[PATCH v1 2/2] builtin/grep.c: integrate with sparse index

[Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>]

Turn on sparse index and remove ensure_full_index().

Change it to only expands the index when using --sparse.

The p2000 tests demonstrate a ~99.4% execution time reduction for
`git grep` using a sparse index.

Test                                           HEAD~1       HEAD
-----------------------------------------------------------------------------
2000.78: git grep --cached bogus (full-v3)     0.019        0.018  (-5.2%)
2000.79: git grep --cached bogus (full-v4)     0.017        0.016  (-5.8%)
2000.80: git grep --cached bogus (sparse-v3)   0.29         0.0015 (-99.4%)
2000.81: git grep --cached bogus (sparse-v4)   0.30         0.0018 (-99.4%)

Optional reading about performance test results
-----------------------------------------------
Notice that because `git-grep` needs to parse blobs in the index, the
index reading time is minuscule comparing to the object parsing time.
And because of this, the p2000 test results cannot clearly reflect the
speedup for index reading: combining with the object parsing time,
the aggregated time difference is extremely close between HEAD~1 and
HEAD.

Hence, the results presenting here are not directly extracted from the
p2000 test results. Instead, to make the performance difference more
visible, the test command is manually ran with GIT_TRACE2_PERF in the
four repos (full-v3, sparse-v3, full-v4, sparse-v4). The numbers here
are then extracted from the time difference between "region_enter" and
"region_leave" of label "do_read_index".

Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
---
 builtin/grep.c                           |  8 ++++++--
 t/perf/p2000-sparse-operations.sh        |  1 +
 t/t1092-sparse-checkout-compatibility.sh | 17 +++++++++++++++++
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/builtin/grep.c b/builtin/grep.c
index 61402e8084..cbaab604fd 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -519,11 +519,15 @@ static int grep_cache(struct grep_opt *opt,
 		strbuf_addstr(&name, repo->submodule_prefix);
 	}
 
+	prepare_repo_settings(repo);
+	repo->settings.command_requires_full_index = 0;
+
 	if (repo_read_index(repo) < 0)
 		die(_("index file corrupt"));
 
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(repo->index);
+	if (grep_sparse)
+		ensure_full_index(repo->index);
+
 	for (nr = 0; nr < repo->index->cache_nr; nr++) {
 		const struct cache_entry *ce = repo->index->cache[nr];
 
diff --git a/t/perf/p2000-sparse-operations.sh b/t/perf/p2000-sparse-operations.sh
index fce8151d41..9a466fcbbe 100755
--- a/t/perf/p2000-sparse-operations.sh
+++ b/t/perf/p2000-sparse-operations.sh
@@ -124,5 +124,6 @@ test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
 test_perf_on_all "git rm -f $SPARSE_CONE/a && git checkout HEAD -- $SPARSE_CONE/a"
+test_perf_on_all git grep --cached bogus
 
 test_done
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index a6a14c8a21..a9bb6734f6 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1972,4 +1972,21 @@ test_expect_success 'sparse index is not expanded: rm' '
 	ensure_not_expanded rm -r deep
 '
 
+test_expect_success 'grep expands index using --sparse' '
+	init_repos &&
+
+	# With --sparse and --cached, do not ignore sparse entries and
+	# expand the index.
+	test_all_match git grep --sparse --cached a
+'
+
+test_expect_success 'grep is not expanded' '
+	init_repos &&
+
+	ensure_not_expanded grep a &&
+	ensure_not_expanded grep a -- deep/* &&
+	# grep does not match anything per se, so ! is used
+	ensure_not_expanded ! grep a -- folder1/*
+'
+
 test_done
-- 
2.37.0