Automatically update MR merge-ref along merge status

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.

4 files changed, 140 insertions, 141 deletions

diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index c72b6e9033d..6e703023ed5 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -1962,57 +1962,6 @@ describe MergeRequest do
     end
   end
 
-  describe '#check_if_can_be_merged' do
-    let(:project) { create(:project, only_allow_merge_if_pipeline_succeeds: true) }
-
-    shared_examples 'checking if can be merged' do
-      context 'when it is not broken and has no conflicts' do
-        before do
-          allow(subject).to receive(:broken?) { false }
-          allow(project.repository).to receive(:can_be_merged?).and_return(true)
-        end
-
-        it 'is marked as mergeable' do
-          expect { subject.check_if_can_be_merged }.to change { subject.merge_status }.to('can_be_merged')
-        end
-      end
-
-      context 'when broken' do
-        before do
-          allow(subject).to receive(:broken?) { true }
-          allow(project.repository).to receive(:can_be_merged?).and_return(false)
-        end
-
-        it 'becomes unmergeable' do
-          expect { subject.check_if_can_be_merged }.to change { subject.merge_status }.to('cannot_be_merged')
-        end
-      end
-
-      context 'when it has conflicts' do
-        before do
-          allow(subject).to receive(:broken?) { false }
-          allow(project.repository).to receive(:can_be_merged?).and_return(false)
-        end
-
-        it 'becomes unmergeable' do
-          expect { subject.check_if_can_be_merged }.to change { subject.merge_status }.to('cannot_be_merged')
-        end
-      end
-    end
-
-    context 'when merge_status is unchecked' do
-      subject { create(:merge_request, source_project: project, merge_status: :unchecked) }
-
-      it_behaves_like 'checking if can be merged'
-    end
-
-    context 'when merge_status is unchecked' do
-      subject { create(:merge_request, source_project: project, merge_status: :cannot_be_merged_recheck) }
-
-      it_behaves_like 'checking if can be merged'
-    end
-  end
-
   describe '#mergeable?' do
     let(:project) { create(:project) }
 
@@ -2026,7 +1975,7 @@ describe MergeRequest do
 
     it 'return true if #mergeable_state? is true and the MR #can_be_merged? is true' do
       allow(subject).to receive(:mergeable_state?) { true }
-      expect(subject).to receive(:check_if_can_be_merged)
+      expect(subject).to receive(:check_mergeability)
       expect(subject).to receive(:can_be_merged?) { true }
 
       expect(subject.mergeable?).to be_truthy
@@ -2040,7 +1989,7 @@ describe MergeRequest do
 
     it 'checks if merge request can be merged' do
       allow(subject).to receive(:mergeable_ci_state?) { true }
-      expect(subject).to receive(:check_if_can_be_merged)
+      expect(subject).to receive(:check_mergeability)
 
       subject.mergeable?
     end
@@ -3108,38 +3057,6 @@ describe MergeRequest do
     end
   end
 
-  describe '#mergeable_to_ref?' do
-    it 'returns true when merge request is mergeable' do
-      subject = create(:merge_request)
-
-      expect(subject.mergeable_to_ref?).to be(true)
-    end
-
-    it 'returns false when merge request is already merged' do
-      subject = create(:merge_request, :merged)
-
-      expect(subject.mergeable_to_ref?).to be(false)
-    end
-
-    it 'returns false when merge request is closed' do
-      subject = create(:merge_request, :closed)
-
-      expect(subject.mergeable_to_ref?).to be(false)
-    end
-
-    it 'returns false when merge request is work in progress' do
-      subject = create(:merge_request, title: 'WIP: The feature')
-
-      expect(subject.mergeable_to_ref?).to be(false)
-    end
-
-    it 'returns false when merge request has no commits' do
-      subject = create(:merge_request, source_branch: 'empty-branch', target_branch: 'master')
-
-      expect(subject.mergeable_to_ref?).to be(false)
-    end
-  end
-
   describe '#merge_participants' do
     it 'contains author' do
       expect(subject.merge_participants).to eq([subject.author])
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index 5c94a87529b..c164c52c975 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -1546,52 +1546,65 @@ describe API::MergeRequests do
     end
   end
 
-  describe "PUT /projects/:id/merge_requests/:merge_request_iid/merge_to_ref" do
-    let(:pipeline) { create(:ci_pipeline_without_jobs) }
+  describe "GET /projects/:id/merge_requests/:merge_request_iid/merge_ref" do
+    before do
+      merge_request.mark_as_unchecked!
+    end
+
+    let(:merge_request_iid) { merge_request.iid }
+
     let(:url) do
-      "/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge_to_ref"
+      "/projects/#{project.id}/merge_requests/#{merge_request_iid}/merge_ref"
     end
 
     it 'returns the generated ID from the merge service in case of success' do
-      put api(url, user), params: { merge_commit_message: 'Custom message' }
-
-      commit = project.commit(json_response['commit_id'])
+      get api(url, user)
 
       expect(response).to have_gitlab_http_status(200)
-      expect(json_response['commit_id']).to be_present
-      expect(commit.message).to eq('Custom message')
+      expect(json_response['commit_id']).to eq(merge_request.merge_ref_head.sha)
     end
 
     it "returns 400 if branch can't be merged" do
-      merge_request.update!(state: 'merged')
+      merge_request.update!(merge_status: 'cannot_be_merged')
 
-      put api(url, user)
+      get api(url, user)
 
       expect(response).to have_gitlab_http_status(400)
-      expect(json_response['message'])
-        .to eq("Merge request is not mergeable to #{merge_request.merge_ref_path}")
+      expect(json_response['message']).to eq('Merge request is not mergeable')
     end
 
-    it 'returns 403 if user has no permissions to merge to the ref' do
-      user2 = create(:user)
-      project.add_reporter(user2)
+    context 'when user has no access to the MR' do
+      let(:project) { create(:project, :private) }
+      let(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
 
-      put api(url, user2)
+      it 'returns 404' do
+        project.add_guest(user)
 
-      expect(response).to have_gitlab_http_status(403)
-      expect(json_response['message']).to eq('403 Forbidden')
+        get api(url, user)
+
+        expect(response).to have_gitlab_http_status(404)
+        expect(json_response['message']).to eq('404 Not found')
+      end
     end
 
-    it 'returns 404 for an invalid merge request IID' do
-      put api("/projects/#{project.id}/merge_requests/12345/merge_to_ref", user)
+    context 'when invalid merge request IID' do
+      let(:merge_request_iid) { '12345' }
 
-      expect(response).to have_gitlab_http_status(404)
+      it 'returns 404' do
+        get api(url, user)
+
+        expect(response).to have_gitlab_http_status(404)
+      end
     end
 
-    it "returns 404 if the merge request id is used instead of iid" do
-      put api("/projects/#{project.id}/merge_requests/#{merge_request.id}/merge", user)
+    context 'when merge request ID is used instead IID' do
+      let(:merge_request_iid) { merge_request.id }
 
-      expect(response).to have_gitlab_http_status(404)
+      it 'returns 404' do
+        get api(url, user)
+
+        expect(response).to have_gitlab_http_status(404)
+      end
     end
   end
 
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index 0ac23050caf..5d492e4b013 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -32,10 +32,8 @@ describe MergeRequests::MergeToRefService do
 
       expect(result[:status]).to eq(:success)
       expect(result[:commit_id]).to be_present
-      expect(result[:source_id]).to eq(merge_request.source_branch_sha)
-      expect(result[:target_id]).to eq(merge_request.target_branch_sha)
       expect(repository.ref_exists?(target_ref)).to be(true)
-      expect(ref_head.id).to eq(result[:commit_id])
+      expect(ref_head.sha).to eq(result[:commit_id])
     end
   end
 
@@ -72,10 +70,6 @@ describe MergeRequests::MergeToRefService do
   let(:merge_request) { create(:merge_request, :simple) }
   let(:project) { merge_request.project }
 
-  before do
-    project.add_maintainer(user)
-  end
-
   describe '#execute' do
     let(:service) do
       described_class.new(project, user, commit_message: 'Awesome message',
@@ -92,6 +86,12 @@ describe MergeRequests::MergeToRefService do
     it_behaves_like 'successfully evaluates pre-condition checks'
 
     context 'commit history comparison with regular MergeService' do
+      before do
+        # The merge service needs an authorized user while merge-to-ref
+        # doesn't.
+        project.add_maintainer(user)
+      end
+
       let(:merge_ref_service) do
         described_class.new(project, user, {})
       end
@@ -136,9 +136,9 @@ describe MergeRequests::MergeToRefService do
         let(:merge_method) { :merge }
 
         it 'returns error' do
-          allow(merge_request).to receive(:mergeable_to_ref?) { false }
+          allow(project).to receive_message_chain(:repository, :merge_to_ref) { nil }
 
-          error_message = "Merge request is not mergeable to #{merge_request.merge_ref_path}"
+          error_message = 'Conflicts detected during merge'
 
           result = service.execute(merge_request)
 
@@ -170,28 +170,5 @@ describe MergeRequests::MergeToRefService do
 
       it { expect(todo).not_to be_done }
     end
-
-    context 'when merge request is WIP state' do
-      it 'fails to merge' do
-        merge_request = create(:merge_request, title: 'WIP: The feature')
-
-        result = service.execute(merge_request)
-
-        expect(result[:status]).to eq(:error)
-        expect(result[:message]).to eq("Merge request is not mergeable to #{merge_request.merge_ref_path}")
-      end
-    end
-
-    it 'returns error when user has no authorization to admin the merge request' do
-      unauthorized_user = create(:user)
-      project.add_reporter(unauthorized_user)
-
-      service = described_class.new(project, unauthorized_user)
-
-      result = service.execute(merge_request)
-
-      expect(result[:status]).to eq(:error)
-      expect(result[:message]).to eq('You are not allowed to merge to this ref')
-    end
   end
 end
diff --git a/spec/services/merge_requests/mergeability_check_service_spec.rb b/spec/services/merge_requests/mergeability_check_service_spec.rb
new file mode 100644
index 00000000000..6ad079d5f28
--- /dev/null
+++ b/spec/services/merge_requests/mergeability_check_service_spec.rb
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe MergeRequests::MergeabilityCheckService do
+  shared_examples_for 'unmergeable merge request' do
+    it 'updates or keeps merge status as cannot_be_merged' do
+      subject
+
+      expect(merge_request.merge_status).to eq('cannot_be_merged')
+    end
+
+    it 'does not change the merge ref HEAD' do
+      expect { subject }.not_to change(merge_request, :merge_ref_head)
+    end
+
+    it 'returns ServiceResponse.error' do
+      result = subject
+
+      expect(result).to be_a(ServiceResponse)
+      expect(result).to be_error
+    end
+  end
+
+  shared_examples_for 'mergeable merge request' do
+    it 'updates or keeps merge status as can_be_merged' do
+      subject
+
+      expect(merge_request.merge_status).to eq('can_be_merged')
+    end
+
+    it 'updates the merge ref' do
+      expect { subject }.to change(merge_request, :merge_ref_head).from(nil)
+    end
+
+    it 'returns ServiceResponse.success' do
+      result = subject
+
+      expect(result).to be_a(ServiceResponse)
+      expect(result).to be_success
+    end
+  end
+
+  describe '#execute' do
+    let(:project) { create(:project, :repository) }
+    let(:merge_request) { create(:merge_request, merge_status: :unchecked, source_project: project, target_project: project) }
+    let(:repo) { project.repository }
+
+    subject { described_class.new(merge_request).execute }
+
+    before do
+      project.add_developer(merge_request.author)
+    end
+
+    it_behaves_like 'mergeable merge request'
+
+    context 'when broken' do
+      before do
+        allow(merge_request).to receive(:broken?) { true }
+        allow(project.repository).to receive(:can_be_merged?) { false }
+      end
+
+      it_behaves_like 'unmergeable merge request'
+    end
+
+    context 'when it has conflicts' do
+      before do
+        allow(merge_request).to receive(:broken?) { false }
+        allow(project.repository).to receive(:can_be_merged?) { false }
+      end
+
+      it_behaves_like 'unmergeable merge request'
+    end
+
+    context 'when MR cannot be merged and has no merge ref' do
+      before do
+        merge_request.mark_as_unmergeable!
+      end
+
+      it_behaves_like 'unmergeable merge request'
+    end
+
+    context 'when MR cannot be merged and has outdated merge ref' do
+      before do
+        MergeRequests::MergeToRefService.new(project, merge_request.author).execute(merge_request)
+        merge_request.mark_as_unmergeable!
+      end
+
+      it_behaves_like 'unmergeable merge request'
+    end
+  end
+end