1 files changed, 49 insertions, 0 deletions
diff --git a/lib/gitlab/git/cross_repo.rb b/lib/gitlab/git/cross_repo.rb
new file mode 100644
index 00000000000..d44657e7db1
--- /dev/null
+++ b/lib/gitlab/git/cross_repo.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class CrossRepo
+      attr_reader :source_repo, :target_repo
+
+      def initialize(source_repo, target_repo)
+        @source_repo = source_repo
+        @target_repo = target_repo
+      end
+
+      def execute(target_ref, &blk)
+        ensuring_ref_in_source(target_ref, &blk)
+      end
+
+      private
+
+      def ensuring_ref_in_source(ref, &blk)
+        return yield ref if source_repo == target_repo
+
+        # If the commit doesn't exist in the target, there's nothing we can do
+        commit_id = target_repo.commit(ref)&.sha
+        return unless commit_id
+
+        # The commit pointed to by ref may exist in the source even when they
+        # are different repositories. This is particularly true of close forks,
+        # but may also be the case if a temporary ref for this comparison has
+        # already been created in the past, and the result hasn't been GC'd yet.
+        return yield commit_id if source_repo.commit(commit_id)
+
+        # Worst case: the commit is not in the source repo so we need to fetch
+        # it. Use a temporary ref and clean up afterwards
+        with_commit_in_source_tmp(commit_id, &blk)
+      end
+
+      # Fetch the ref into source_repo from target_repo, using a temporary ref
+      # name that will be deleted once the method completes. This is a no-op if
+      # fetching the source branch fails
+      def with_commit_in_source_tmp(commit_id, &blk)
+        tmp_ref = "refs/#{::Repository::REF_TMP}/#{SecureRandom.hex}"
+
+        yield commit_id if source_repo.fetch_source_branch!(target_repo, commit_id, tmp_ref)
+      ensure
+        source_repo.delete_refs(tmp_ref) # best-effort
+      end
+    end
+  end
+end