In order to compute whether objects reachable from a set of tips are all
connected, we do a revision walk with these tips as positive references
and `--not --all`. `--not --all` will cause the revision walk to load
all preexisting references as uninteresting, which can be very expensive
in repositories with many references.

Benchmarking the git-rev-list(1) command highlights that by far the most
expensive single phase is initial sorting of the input revisions: after
all references have been loaded, we first sort commits by author date.
In a real-world repository with about 2.2 million references, it makes
up about 40% of the total runtime of git-rev-list(1).

Ultimately, the connectivity check shouldn't really bother about the
order of input revisions at all. We only care whether we can actually
walk all objects until we hit the cut-off point. So sorting the input is
a complete waste of time.

Introduce a new "--unsorted-input" flag to git-rev-list(1) which will
cause it to not sort the commits and adjust the connectivity check to
always pass the flag. This results in the following speedups, executed
in a clone of gitlab-org/gitlab [1]:

    Benchmark #1: git rev-list  --objects --quiet --not --all --not $(cat newrev)
      Time (mean ± σ):      7.639 s ±  0.065 s    [User: 7.304 s, System: 0.335 s]
      Range (min … max):    7.543 s …  7.742 s    10 runs

    Benchmark #2: git rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.995 s ±  0.044 s    [User: 4.657 s, System: 0.337 s]
      Range (min … max):    4.909 s …  5.048 s    10 runs

    Summary
      'git rev-list --unsorted-input --objects --quiet --not --all --not $(cat newrev)' ran
        1.53 ± 0.02 times faster than 'git rev-list  --objects --quiet --not --all --not $newrev'

[1]: https://gitlab.com/gitlab-org/gitlab.git. Note that not all refs
     are visible to clients.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 connected.c | 1 +
 revision.c  | 4 +++-
 revision.h  | 1 +
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/connected.c b/connected.c
index b18299fdf0..b5f9523a5f 100644
--- a/connected.c
+++ b/connected.c
@@ -106,6 +106,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 	if (opt->progress)
 		strvec_pushf(&rev_list.args, "--progress=%s",
 			     _("Checking connectivity"));
+	strvec_push(&rev_list.args, "--unsorted-input");
 
 	rev_list.git_cmd = 1;
 	rev_list.env = opt->env;
diff --git a/revision.c b/revision.c
index cddd0542a6..7eb09009ba 100644
--- a/revision.c
+++ b/revision.c
@@ -2256,6 +2256,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--author-date-order")) {
 		revs->sort_order = REV_SORT_BY_AUTHOR_DATE;
 		revs->topo_order = 1;
+	} else if (!strcmp(arg, "--unsorted-input")) {
+		revs->unsorted_input = 1;
 	} else if (!strcmp(arg, "--early-output")) {
 		revs->early_output = 100;
 		revs->topo_order = 1;
@@ -3584,7 +3586,7 @@ int prepare_revision_walk(struct rev_info *revs)
 
 	if (!revs->reflog_info)
 		prepare_to_use_bloom_filter(revs);
-	if (revs->no_walk != REVISION_WALK_NO_WALK_UNSORTED)
+	if (revs->no_walk != REVISION_WALK_NO_WALK_UNSORTED && !revs->unsorted_input)
 		commit_list_sort_by_date(&revs->commits);
 	if (revs->no_walk)
 		return 0;
diff --git a/revision.h b/revision.h
index fbb068da9f..ef6998509a 100644
--- a/revision.h
+++ b/revision.h
@@ -134,6 +134,7 @@ struct rev_info {
 			simplify_history:1,
 			show_pulls:1,
 			topo_order:1,
+			unsorted_input:1,
 			simplify_merges:1,
 			simplify_by_decoration:1,
 			single_worktree:1,
-- 
2.32.0