Subject: Re: [PATCH v2] builtin/refs: add '--skip-reflog' flag to bypass reflog migration

From: Junio C Hamano <[email-masked]>
Date: Tue, 11 Feb 2025 09:48:22 -0800

Karthik Nayak <[email-masked]> writes:

> The 'git-refs(1)' migrate subcommand, which transfers repositories
> between reference backends, currently migrates reflogs by default as of
> 246cebe320 (refs: add support for migrating reflogs, 2024-12-16).
>
> While this behavior is desirable for most client-side repositories,
> server-side repositories are not expected to contain reflogs. However,
> due to historical reasons, some may still have them. This could be
> caused, for example, by bugs, misconfiguration, or an administrator
> enabling reflogs on the server for debugging purposes.
>
> To address this, introduce the --skip-reflog flag, allowing users to
> bypass reflog migration. This ensures that the repository ends up in the
> expected state after migration.
>
> Helped-by: Patrick Steinhardt <[email-masked]>
> Signed-off-by: Karthik Nayak <[email-masked]>
> ---
> Changes in v2:
> - Fix typo in commit mesasge and clarify the intent.
> - Modify the test to use `test_line_count` and `test_must_be_empty`.
> - Link to v1: https://lore.kernel.org/r/20250207-477-refs-migrate[email-masked]
> ---
> Range-diff versus v1:
>
> 1:  ce14d3d07e ! 1:  6b83089348 builtin/refs: add '--skip-reflog' flag to bypass reflog migration
>     @@ Commit message
>      
>          The 'git-refs(1)' migrate subcommand, which transfers repositories
>          between reference backends, currently migrates reflogs by default as of
> ...
>      +		'
>       	done
>       done
> ---
>  builtin/refs.c          |  3 +++
>  refs.c                  |  8 +++++---
>  refs.h                  |  5 ++++-
>  t/t1460-refs-migrate.sh | 19 +++++++++++++++++--
>  4 files changed, 29 insertions(+), 6 deletions(-)

This is tangent that is totally unrelated to the theme of this
patch, but I find that the placement of range-diff makes it very
hard to follow.

After skimming the proposed log message, the next thing I would want
to see is the list of paths that are modified, before deciding I
want to review the patch now.  Once I decide to read it _now_, the
changes from the previous iteration and range-diff becomes relevant.

Is it just me who decides in what order to review the patches and
then reviews them in that order?

Anyway.

> diff --git a/builtin/refs.c b/builtin/refs.c
> index a29f19583474518ee0942ea53c39cbdf9661c5e2..30be0254c14dd3d07693d70c25dddc9990756e9c 100644
> --- a/builtin/refs.c
> +++ b/builtin/refs.c
> @@ -30,6 +30,9 @@ static int cmd_refs_migrate(int argc, const char **argv, const char *prefix,
>  		OPT_BIT(0, "dry-run", &flags,
>  			N_("perform a non-destructive dry-run"),
>  			REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN),
> +		OPT_BIT(0, "skip-reflog", &flags,
> +			N_("skip migrating reflogs"),
> +			REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG),
>  		OPT_END(),
>  	};
>  	struct strbuf errbuf = STRBUF_INIT;

OK.

> diff --git a/refs.c b/refs.c
> index f4094a326a9f88f979654b668cc9c3d27d83cb5d..5e8f5c06fa68d16c93ee11edd9742995eea994b6 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -3035,9 +3035,11 @@ int repo_migrate_ref_storage_format(struct repository *repo,
>  	if (ret < 0)
>  		goto done;
>  
> -	ret = refs_for_each_reflog(old_refs, migrate_one_reflog, &data);
> -	if (ret < 0)
> -		goto done;
> +	if (!(flags & REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG)) {
> +		ret = refs_for_each_reflog(old_refs, migrate_one_reflog, &data);
> +		if (ret < 0)
> +			goto done;
> +	}
>  
>  	ret = ref_transaction_commit(transaction, errbuf);
>  	if (ret < 0)
> diff --git a/refs.h b/refs.h
> index a0cdd99250e8286b55808b697b0a94afac5d8319..ccee8fc6705e6e93a1c6234e7395180f8dfd815b 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -1157,8 +1157,11 @@ int is_pseudo_ref(const char *refname);
>   *   - REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN: perform a dry-run migration
>   *     without touching the main repository. The result will be written into a
>   *     temporary ref storage directory.
> + *
> + *   - REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG: skip migration of reflogs.
>   */
> -#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN (1 << 0)
> +#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN      (1 << 0)
> +#define REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG (1 << 1)

These are quite a mouthful, but they are only used between the refs
subsystem and "git refs" command, so being overly verbose and
specific is a good thing.

>  /*
>   * Migrate the ref storage format used by the repository to the
> diff --git a/t/t1460-refs-migrate.sh b/t/t1460-refs-migrate.sh
> index a6d9b35a46eb59350aa0d59d982a2fbfaecf1448..28c0024a4c8cb8282bf586840265edba442f5056 100755
> --- a/t/t1460-refs-migrate.sh
> +++ b/t/t1460-refs-migrate.sh
> @@ -9,14 +9,16 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
>  
>  # Migrate the provided repository from one format to the other and
>  # verify that the references and logs are migrated over correctly.
> -# Usage: test_migration <repo> <format> <skip_reflog_verify>
> +# Usage: test_migration <repo> <format> <skip_reflog_verify> <...options>
>  #   <repo> is the relative path to the repo to be migrated.
>  #   <format> is the ref format to be migrated to.
>  #   <skip_reflog_verify> (true or false) whether to skip reflog verification.
> +#   <...options> are other options be passed directly to 'git refs migrate'.

Yuck.  I've never seen preceding three-dots.  Makes readers wonder
how this thing behaves differently from the usual <options...>.

Doubly yuck is ...

>  test_migration () {
>  	repo=$1 &&
>  	format=$2 &&
>  	skip_reflog_verify=${3:-false} &&
> +	shift $(( $# >= 3 ? 3 : 2 )) &&

... this thing.  The above usage declares that <skip_reflog_verify>,
just like <repo> and <format>, are required so I do not see why we
need to be conditional here.  If you want to make it optional,
writing it this way instead ...

	repo=$1 &&
        format=$2 &&
	shift 2 &&

        skip_reflog_verify=false &&
	if test $# -gt 1
	then
		skip_reflog_verify=$1
		shift
	fi

... would make it easier to extend when you ever want to add the
next optional positional parameter.

>  	git -C "$repo" for-each-ref --include-root-refs \
>  		--format='%(refname) %(objectname) %(symref)' >expect &&
>  	if ! $skip_reflog_verify
> @@ -25,7 +27,7 @@ test_migration () {
>  	   git -C "$repo" reflog list >expect_log_list
>  	fi &&
>  
> -	git -C "$repo" refs migrate --ref-format="$2" &&
> +	git -C "$repo" refs migrate --ref-format="$format" $@ &&

It is a good change to use the named variable we captured upfront,
but did you mean to leave $@ unquoted?

Unless you want to use "$@" to invoke the magic "pass each parameter
separately, even if it has $IFS whitespace in it" semantics,
choosing between $* (when you want to split a parameter into many
when it has $IFS whitespaces) and "$*" (when you want everything in
one string) is preferred.

>  	git -C "$repo" for-each-ref --include-root-refs \
>  		--format='%(refname) %(objectname) %(symref)' >actual &&
> @@ -241,6 +243,19 @@ do
>  				test_cmp expect.reflog actual.reflog
>  			)
>  		'
> +
> +		test_expect_success "$from_format -> $to_format: skip reflog with --skip-reflog" '
> +			test_when_finished "rm -rf repo" &&
> +			git init --ref-format=$from_format repo &&
> +			test_commit -C repo initial &&
> +			# we see that the repository contains reflogs.
> +			git -C repo reflog --all >reflogs &&
> +			test_line_count = 2 reflogs &&
> +			test_migration repo "$to_format" true --skip-reflog &&
> +			# there should be no reflogs post migration.
> +			git -C repo reflog --all >reflogs &&
> +			test_must_be_empty reflogs
> +		'
>  	done
>  done
>  
>
> ---
>
> base-commit: bc204b742735ae06f65bb20291c95985c9633b7f
> change-id: 20250207-477-refs-migrate-add-a-flag-to-ignore-reflogs-during-migration-8066f6df50ac
>
> Thanks
> - Karthik