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last-modified: check pathspec against Bloom filter first

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When git-last-modified(1) starts, it builds a list of all the paths
matching the pathspec it needs to find the last modifying commit for.
For example, every file and subdirectory listed by:

    $ git last-modified -t --max-depth=0 -- src/

As it resolves a commit for each path during the revision walk, it drops
that path from the list.

To avoid diffing trees for every commit, Bloom filters are used when
available. For each remaining path, the commit's Bloom filter is checked
to see whether the commit changed that path. The Bloom filter says
either "no" or "maybe", and only in the latter case is the diff
calculated.

git-log(1) does this differently. It does not expand the pathspec but
checks the Bloom filter against the pathspec itself. This way, commits
not touching any path matching the pathspec can be discarded as a whole.

Apply this same check to git-last-modified(1). In a previous commit the
function revs_maybe_changed_in_bloom(), used by git-log(1), was made
public. Use this as a pre-filter in git-last-modified(1). After this
pre-filter, paths are still checked one-by-one to only find those which
don't have a "last commit" yet.

With `--show-trees` the list holds more than the paths matching the
pathspec. It also holds each parent tree entry, up to the root. Each of
those can resolve to a different commit. Thus for the pathspec "a/b/c",
the list will also hold "a" and "a/b".

When a commit touches "a/other", that commit could be the last commit
for "a", but revs_maybe_changed_in_bloom() would discard it, because it
doesn't match the full pathspec.

Instead, when `--show-trees` is given, use
revs_maybe_changed_in_bloom_with_parents(), which indicates the commit
maybe changed any of the paths leading up to the path in the pathspec.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Toon Claes
2026-08-07 20:26:51 +02:00
committed by Junio C Hamano
parent 7fc4c80a86
commit 885ea73988
2 changed files with 33 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
builtin/last-modified.c
View File

@@ -272,6 +272,18 @@ static bool maybe_changed_path(struct last_modified *lm,
if (!filter)
return true;
/*
* With --show-trees we also track the tree entries containing the
* paths, so a change to any of those parent directories matters too.
*/
if (lm->show_trees) {
if (!revs_maybe_changed_in_bloom_with_parents(&lm->rev, filter))
return false;
} else {
if (!revs_maybe_changed_in_bloom(&lm->rev, filter))
return false;
}
hashmap_for_each_entry(&lm->paths, &iter, ent, hashent) {
if (active && !bitmap_get(active, ent->diff_idx))
continue;

21
t/t8020-last-modified.sh
View File

@@ -269,6 +269,27 @@ test_expect_success 'last-modified merge undoes changes' '
EOF
'
test_expect_success 'last-modified with Bloom filters and --show-trees' '
test_when_finished rm -rf bloom &&
git init bloom &&
(
cd bloom &&
mkdir d &&
test_commit base-a d/a &&
test_commit base-b d/b &&
test_commit touch-a d/a &&
test_commit touch-b d/b &&
git commit-graph write --reachable --changed-paths &&
git -c core.commitGraph=false last-modified -t HEAD -- d/a \
>expect &&
git -c core.commitGraph=true last-modified -t HEAD -- d/a \
>actual &&
test_cmp expect actual
)
'
test_expect_success 'cannot run last-modified on two commits' '
test_must_fail git last-modified HEAD HEAD~1 2>err &&
test_grep "last-modified can only operate on one commit at a time" err