Re: [PATCH v2 3/7] rebase: store orig_head as a commit

From: Junio C Hamano <gitster@pobox.com>
To: "Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org,
	Philippe Blain <levraiphilippeblain@gmail.com>,
	Denton Liu <liu.denton@gmail.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Phillip Wood <phillip.wood123@gmail.com>,
	Elijah Newren <newren@gmail.com>,
	Jonathan Tan <jonathantanmy@google.com>,
	Phillip Wood <phillip.wood@dunelm.org.uk>
Subject: Re: [PATCH v2 3/7] rebase: store orig_head as a commit
Date: Wed, 07 Sep 2022 11:12:06 -0700	[thread overview]
Message-ID: <xmqqzgfbuk95.fsf@gitster.g> (raw)
In-Reply-To: 9daee95d434155742dbb19271eea8c0bc05eb365.1662561470.git.gitgitgadget@gmail.com

"Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com> writes:

> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 56e4214b441..a3cf1ef5923 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -68,7 +68,7 @@ struct rebase_options {
>  	const char *upstream_name;
>  	const char *upstream_arg;
>  	char *head_name;
> -	struct object_id orig_head;
> +	struct commit *orig_head;

OK.

> @@ -261,13 +261,13 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
>  	struct replay_opts replay = get_replay_opts(opts);
>  	struct string_list commands = STRING_LIST_INIT_DUP;
>  
> -	if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head,
> +	if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head->object.oid,
>  				&revisions, &shortrevisions))
>  		return -1;

This step has ton of changes like this, which are fallouts from the
change of type of a member that used to be an oid to a full blown
commit object.  It is expected and I will strip all of them from my
quotes.  They all look correct, and otherwise the compiler would
complain ;-).

> @@ -448,7 +447,8 @@ static int read_basic_state(struct rebase_options *opts)
>  	} else if (!read_oneliner(&buf, state_dir_path("head", opts),
>  				  READ_ONELINER_WARN_MISSING))
>  		return -1;
> -	if (get_oid(buf.buf, &opts->orig_head))
> +	opts->orig_head = lookup_commit_reference_by_name(buf.buf);

This is not exactly a new problem, but I noticed it while looking
for more iffy uses of lookup_commit_reference_by_name(), so...

At this point in the codepath, we expect buf.buf has full-hex object
name and nothing else; the original should have used get_oid_hex()
to highlight that fact.  lookup_commit_reference_by_name() allows
object names that are not written as full-hex object name, and it
may get confused if a branch or tag with 40-hex (or 64-hex in a
repository with newhash) name exists.  It would be a more sensible
conversion to use get_oid_hex() to turn buf.buf into an object name
and then use lookup_commit_reference() on it.

> @@ -866,15 +866,11 @@ static int is_linear_history(struct commit *from, struct commit *to)
>  
>  static int can_fast_forward(struct commit *onto, struct commit *upstream,
>  			    struct commit *restrict_revision,
> -			    struct object_id *head_oid, struct object_id *merge_base)
> +			    struct commit *head, struct object_id *merge_base)
>  {
> -	struct commit *head = lookup_commit(the_repository, head_oid);
>  	struct commit_list *merge_bases = NULL;
>  	int res = 0;
>  
> -	if (!head)
> -		goto done;
> -

This one benefits from being able to avoid its own lookup_commit()
because the caller already has it in the desired form.

This is not a comment on the new code, but it does make readers
wonder if the conversion changes behaviour.  lookup_commit() takes
an object name and requires it to be a commit object's name, doesn't
it?  If we gave a tag to the program, the old code would have had
the object name of that tag in head_oid and at this point and
lookup_commit() noticed and would have stopped you from fast
forwarding your branch to the tag, which was a good thing.  In the
new code, since we turn the object name we take from the user into a
commit object way before the control reaches this place, we won't
get such an error here, but if we fast-forward to the object, we
will still fast forward to the commit that is pointed by the tag,
so the new behaviour is even better, perhaps?

> @@ -1610,17 +1606,17 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  		/* Is it a local branch? */
>  		strbuf_reset(&buf);
>  		strbuf_addf(&buf, "refs/heads/%s", branch_name);
> -		if (!read_ref(buf.buf, &options.orig_head)) {
> +		options.orig_head = lookup_commit_reference_by_name(buf.buf);
> +		if (options.orig_head) {
>  			die_if_checked_out(buf.buf, 1);
>  			options.head_name = xstrdup(buf.buf);
>  		/* If not is it a valid ref (branch or commit)? */

This is iffy, or it may be just wrong.

The old code is checking if "refs/heads/$branch_name" is a branch
and does the check.  If you had a branch "refs/heads/A" (whose ref
is at "refs/heads/refs/heads/A") but do not have a branch "A", and
if you fed "A" to this part of the code in buf.buf, then the
original code would not have been fooled by the presence of such a
funny branch.  New code (incorrectly) does because it prefixes
"refs/heads/" to "A" and asks to turn string "refs/heads/A" into a
commit object, triggering the usual ref dwim rules.

We end up setting options.head_name to a wrong thing (in this case,
the user said "A", we turned it into a refname "refs/heads/A" that
does not exist, and set options.orig_head to the commit object
pointed by the ref "refs/heads/refs/heads/A", and we use that commit
as orig_head, but use an incorrect head_name).

I didn't look as carefully as this one, but there may be similarly
iffy uses of lookup_commit_reference_by_name() introduced by this
patch that used to be more strict/exact; they may need to be fixed.

>  		} else {
> -			struct commit *commit =
> +			options.orig_head =
>  				lookup_commit_reference_by_name(branch_name);
> -			if (!commit)
> +			if (!options.orig_head)
>  				die(_("no such branch/commit '%s'"),
>  				    branch_name);
> -			oidcpy(&options.orig_head, &commit->object.oid);
>  			options.head_name = NULL;

This side, which is "this is what happens to a random object name
that is not a branch name", is perfectly fine.