Re: [PATCH v2] sequencer: refuse to pick or revert shallow boundary commits

> From: Nakul V S <vsnakul@gmail.com>
>
> In a shallow repository, the commits at the edge of the downloaded
> history are recorded in $GIT_DIR/shallow and have their parents hidden
> from the revision machinery.  Such a "shallow boundary" commit looks
> like a root commit that added every file in its tree at once.
>
> Reverting it therefore removes every tracked file, and cherry-picking
> or rebasing it adds all of them again.  In a "git clone --depth=1" this
> is the only commit there is, so "git revert HEAD" empties the working
> tree, which is almost never what the user meant.
>
> Refuse to do so by default for "revert", "cherry-pick" and "rebase".
> For "revert" and "cherry-pick" only, add --allow-shallow-boundary to
> override the check; it is remembered across --continue.  "rebase"
> always refuses, and the error message suggests "git fetch --unshallow"
> instead.
>
> The check is made when the list of commits to replay is built, not when
> each commit is picked, so a command that is going to be refused does so
> before it has done anything: no commit is created, HEAD and the working
> tree are untouched, and no sequencer or rebase state is left behind for
> the user to clean up.
>
> The "apply" backend of "rebase" does not go through the sequencer, so
> it checks the commits "git format-patch" is about to turn into patches
> before it rewinds HEAD.  This is skipped altogether in a repository
> that is not shallow.
>
> A "pick" or "revert" line for a shallow boundary commit that the user
> types into the todo list by hand is not checked, as the check is not
> made again after the todo list has been edited.  "git commit --amend"
> is also left alone for now.
>
> Signed-off-by: Nakul V S <vsnakul@gmail.com>
> ---
>     sequencer: refuse to pick or revert shallow boundary commits by default
>     
>     Changes since v1:
>     
>      * Also refuse cherry-pick and rebase (merge, interactive and --apply
>        backends), as suggested by Phillip.
>      * Check when the list of commits is built rather than in
>        do_pick_commit(), so a refused command leaves no state behind. Uses a
>        single shared helper.
>      * --allow-shallow-boundary is now accepted by cherry-pick as well, and
>        is saved across --continue.
>      * Dropped the option from the revert synopsis.
>      * rebase --autostash restores the user's changes if the rebase is
>        refused.
>      * Rebased onto current master.
>     
>     Not covered here: git commit --amend, and pick/revert lines typed into
>     the rebase todo by hand.
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2273%2Fvsnakul%2Fsequencer-shallow-boundary-check-v2
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2273/vsnakul/sequencer-shallow-boundary-check-v2
> Pull-Request: https://github.com/gitgitgadget/git/pull/2273
>
> Range-diff vs v1:

> diff --git a/shallow.c b/shallow.c
> index 8e244a5669..bc31ff5232 100644
> --- a/shallow.c
> +++ b/shallow.c
> @@ -92,6 +92,15 @@ int is_repository_shallow(struct repository *r)
>  	return r->parsed_objects->is_shallow;
>  }
>  
> +int is_shallow_boundary_commit(struct repository *r, const struct object_id *oid)
> +{
> +	struct commit_graft *graft;
> +	if (!is_repository_shallow(r))
> +		return 0;
> +	graft = lookup_commit_graft(r, oid);
> +	return graft && graft->nr_parent < 0;
> +}
> +