Re: [PATCH] shallow.c: use 'reset_repository_shallow' when appropriate

> In bd0b42aed3 (fetch-pack: do not take shallow lock unnecessarily,
> 2019-01-10), the author noted that 'is_repository_shallow' produces
> visible side-effect(s) by setting 'is_shallow' and 'shallow_stat'.
>
> This is a problem for e.g., fetching with '--update-shallow' in a
> shallow repsoitory with 'fetch.writeCommitGraph' enabled, since the

> update to '.git/shallow' will cause Git to think that the repository
> isn't shallow when it is, thereby circumventing the commit-graph
> compatability check.
>
> This causes problems in shallow repositories with at least shallow refs
> that have at least one ancestor (since the client won't have those
> objects, and therefore can't take the reachability closure over commits
> when writing a commit-graph).

> Address this by introducing 'reset_repository_shallow()', and calling
> it whenever the shallow files is updated. This happens in two cases:
>
>   * during 'update_shallow', when either the repository is
>     un-shallowing, or after commit_lock_file, when the contents of
>     .git/shallow is changing, and
>
>   * in 'prune_shallow', when the repository can go from shallow to
>     un-shallow when the shallow file is updated, forcing
>     'is_repository_shallow' to re-evaluate whether the repository is
>     still shallow after fetching in the above scenario.
>
> As a result of the second change, 'prune_shallow' can now only be called
> once (since 'check_shallow_file_for_update' will die after calling
> 'reset_repository_shallow'). But, this is OK since we only call
> 'prune_shallow' at most once per process.
>
> Helped-by: Jonathan Tan <jonathantanmy@google.com>
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>
> Here's a cleaned up version of the patch that we were originally
> discussing in
> https://lore.kernel.org/git/20200414235057.GA6863@syl.local/, which
> addresses some of Jonathan's feedback and adds a test to make sure that
> the new behavior is working correctly.
>
>  commit.h                 |  1 +
>  fetch-pack.c             |  1 +
>  shallow.c                | 15 ++++++++-------
>  t/t5537-fetch-shallow.sh | 36 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 46 insertions(+), 7 deletions(-)
>
> diff --git a/commit.h b/commit.h
> index 008a0fa4a0..ee1ba139d4 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -251,6 +251,7 @@ int register_shallow(struct repository *r, const struct object_id *oid);
>  int unregister_shallow(const struct object_id *oid);
>  int for_each_commit_graft(each_commit_graft_fn, void *);
>  int is_repository_shallow(struct repository *r);
> +void reset_repository_shallow(struct repository *r);
>  struct commit_list *get_shallow_commits(struct object_array *heads,
>  					int depth, int shallow_flag, int not_shallow_flag);
>  struct commit_list *get_shallow_commits_by_rev_list(
> diff --git a/fetch-pack.c b/fetch-pack.c
> index 1734a573b0..684868bc17 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -1632,6 +1632,7 @@ static void update_shallow(struct fetch_pack_args *args,
>  			rollback_lock_file(&shallow_lock);
>  		} else
>  			commit_lock_file(&shallow_lock);
> +		reset_repository_shallow(the_repository);
>  		alternate_shallow_file = NULL;
>  		return;
>  	}

> +void reset_repository_shallow(struct repository *r)
> +{
> +	r->parsed_objects->is_shallow = -1;
> +	stat_validity_clear(r->parsed_objects->shallow_stat);
> +}

> @@ -362,6 +361,7 @@ void setup_alternate_shallow(struct lock_file *shallow_lock,
>  		 * shallow file".
>  		 */
>  		*alternate_shallow_file = "";
> +	reset_repository_shallow(the_repository);
>  	strbuf_release(&sb);
>  }

> @@ -414,6 +414,7 @@ void prune_shallow(unsigned options)
>  	} else {
>  		unlink(git_path_shallow(the_repository));
>  		rollback_lock_file(&shallow_lock);
> +		reset_repository_shallow(the_repository);
>  	}

> +	cat <<EOF >expect.refs &&
> +refs/remotes/shallow/master
> +refs/remotes/shallow/no-shallow
> +refs/tags/heavy-tag
> +refs/tags/heavy-tag-for-graph
> +refs/tags/light-tag
> +refs/tags/light-tag-for-graph
> +EOF