Re: [RFC PATCH] Not computing changed path filter for root commits

[Taylor Blau <me@ttaylorr.com>]

On Fri, Sep 15, 2023 at 10:29:12PM +0200, SZEDER Gábor wrote:
> On Mon, Sep 11, 2023 at 03:31:56PM -0700, Jonathan Tan wrote:
> > SZEDER Gábor suggested [2] that we change the revision walk to read
> > changed path filters also for root commits, but I don't think that's
> > possible - we have to tie reading changed path filters to when we read
> > trees, and right now, we don't seem to read trees when evaluating root
> > commits (rev_compare_tree() in revision.c is in the only code path that
> > uses changed path filters, and it itself is only called per-parent and
> > thus not called for root commits).
>
> When encountering a root commit during a pathspec-limited revision
> walk we call rev_same_tree_as_empty() instead of rev_compare_tree().
> All that's missing there is checking the Bloom filter and accounting
> for false positives.

I think that we'd want something like this, though I would definitely
appreciate a second set of eyes since I am not 100% confident in my
set of changes here:

--- 8< ---
diff --git a/revision.c b/revision.c
index 2f4c53ea20..1d36df49e2 100644
--- a/revision.c
+++ b/revision.c
@@ -837,14 +837,24 @@ static int rev_compare_tree(struct rev_info *revs,
 static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit)
 {
 	struct tree *t1 = repo_get_commit_tree(the_repository, commit);
+	int bloom_ret = 1;

 	if (!t1)
 		return 0;

+	if (revs->bloom_keys_nr) {
+		bloom_ret = check_maybe_different_in_bloom_filter(revs, commit);
+		if (!bloom_ret)
+			return 1;
+	}
+
 	tree_difference = REV_TREE_SAME;
 	revs->pruning.flags.has_changes = 0;
 	diff_tree_oid(NULL, &t1->object.oid, "", &revs->pruning);

+	if (bloom_ret == 1 && tree_difference == REV_TREE_SAME)
+		count_bloom_filter_false_positive++;
+
 	return tree_difference == REV_TREE_SAME;
 }

diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index fa9d32facf..3a45cb997b 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -162,7 +162,7 @@ test_expect_success 'setup - add commit-graph to the chain with Bloom filters' '

 test_bloom_filters_used_when_some_filters_are_missing () {
 	log_args=$1
-	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":9"
+	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":10"
 	setup "$log_args" &&
 	grep -q "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.perf" &&
 	test_cmp log_wo_bloom log_w_bloom
--- >8 ---

Thanks,
Taylor