Subject: Re: [PATCH 8/8] commit-graph: drop COMMIT_GRAPH_WRITE_CHECK_OIDS flag

From: Taylor Blau <[email-masked]>
Date: Wed, 13 May 2020 15:32:28 -0600

On Thu, May 07, 2020 at 04:40:05PM -0400, Jeff King wrote:
> On Mon, May 04, 2020 at 07:14:03PM -0600, Taylor Blau wrote:
>
> > If callers do wish to retain this behavior, they can easily work around
> > this change by doing the following:
> >
> >     git for-each-ref --format='%(objectname) %(objecttype) %(*objecttype)' |
> >     awk '/commit/ { print $1 }' |
> >     git commit-graph write --stdin-commits
>
> I know this came from my earlier email, but I think that recipe actually
> shows how to make your input work even if --check-oids were the default.
> If you really want the --check-oids behavior, you'd want the opposite:
> to complain about those ones. So it's something like:
>
>      git for-each-ref --format='%(objectname) %(objecttype) %(*objecttype)' |
>      awk '
>        !/commit/ { print "not-a-commit:"$1 }
>         /commit/ { print $1 }
>      ' |
>      git commit-graph write --stdin-commits
>
> > diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> > index 53a650225a..fcac7d12e1 100644
> > --- a/Documentation/git-commit-graph.txt
> > +++ b/Documentation/git-commit-graph.txt
> > @@ -47,8 +47,10 @@ with `--stdin-commits` or `--reachable`.)
> >  +
> >  With the `--stdin-commits` option, generate the new commit graph by
> >  walking commits starting at the commits specified in stdin as a list
> > -of OIDs in hex, one OID per line. (Cannot be combined with
> > -`--stdin-packs` or `--reachable`.)
> > +of OIDs in hex, one OID per line. OIDs that resolve to non-commits
> > +(either directly, or by peeling tags) are silently ignored. OIDs that
> > +are malformed, or do not exist generate an error. (Cannot be combined
> > +with `--stdin-packs` or `--reachable`.)
>
> Yeah, I think these semantics are good.
>
> > diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
> > index 9eec68572f..3637d079fb 100644
> > --- a/builtin/commit-graph.c
> > +++ b/builtin/commit-graph.c
> > @@ -153,13 +153,14 @@ static int read_one_commit(struct oidset *commits, struct progress *progress,
> >
> >  	display_progress(progress, oidset_size(commits) + 1);
> >
> > +	if (oid_object_info(the_repository, &oid, NULL) < 0) {
> > +		error(_("object %s does not exist"), hash);
> > +		return 1;
> > +	}
> > +
> >  	result = lookup_commit_reference_gently(the_repository, &oid, 1);
> >  	if (result)
> >  		oidset_insert(commits, &result->object.oid);
> > -	else {
> > -		error(_("invalid commit object id: %s"), hash);
> > -		return 1;
> > -	}
> >  	return 0;
> >  }
>
> We can avoid the object-existence check entirely if
> lookup_commit_reference_gently() gives us an answer. And we'd expect
> that to be the common path.
>
> Also, using has_object_file() is cheaper than oid_object_info(), since
> it doesn't have to resolve the type for deltas.
>
> So perhaps:
>
>   result = lookup_commit_reference_gently(...);
>   if (result)
>           oidset_insert(...);
>   else if (has_object_file(&oid))
>           ; /* not a commit; quietly ignore;
>   else
>           return error(no such object...);
>
> That said, I think this technique misses some cases of corruption.
> You're checking that the outer-most object exists, but not any
> intermediate peeled objects. I.e., lookup_commit_reference_gently()
> might have failed for two reasons:
>
>   - an object it peeled to didn't exist
>
>   - an object it peeled to wasn't a commit
>
> To do it thoroughly, I think you'd have to call deref_tag() yourself and
> distinguish NULL there (an error) from a result where obj->type isn't
> OBJ_COMMIT (quietly ignore).

Makes sense. I initially worried a little bit about what to call the
error message (i.e., is is "this object doesn't exist" or "this object
exists but peels to a non-commit, or an otherwise missing object"). But,
I think just saying "invalid object: <hash>" is good enough here.

> >  enum commit_graph_write_flags {
> > -	COMMIT_GRAPH_WRITE_APPEND     = (1 << 0),
> > -	COMMIT_GRAPH_WRITE_PROGRESS   = (1 << 1),
> > -	COMMIT_GRAPH_WRITE_SPLIT      = (1 << 2),
> > -	/* Make sure that each OID in the input is a valid commit OID. */
> > -	COMMIT_GRAPH_WRITE_CHECK_OIDS = (1 << 3),
> > -	COMMIT_GRAPH_WRITE_BLOOM_FILTERS = (1 << 4),
> > +	COMMIT_GRAPH_WRITE_APPEND        = (1 << 0),
> > +	COMMIT_GRAPH_WRITE_PROGRESS      = (1 << 1),
> > +	COMMIT_GRAPH_WRITE_SPLIT         = (1 << 2),
> > +	COMMIT_GRAPH_WRITE_BLOOM_FILTERS = (1 << 3)
>
> As much as I love looking at matched-indentation lists, I think this
> diff is a good example of why it's not worth doing. It's much easier to
> see what's going on if the first three items aren't touched. I'd
> actually even leave BLOOM_FILTERS where it is, and accept the hole which
> could be refilled later.
>
> Your patch also loses the trailing comma after the final BLOOM_FILTERS
> entry.

Fixed both, thanks. I'll build these eight new patches and send them
shortly, thanks for your review.

> -Peff

Thanks,
Taylor