Re: [RFH] sequencer: simplify logic around stopped-sha file

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Thu, 24 Sep 2020, Junio C Hamano wrote:

> While I was auditing the use of get_oid_committish(), I noticed that
> an abbreviated object name is written to rebase-merge/stopped-sha
> only to be read later and expanded to a full object name to be
> passed to record_in_rewritten().  Nobody seems to expose this to the
> end-user and it is unclear if there is a point in keeping it short
> by abbreviating and risking to make it ambiguous as the rebase
> progresses and creates new objects.
>
> The attached patch tries to simplify the logic involved around this
> file, to write and read full object names to/from the file, and the
> result seems to pass testsuite---which in the ideal world would be
> sufficient signal that this change is safe and sane, but it could be
> that original authors thought that a change to stop abbreviating is
> nonsense and not worth protecting the code against, hence RFH here.
>
> Is there something obvious I am not seeing that makes this change a
> bad idea (other than "somebody may be in the middle of a rebase and
> all of a sudden, version of Git gets updated to contain this one,
> which is unable to read abbreviated object name the current version
> left on disk", which I am deliberately ignoring)?

At least in my understanding, you are not missing anything:

- this file is an implementation detail,

- it is not exposed directly via any user-visible interface,

- any reader will _have_ to be prepared for an unabbreviated object ID (in
  the highly unlikely case that an object ID would be ambiguous if
  abbreviated even by one hex character),

- and most importantly: just like we expand the commit IDs in the todo
  list, we actually want to expand them in `stopped-sha` because it _is_
  possible that a new object is written that makes the previous
  unambiguously abbreviated object ID now ambiguous (e.g. when the user
  commits in a separate worktree while the rebase is interrupted, before
  continuing the rebase).

In short: ACK

Thank you,
Dscho

>
> Thanks.
>
>
>  sequencer.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/sequencer.c b/sequencer.c
> index fd7701c88a..7dc9088d09 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -120,7 +120,7 @@ static GIT_PATH_FUNC(rebase_path_author_script, "rebase-merge/author-script")
>  static GIT_PATH_FUNC(rebase_path_amend, "rebase-merge/amend")
>  /*
>   * When we stop at a given patch via the "edit" command, this file contains
> - * the abbreviated commit name of the corresponding patch.
> + * the commit object name of the corresponding patch.
>   */
>  static GIT_PATH_FUNC(rebase_path_stopped_sha, "rebase-merge/stopped-sha")
>  /*
> @@ -3012,11 +3012,12 @@ static int make_patch(struct repository *r,
>  {
>  	struct strbuf buf = STRBUF_INIT;
>  	struct rev_info log_tree_opt;
> -	const char *subject, *p;
> +	const char *subject;
> +	char hex[GIT_MAX_HEXSZ + 1];
>  	int res = 0;
>
> -	p = short_commit_name(commit);
> -	if (write_message(p, strlen(p), rebase_path_stopped_sha(), 1) < 0)
> +	oid_to_hex_r(hex, &commit->object.oid);
> +	if (write_message(hex, strlen(hex), rebase_path_stopped_sha(), 1) < 0)
>  		return -1;
>  	res |= write_rebase_head(&commit->object.oid);
>
> @@ -4396,7 +4397,7 @@ int sequencer_continue(struct repository *r, struct replay_opts *opts)
>
>  		if (read_oneliner(&buf, rebase_path_stopped_sha(),
>  				  READ_ONELINER_SKIP_IF_EMPTY) &&
> -		    !get_oid_committish(buf.buf, &oid))
> +		    !get_oid_hex(buf.buf, &oid))
>  			record_in_rewritten(&oid, peek_command(&todo_list, 0));
>  		strbuf_release(&buf);
>  	}
>