Bring back Rugged implementation of CommitIsAncestor

This brings back some of the changes in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20095.

For users using Gitaly on top of NFS, accessing the Git data directly
via Rugged is more performant than Gitaly. This merge request introduces
the feature flag `rugged_commit_is_ancestor` to activate the Rugged
path.

Part of four Rugged changes identified in
https://gitlab.com/gitlab-org/gitlab-ce/issues/57317.

2 files changed, 27 insertions, 2 deletions

diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 2bfff8397e8..912253252a8 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -11,7 +11,7 @@ module Gitlab
       include Gitlab::Git::WrapsGitalyErrors
       include Gitlab::EncodingHelper
       include Gitlab::Utils::StrongMemoize
-      include Gitlab::Git::RuggedImpl::Repository
+      prepend Gitlab::Git::RuggedImpl::Repository
 
       SEARCH_CONTEXT_LINES = 3
       REV_LIST_COMMIT_LIMIT = 2_000
diff --git a/lib/gitlab/git/rugged_impl/repository.rb b/lib/gitlab/git/rugged_impl/repository.rb
index 135c47017b3..d4500573235 100644
--- a/lib/gitlab/git/rugged_impl/repository.rb
+++ b/lib/gitlab/git/rugged_impl/repository.rb
@@ -10,7 +10,9 @@ module Gitlab
   module Git
     module RuggedImpl
       module Repository
-        FEATURE_FLAGS = %i(rugged_find_commit).freeze
+        extend ::Gitlab::Utils::Override
+
+        FEATURE_FLAGS = %i(rugged_find_commit rugged_tree_entries rugged_tree_entry rugged_commit_is_ancestor).freeze
 
         def alternate_object_directories
           relative_object_directories.map { |d| File.join(path, d) }
@@ -41,6 +43,29 @@ module Gitlab
           obj = rugged.rev_parse(revspec)
           Ref.dereference_object(obj)
         end
+
+        override :ancestor?
+        def ancestor?(from, to)
+          if Feature.enabled?(:rugged_commit_is_ancestor)
+            rugged_is_ancestor?(from, to)
+          else
+            super
+          end
+        end
+
+        def rugged_is_ancestor?(ancestor_id, descendant_id)
+          return false if ancestor_id.nil? || descendant_id.nil?
+
+          rugged_merge_base(ancestor_id, descendant_id) == ancestor_id
+        rescue Rugged::OdbError
+          false
+        end
+
+        def rugged_merge_base(from, to)
+          rugged.merge_base(from, to)
+        rescue Rugged::ReferenceError
+          nil
+        end
       end
     end
   end