[PATCH 0/3] tag: keep the message file in case ref transaction fails

The ref transaction can fail after the message has been written using the editor. The ref transaction is attempted after the message file (`TAG_EDITMSG`) has been unlinked, so there is no backup tag message file to retry the command.[1]

This is unfortunate if someone has written more than e.g. “v1.99.4” in the editor. (I don’t know if people write long tag messages in practice.)

Hold on to the tag message file until after the ref transaction in order to preserve the backup.

† 1: On commit 91428f078b (The eighteenth batch, 2023-05-10)

§ Reproduction script

``` cd /tmp dir=$(mktemp -d) cd $dir git init git commit --allow-empty -mInit git tag release/v1 # Fails git tag -a release ```

Error message:

``` fatal: cannot lock ref 'refs/tags/release': 'refs/tags/release/v1' exists; cannot create 'refs/tags/release' ```

Better error message and behavior:

``` The tag message has been left in .git/TAG_EDITMSG fatal: cannot lock ref 'refs/tags/release': 'refs/tags/release/v1' exists; cannot create 'refs/tags/release' ```

§ Alternatives considered

My first thought was to find a way to “dry run” the ref update before opening the editor (the edge case of the ref update command succeeding the first time but not the second *real* time seemed incredibly unlikely to happen by happenstance, so I saw no reason to consider that). However that seemed like it would involve more code and conditionals, and I don’t know if the dry-run mode is even supported.

A benefit of this alternative approach would be to error out immediately instead of opening the editor. But trying to create a tag which collides with an existing “namespace” seems very unlikely to happen in practice.[2] Losing a file is much worse than being inconvenienced to retry the command, so I decided to just focus on the former problem.

Most importantly though this approach was within my ability to implement.

† 2: Just observe my “Reproduction script”: one tries to create `release` after
    someone else made `release/v1`. But what is just “release”? What follows
    (next version) that? But why am I arguing against my change…

§ CI

https://github.com/LemmingAvalanche/git/actions/runs/4972149825

§ Carbon copy

The suggested contacts seemed too long:

``` $ ./contrib/contacts/git-contacts v2.40.1.. Junio C Hamano <> René Scharfe <> Jonathan Tan <> Jonathan Nieder <> Jeff King <> Stefan Beller <> Denton Liu <> Robert Dailey <> ```

So I added only Jeff King based on commit 3927bbe9a4 (tag: delete TAG_EDITMSG only on successful tag, 2008-12-06).

§ Patches

1. Test the happy case
2. Test the unhappy case (fails without the next patch)
3. The change

—

Cheers.

Kristoffer Haugsbakk (3):
  t/t7004-tag: add regression test for existing behavior
  t/t7004-tag: add failing tag message file test
  tag: keep the message file in case ref transaction fails

 builtin/tag.c  | 24 +++++++++++++++---------
 t/t7004-tag.sh | 19 +++++++++++++++++++
 2 files changed, 34 insertions(+), 9 deletions(-)