Re: [PATCH v2 08/10] sparse-index: complete partial expansion

[Derrick Stolee <derrickstolee@github.com>]

On 5/23/2022 6:48 PM, Junio C Hamano wrote:
> Derrick Stolee <derrickstolee@github.com> writes:
> 
>>> I suspect that is a situation that is not so uncommon.  Working
>>> inside a narrow cone of a wide tree, performing a merge would
>>> hopefully allow many subtrees that are outside of the cones of our
>>> interest merged without getting expanded at all (e.g. only the other
>>> side touched these subtrees we are not working on, so their version
>>> will become the merge result), while changes to some paths in the
>>> cone of our interest may result in true conflicts represented as
>>> cache entries at higher stages, needing conflict resolution
>>> concluded with "git add".  Having to expand these subtrees that we
>>> managed to merge while still collapsed, only because we have
>>> conflicts in some other parts of the tree, feels somewhat sad.
>>
>> You are correct that conflicts outside of the sparse-checkout cone will
>> cause index expansion. That happens during the 'git merge' command, but
>> the index will continue to fail to collapse as long as those conflicts
>> still exist in the index.
>>
>> When there are conflicts like this during the merge, then the index
>> expansion is not as large of a portion of the command as normal, because
>> the conflict resolution also takes some time to compute. The commands
>> afterwards do take longer purely because of the expanded index.
> 
> I was imagining a situation more like "tech-writers only have
> Documentation/ inside the cone of interest, attempt a pull from
> somebody else, have conflicts inside Documentation/, but everything
> else could be resolved cleanly without expanding the index".  If the
> puller's tree is based on the pristine upstream release tag, and the
> pullee's tree is based on a slightly newer version of upstream
> snapshot, everything that happened outside Documentation/ in their
> trees would fast-forward, so such a merge wouldn't have to expand
> directories like "builtin/" or "contrib/" in the index and instead
> can merge at the tree level, right?
> 
> On the other hand, ...
> 
>> However, this state is also not as common as you might think. If a user
>> has a sparse-checkout cone specified, then they are unlikely to change
>> files outside of the sparse-checkout cone. They would not be the reason
>> that those files have a conflict. The conflicts would exist only if they
>> are merging branches that had conflicts outside of the cone. Typically,
>> any merge of external changes like this are of the form of "git pull" or
>> "git rebase", in which case the conflicts are still "local" to the
>> developer's changes.
> 
> ... you seem to be talking about the opposite case (e.g. in the
> above paragraph), where a conflict happens outside the cone of
> interest of the person who is making a merge.  So, I am a bit
> puzzled.

Hm. We must be getting mixed up with each other. Let me try again
from the beginning.

When the conflict happens inside of the sparse-checkout cone,
then the sparse index is not expanded. This is checked by the
test 'sparse-index is not expanded: merge conflict in cone' in
t1092.

Most merges get to "fast forward" changes that are outside of
the sparse-checkout cone because the only interesting changes
that could lead to a conflict are those created by the current
user (and hence within the sparse-checkout cone).

So, the typical case of a tech writer only editing "Documentation/"
should only see conflicts within "Documentation/".

The case where we would see conflicts outside of the cone include
cases where long-lived branches are being merged by someone with
a small cone. I can imagine an automated process using an empty
sparse-checkout cone to occasionally merge a "deployed" and a
"develop" branch, and it gets conflicts when someone ships a
hotfix directly to "deployed" without first going through "develop".
Any conflict is likely to cause index expansion in this case.

Let's re-introduce the patch section we are talking about:

+	if (pl && !pl->use_cone_patterns) {
 		pl = NULL;
+	} else {
+		/*
+		 * We might contract file entries into sparse-directory
+		 * entries, and for that we will need the cache tree to
+		 * be recomputed.
+		 */
+		cache_tree_free(&istate->cache_tree);
+
+		/*
+		 * If there is a problem creating the cache tree, then we
+		 * need to expand to a full index since we cannot satisfy
+		 * the current request as a sparse index.
+		 */
+		if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
+			pl = NULL;
+	}

If a user is in a conflict state and modifies their sparse-checkout
cone, then we will hit this "recompute the cache-tree" state, fail,
and cause full index expansion. I think that combination (have a
conflict AND modify sparse-checkout cone) is rare enough to not
optimize for (yet).

Does that make the situation more clear?

Thanks,
-Stolee