auto
-nH --null -e "git describe" \{\} +" in "<guix-repo>/gnu/packages", in
Emacs:

- ./audio.scm:751: ;; Ardour expects this file to exist at build time.  The
                   revision is the output of git describe HEAD | sed
                   's/^[A-Za-z]*+//'

- ./build-tools.scm:589: (substitute* "src/tup/link.sh" (("`git
  describe`") ,version))
 
- ./linux.scm:7263: ;; the checkout lacks a .git directory, breaking ‘git
  describe’.
  
- ./axoloti.scm:500: ;; TODO: this is the output of: git describe --long
  --tags --dirty --always

IMHO "git describe" should never be used to obtain the last revision
for the reasons I explained in my previous message (see a quote below):
IF you get it right is ONLY by chance (probably it's most of the times),
not by **design**; executive summary:

1. "git describe [--tag]" have a bug and doesn't traverse the graph in
topological order; for the Guix git repo this means that now the last
"git describe" tell us something like "v1.3.0-53609-gc70c513317" (the
number of commits and the commit hash may vary depending on last "git
pull"), not something like...

2. is NOT guaranteed that the last tag reported by "git describe
[--tag]" (even if the above mentioned bug is resolved) is the one
corresponding to a released revision of the software, since tags (even
annotated one) can be added by repo committers for any reason they find
useful; i.e. the last tag commited gor the Guix repo is
base-for-issue-62196.  If and ONLY IF committers use a recognised
pattern for the tag - i.e. v<semver> - we can get the last (tagged)
revision from git (see below for alternative to "

Giovanni Biscuolo <g@xelera.eu> writes:

[...]

> The upstream bug report (and a reproducer) is this one:
> «Subject: [BUG] `git describe` doesn't traverse the graph in topological
> order»
> https://lore.kernel.org/git/ZNffWAgldUZdpQcr@farprobe/
>
> Another user also reported the issue and a reproducer:
> https://public-inbox.org/git/PH0PR08MB773203CE3206B8DEFB172B2F94839@PH0PR08MB7732.namprd08.prod.outlook.com/
>
> The "executive summary" is that "git describe" gets the count of "fewest
> commits different from the input commit-ish" wrong (see anso previous
> messages in this thread for details).
>
> Anyway, even if this bug was solved, I'd warmly suggest NOT to base the
> check for the latest stable Guix commit (usually tagged as v[0-9]*) on
> the result of "git describe".
>
> Today, if "guix describe"

I mean "git describe", sorry!

> had no bugs, the correct result would be:
> "base-for-issue-62196"... AFAIU :-)
>
> This is a reproducer:
>
> --8<---------------cut here---------------start------------->8---
>
> $ git describe $(git rev-list --tags --max-count=1)
> base-for-issue-62196
>
> --8<---------------cut here---------------end--------------->8---
>
> To get the value corresponding to the latest tagged version, we should
> testrict the list of tags to the ones matching the "v[0-9]*" regexp:
>
> --8<---------------cut here---------------start------------->8---
>
> $ git describe $(git rev-list --tags="v[0-9]*" --max-count=1)
> v1.4.0
>
> --8<---------------cut here---------------end--------------->8---

More efficient alternative:

--8<---------------cut here---------------start------------->8---

$ git tag --list 'v*' --sort=-creatordate | head -1
v1.4.0

--8<---------------cut here---------------end--------------->8---

[...]

Should we add some notes (a footnote?) in our Guix manual?

WDYT?

-- 
Giovanni Biscuolo

Xelera IT Infrastructures

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Re: bug#63775: git describe on current master says: v1.3.0-38775-g6192acf8b7

[Simon Tournier]

Hi,

On sam., 03 févr. 2024 at 19:43, Giovanni Biscuolo <g@xelera.eu> wrote:

> This is a git bug, not an issue with our repo, and for this reason (I
> hope) I'm closing this bug; please see below.

Here the explanation of the bug of “git describe”:

    https://lore.kernel.org/git/20191008123156.GG11529@szeder.dev/
    
      $ git describe d1a251a1fa
      v2.23.0-135-gd1a251a1fa
      $ git log --oneline v2.23.0..d1a251a1fa | wc -l
      59

    Uh-oh, 59 != 135.

    This is happening because:

      - Git is too fast ;) and the committer date has a one second
        granularity, so scripts can easily create subsequent commits with
        the same committer date.  Case in point are the two subsequent
        merge commits f3c19f85c5 and 4a3ed2bec6 at the bottom of this
        simplified history snippet (kind of a hand-edited 'git log --graph
        --format="%h %cd %s"'):

        *   d1a251a1fa 2019-09-09 12:26:36 -0700 Merge branch 'en/checkout-mismerge-fix'
        |\
        * | ... a big chunk of history simplified away ...
        | * acb7da05ac 2019-08-16 09:58:00 -0700 checkout: remove duplicate code
        * | a5e4be2f68 2019-04-25 16:41:15 +0900 Merge branch 'ab/commit-graph-fixes'
        * | f3c19f85c5 2019-04-25 16:41:14 +0900 Merge branch 'ab/gc-reflog'
        |/
        *   4a3ed2bec6 2019-04-25 16:41:14 +0900 Merge branch 'nd/checkout-m'

      - 'git describe' implements its own history traversal: it iterates
        over all parents of a commit, adds any yet unseen parents to a
        commit list ordered by date, and then continues with the first,
        i.e. most recent commit from that list.  While doing so it uses
        several bits in 'commit->object.flags' to track reachability
        information from several candidate tags at once, and copies these
        flags from each commit to its parents; this is important to
        compute the correct number of additional commits.  Another
        important thing is the implementation detail that
        commit_list_insert_by_date() inserts a new commit after all other
        commits with the same date that are already in the list.


Thanks Giovanni for pointing this out.

Cheers,
simon