Add diff_type column

This will be used to determine which type of MR diff will be
created/fetched.

app/models/merge_request.rb

@@ -50,12 +50,15 @@ class MergeRequest < ApplicationRecord
       end
     end
 
-  has_many :merge_request_diffs
+  has_many :merge_request_diffs,
+    -> { regular }, inverse_of: :merge_request
   has_many :merge_request_context_commits, inverse_of: :merge_request
   has_many :merge_request_context_commit_diff_files, through: :merge_request_context_commits, source: :diff_files
 
   has_one :merge_request_diff,
-    -> { order('merge_request_diffs.id DESC') }, inverse_of: :merge_request
+    -> { regular.order('merge_request_diffs.id DESC') }, inverse_of: :merge_request
+  has_one :merge_head_diff,
+    -> { merge_head }, inverse_of: :merge_request, class_name: 'MergeRequestDiff'
   has_one :cleanup_schedule, inverse_of: :merge_request
 
   belongs_to :latest_merge_request_diff, class_name: 'MergeRequestDiff'

app/models/merge_request_diff.rb

@@ -32,6 +32,7 @@ class MergeRequestDiff < ApplicationRecord
   has_many :merge_request_diff_commits, -> { order(:merge_request_diff_id, :relative_order) }
 
   validates :base_commit_sha, :head_commit_sha, :start_commit_sha, sha: true
+  validates :merge_request_id, uniqueness: { scope: :diff_type }, if: :merge_head?
 
   state_machine :state, initial: :empty do
     event :clean do
@@ -50,6 +51,11 @@ class MergeRequestDiff < ApplicationRecord
     state :overflow_diff_lines_limit
   end
 
+  enum diff_type: {
+    regular: 1,
+    merge_head: 2
+  }
+
   scope :with_files, -> { without_states(:without_files, :empty) }
   scope :viewable, -> { without_state(:empty) }
   scope :by_commit_sha, ->(sha) do

db/migrate/20210107105306_add_diff_type_to_merge_request_diffs.rb

+# frozen_string_literal: true
+
+class AddDiffTypeToMergeRequestDiffs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  UNIQUE_INDEX_NAME = 'index_merge_request_diffs_on_unique_merge_request_id'
+
+  def up
+    unless column_exists?(:merge_request_diffs, :diff_type)
+      with_lock_retries do
+        add_column :merge_request_diffs, :diff_type, :integer, null: false, limit: 2, default: 1
+      end
+    end
+
+    add_concurrent_index :merge_request_diffs, :merge_request_id, unique: true, where: 'diff_type = 2', name: UNIQUE_INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name(:merge_request_diffs, UNIQUE_INDEX_NAME)
+
+    if column_exists?(:merge_request_diffs, :diff_type)
+      with_lock_retries do
+        remove_column :merge_request_diffs, :diff_type
+      end
+    end
+  end
+end

db/schema_migrations/20210107105306

+f76ce27a82f4773dcda324d79cc93a044f21317dbb9fdff035879502b5752da3
\ No newline at end of file

db/structure.sql

@@ -14011,6 +14011,7 @@ CREATE TABLE merge_request_diffs (
     stored_externally boolean,
     files_count smallint,
     sorted boolean DEFAULT false NOT NULL,
+    diff_type smallint DEFAULT 1 NOT NULL,
     CONSTRAINT check_93ee616ac9 CHECK ((external_diff_store IS NOT NULL))
 );
 
@@ -22221,6 +22222,8 @@ CREATE INDEX index_merge_request_diffs_on_external_diff_store ON merge_request_d
 
 CREATE INDEX index_merge_request_diffs_on_merge_request_id_and_id ON merge_request_diffs USING btree (merge_request_id, id);
 
+CREATE UNIQUE INDEX index_merge_request_diffs_on_unique_merge_request_id ON merge_request_diffs USING btree (merge_request_id) WHERE (diff_type = 2);
+
 CREATE INDEX index_merge_request_metrics_on_first_deployed_to_production_at ON merge_request_metrics USING btree (first_deployed_to_production_at);
 
 CREATE INDEX index_merge_request_metrics_on_latest_closed_at ON merge_request_metrics USING btree (latest_closed_at) WHERE (latest_closed_at IS NOT NULL);

spec/factories/merge_request_diffs.rb

@@ -10,12 +10,18 @@ FactoryBot.define do
     head_commit_sha { Digest::SHA1.hexdigest(SecureRandom.hex) }
     start_commit_sha { Digest::SHA1.hexdigest(SecureRandom.hex) }
 
+    diff_type { :regular }
+
     trait :external do
       external_diff { fixture_file_upload("spec/fixtures/doc_sample.txt", "plain/txt") }
       stored_externally { true }
       importing { true } # this avoids setting the state to 'empty'
     end
 
+    trait :merge_head do
+      diff_type { :merge_head }
+    end
+
     factory :external_merge_request_diff, traits: [:external]
   end
 end

spec/lib/gitlab/import_export/all_models.yml

@@ -146,6 +146,7 @@ merge_requests:
 - merge_user
 - merge_request_diffs
 - merge_request_diff
+- merge_head_diff
 - merge_request_context_commits
 - merge_request_context_commit_diff_files
 - events

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -220,6 +220,7 @@ MergeRequestDiff:
 - commits_count
 - files_count
 - sorted
+- diff_type
 MergeRequestDiffCommit:
 - merge_request_diff_id
 - relative_order

spec/models/merge_request_diff_spec.rb

@@ -16,6 +16,8 @@ RSpec.describe MergeRequestDiff do
   describe 'validations' do
     subject { diff_with_commits }
 
+    it { is_expected.not_to validate_uniqueness_of(:diff_type).scoped_to(:merge_request_id) }
+
     it 'checks sha format of base_commit_sha, head_commit_sha and start_commit_sha' do
       subject.base_commit_sha = subject.head_commit_sha = subject.start_commit_sha = 'foobar'
 
@@ -23,6 +25,24 @@ RSpec.describe MergeRequestDiff do
       expect(subject.errors.count).to eq 3
       expect(subject.errors).to all(include('is not a valid SHA'))
     end
+
+    it 'does not validate uniqueness by default' do
+      expect(build(:merge_request_diff, merge_request: subject.merge_request)).to be_valid
+    end
+
+    context 'when merge request diff is a merge_head type' do
+      it 'is valid' do
+        expect(build(:merge_request_diff, :merge_head, merge_request: subject.merge_request)).to be_valid
+      end
+
+      context 'when merge_head diff exists' do
+        let(:existing_merge_head_diff) { create(:merge_request_diff, :merge_head) }
+
+        it 'validates uniqueness' do
+          expect(build(:merge_request_diff, :merge_head, merge_request: existing_merge_head_diff.merge_request)).not_to be_valid
+        end
+      end
+    end
   end
 
   describe 'create new record' do