Subject: Re: rev-list with multiple commits sharing same patch-id

From: Arnaud Morin <[email-masked]>
Date: Tue, 12 Jan 2021 15:34:38 +0000

So instead of:
id = has_commit_patch_id(commit, &ids);

We should use some sort of iterator, in order to call:
id->commit->object.flags |= cherry_flag;

for all commits which are related to this patch?

-- 
Arnaud Morin

On 12.01.21 - 10:11, Jeff King wrote:
> On Tue, Jan 12, 2021 at 09:17:37AM -0500, Jeff King wrote:
> 
> > It looks like this was broken in v2.10.0, via dfb7a1b4d0 (patch-ids:
> > stop using a hand-rolled hashmap implementation, 2016-07-29).
> > 
> > I think the issue is that it is allowing duplicate entries in the
> > hashmap. The algorithm is something like:
> > 
> >   - iterate over left-hand commits, inserting patch-id for each into
> >     hashmap
> > 
> >   - iterate over right-hand commits, seeing if any are present in
> >     hashmap. If so, we exclude the commit _and_ mark the patch-id as
> >     "seen"
> > 
> >   - iterate again over left-hand commits, omitting any whose patch-ids
> >     are marked as "seen"
> > 
> > So if two commits on the left-hand side have the same patch-id, if we
> > insert two entries into the hashmap, then only one of them is going to
> > get its "seen" flag marked in the middle step.
> 
> Yeah, that's definitely it. Here's what the fix would look like directly
> on top of dfb7a1b4d0:
> 
> diff --git a/patch-ids.c b/patch-ids.c
> index db31fa647a..a8ed4f0872 100644
> --- a/patch-ids.c
> +++ b/patch-ids.c
> @@ -66,12 +66,24 @@ struct patch_id *add_commit_patch_id(struct commit *commit,
>  				     struct patch_ids *ids)
>  {
>  	struct patch_id *key = xcalloc(1, sizeof(*key));
> +	struct patch_id *old;
>  
>  	if (init_patch_id_entry(key, commit, ids)) {
>  		free(key);
>  		return NULL;
>  	}
>  
> +	/*
> +	 * Don't allow duplicates. Note that we can't use hashmap_put here; we
> +	 * must retain the _old_ struct, because some commit->util is pointing
> +	 * to it.
> +	 */
> +	old = hashmap_get(&ids->patches, key, ids);
> +	if (old) {
> +		free(key);
> +		return old;
> +	}
> +
>  	hashmap_add(&ids->patches, key);
>  	return key;
>  }
> diff --git a/t/t6007-rev-list-cherry-pick-file.sh b/t/t6007-rev-list-cherry-pick-file.sh
> index 28d4f6b259..0c5f1dcc54 100755
> --- a/t/t6007-rev-list-cherry-pick-file.sh
> +++ b/t/t6007-rev-list-cherry-pick-file.sh
> @@ -207,4 +207,16 @@ test_expect_success '--count --left-right' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success '--cherry-pick with duplicates on each side' '
> +	git checkout -b dup-orig &&
> +	test_commit dup-base &&
> +	git revert dup-base &&
> +	git cherry-pick dup-base &&
> +	git checkout -b dup-side HEAD~3 &&
> +	test_tick &&
> +	git cherry-pick -3 dup-orig &&
> +	git rev-list --cherry-pick dup-orig...dup-side >actual &&
> +	test_must_be_empty actual
> +'
> +
>  test_done
> 
> 
> Unfortunately we can't do the same thing now. The "seen" flag went away
> in 683f17ec44 (patch-ids: replace the seen indicator with a commit
> pointer, 2016-07-29), in favor of having each patch_id struct point to a
> "struct commit". We could revert that, or turn that pointer into a list,
> but it gets worse. Later, b3dfeebb92 (rebase: avoid computing
> unnecessary patch IDs, 2016-07-29) built on that by lazily computing the
> patch-ids. So each patch_id struct must point to exactly one commit!
> 
> We'll have to either:
> 
>   - split them apart into two structs at the time of the lazy computation
> 
> or
> 
>   - accept having duplicate patch_ids, but on lookup make sure we find
>     _all_ matches, not just the first one.
> 
> Hmm, that last one might not be too bad, since we have
> hashmap_get_next() (it does mean iterating over a chained bucket
> linearly, but that's already the worst-case for a hash lookup).
> 
> -Peff