[PATCH v2 3/3] remote: make add_missing_tags() linear

[Derrick Stolee via GitGitGadget]

From: Derrick Stolee <dsto...@microsoft.com>

The add_missing_tags() method currently has quadratic behavior.
This is due to a linear number (based on number of tags T) of
calls to in_merge_bases_many, which has linear performance (based
on number of commits C in the repository).

Replace this O(T * C) algorithm with an O(T + C) algorithm by
using get_reachable_subset(). We ignore the return list and focus
instead on the reachable_flag assigned to the commits we care
about, because we need to interact with the tag ref and not just
the commit object.

Signed-off-by: Derrick Stolee <dsto...@microsoft.com>
---
 remote.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/remote.c b/remote.c
index 81f4f01b00..b850f2feb3 100644
--- a/remote.c
+++ b/remote.c
@@ -1205,9 +1205,36 @@ static void add_missing_tags(struct ref *src, struct ref 
**dst, struct ref ***ds
         * sent to the other side.
         */
        if (sent_tips.nr) {
+               const int reachable_flag = 1;
+               struct commit_list *found_commits;
+               struct commit **src_commits;
+               int nr_src_commits = 0, alloc_src_commits = 16;
+               ALLOC_ARRAY(src_commits, alloc_src_commits);
+
                for_each_string_list_item(item, &src_tag) {
                        struct ref *ref = item->util;
+                       struct commit *commit;
+
+                       if (is_null_oid(&ref->new_oid))
+                               continue;
+                       commit = lookup_commit_reference_gently(the_repository,
+                                                               &ref->new_oid,
+                                                               1);
+                       if (!commit)
+                               /* not pushing a commit, which is not an error 
*/
+                               continue;
+
+                       ALLOC_GROW(src_commits, nr_src_commits + 1, 
alloc_src_commits);
+                       src_commits[nr_src_commits++] = commit;
+               }
+
+               found_commits = get_reachable_subset(sent_tips.tip, 
sent_tips.nr,
+                                                    src_commits, 
nr_src_commits,
+                                                    reachable_flag);
+
+               for_each_string_list_item(item, &src_tag) {
                        struct ref *dst_ref;
+                       struct ref *ref = item->util;
                        struct commit *commit;
 
                        if (is_null_oid(&ref->new_oid))
@@ -1223,7 +1250,7 @@ static void add_missing_tags(struct ref *src, struct ref 
**dst, struct ref ***ds
                         * Is this tag, which they do not have, reachable from
                         * any of the commits we are sending?
                         */
-                       if (!in_merge_bases_many(commit, sent_tips.nr, 
sent_tips.tip))
+                       if (!(commit->object.flags & reachable_flag))
                                continue;
 
                        /* Add it in */
@@ -1231,7 +1258,12 @@ static void add_missing_tags(struct ref *src, struct ref 
**dst, struct ref ***ds
                        oidcpy(&dst_ref->new_oid, &ref->new_oid);
                        dst_ref->peer_ref = copy_ref(ref);
                }
+
+               clear_commit_marks_many(nr_src_commits, src_commits, 
reachable_flag);
+               free(src_commits);
+               free_commit_list(found_commits);
        }
+
        string_list_clear(&src_tag, 0);
        free(sent_tips.tip);
 }
-- 
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