Move all rugged operation for ff_merge inside Gitlab::Git

We also delete some unused code related to the aforementioned feature.

app/models/repository.rb

@@ -862,22 +862,12 @@ class Repository
   end
 
   def ff_merge(user, source, target_branch, merge_request: nil)
-    our_commit = rugged.branches[target_branch].target
-    their_commit =
-      if source.is_a?(Gitlab::Git::Commit)
-        source.raw_commit
-      else
-        rugged.lookup(source)
-      end
+    their_commit_id = commit(source)&.id
+    raise 'Invalid merge source' if their_commit_id.nil?
 
-    raise 'Invalid merge target' if our_commit.nil?
-    raise 'Invalid merge source' if their_commit.nil?
+    merge_request&.update(in_progress_merge_commit_sha: their_commit_id)
 
-    with_branch(user, target_branch) do |start_commit|
-      merge_request&.update(in_progress_merge_commit_sha: their_commit.oid)
-
-      their_commit.oid
-    end
+    with_cache_hooks { raw.ff_merge(user, their_commit_id, target_branch) }
   end
 
   def revert(

lib/gitlab/git/repository.rb

@@ -745,6 +745,16 @@ module Gitlab
         nil
       end
 
+      def ff_merge(user, source_sha, target_branch)
+        OperationService.new(user, self).with_branch(target_branch) do |our_commit|
+          raise ArgumentError, 'Invalid merge target' unless our_commit
+
+          source_sha
+        end
+      rescue Rugged::ReferenceError
+        raise ArgumentError, 'Invalid merge source'
+      end
+
       def revert(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:)
         OperationService.new(user, self).with_branch(
           branch_name,

lib/gitlab/git_access.rb

@@ -215,10 +215,6 @@ module Gitlab
       ).exec
     end
 
-    def matching_merge_request?(newrev, branch_name)
-      Checks::MatchingMergeRequest.new(newrev, branch_name, project).match?
-    end
-
     def deploy_key
       actor if deploy_key?
     end

spec/lib/gitlab/git/repository_spec.rb

@@ -1564,6 +1564,60 @@ describe Gitlab::Git::Repository, seed_helper: true do
     end
   end
 
+  describe '#ff_merge' do
+    let(:repository) do
+      Gitlab::Git::Repository.new('default', TEST_MUTABLE_REPO_PATH, '')
+    end
+    let(:branch_head) { '6d394385cf567f80a8fd85055db1ab4c5295806f' }
+    let(:source_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
+    let(:user) { build(:user) }
+    let(:target_branch) { 'test-ff-target-branch' }
+
+    before do
+      repository.create_branch(target_branch, branch_head)
+    end
+
+    after do
+      ensure_seeds
+    end
+
+    subject { repository.ff_merge(user, source_sha, target_branch) }
+
+    it 'performs a ff_merge' do
+      expect(subject.newrev).to eq(source_sha)
+      expect(subject.repo_created).to be(false)
+      expect(subject.branch_created).to be(false)
+
+      expect(repository.commit(target_branch).id).to eq(source_sha)
+    end
+
+    context 'with a non-existing target branch' do
+      subject { repository.ff_merge(user, source_sha, 'this-isnt-real') }
+
+      it 'throws an ArgumentError' do
+        expect { subject }.to raise_error(ArgumentError)
+      end
+    end
+
+    context 'with a non-existing source commit' do
+      let(:source_sha) { 'f001' }
+
+      it 'throws an ArgumentError' do
+        expect { subject }.to raise_error(ArgumentError)
+      end
+    end
+
+    context 'when the source sha is not a descendant of the branch head' do
+      let(:source_sha) { '1a0b36b3cdad1d2ee32457c102a8c0b7056fa863' }
+
+      it "doesn't perform the ff_merge" do
+        expect { subject }.to raise_error(Gitlab::Git::CommitError)
+
+        expect(repository.commit(target_branch).id).to eq(branch_head)
+      end
+    end
+  end
+
   def create_remote_branch(repository, remote_name, branch_name, source_branch_name)
     source_branch = repository.branches.find { |branch| branch.name == source_branch_name }
     rugged = repository.rugged