UI Bug Fix: Expand the early checks when using /merge quick action

At the moment we do a check of the `mergable_state?` method to see
if it can be merged, and later when we try to merge, the
`mergeable?` method is called. But sometimes this means we return
that the merge request was merged, when it actually isn't.

Issue: gitlab.com/gitlab-org/gitlab/-/issues/354453
MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/82170

Changelog: fixed
EE: true

app/services/merge_requests/merge_orchestration_service.rb

@@ -26,7 +26,7 @@ module MergeRequests
 
     def can_merge_immediately?(merge_request)
       merge_request.can_be_merged_by?(current_user) &&
-        merge_request.mergeable_state?
+        merge_request.mergeable?
     end
 
     def can_merge_automatically?(merge_request)

ee/spec/factories/merge_requests.rb

@@ -8,6 +8,12 @@ FactoryBot.modify do
       end
     end
 
+    trait :blocked do
+      after :create do |merge_request, _|
+        create(:merge_request_block, blocked_merge_request: merge_request)
+      end
+    end
+
     trait :on_train do
       transient do
         train_creator { author }

ee/spec/services/quick_actions/interpret_service_spec.rb

@@ -87,6 +87,22 @@ RSpec.describe QuickActions::InterpretService do
     end
   end
 
+  shared_examples 'failed command' do |error_msg|
+    let(:match_msg) { error_msg ? eq(error_msg) : be_empty }
+
+    it 'populates {} if content contains an unsupported command' do
+      _, updates, _ = service.execute(content, issuable)
+
+      expect(updates).to be_empty
+    end
+
+    it "returns #{error_msg || 'an empty'} message" do
+      _, _, message = service.execute(content, issuable)
+
+      expect(message).to match_msg
+    end
+  end
+
   describe '#execute' do
     let(:merge_request) { create(:merge_request, source_project: project) }
 
@@ -1227,6 +1243,36 @@ RSpec.describe QuickActions::InterpretService do
       end
     end
 
+    context 'when the merge request is not approved' do
+      it_behaves_like 'failed command', 'Could not apply merge command.' do
+        let(:content) { '/merge' }
+        let(:issuable) { merge_request }
+        let!(:rule) { create(:any_approver_rule, merge_request: merge_request, approvals_required: 1) }
+      end
+    end
+
+    context 'when the merge request is blocked' do
+      it_behaves_like 'failed command', 'Could not apply merge command.' do
+        before do
+          stub_licensed_features(blocking_merge_requests: true)
+        end
+
+        let(:content) { '/merge' }
+        let(:issuable) { create(:merge_request, :blocked, source_project: project) }
+      end
+    end
+
+    context 'when merge request has denied policies' do
+      it_behaves_like 'failed command', 'Could not apply merge command.' do
+        before do
+          allow(merge_request).to receive(:has_denied_policies?).and_return(true)
+        end
+
+        let(:content) { '/merge' }
+        let(:issuable) { merge_request }
+      end
+    end
+
     context 'approved merge request can be merged' do
       before do
         merge_request.update!(approvals_before_merge: 1)

spec/services/merge_requests/merge_orchestration_service_spec.rb

@@ -64,7 +64,7 @@ RSpec.describe MergeRequests::MergeOrchestrationService do
 
     context 'when merge request is not mergeable' do
       before do
-        allow(merge_request).to receive(:mergeable_state?) { false }
+        allow(merge_request).to receive(:mergeable?) { false }
       end
 
       it 'does nothing' do
@@ -87,7 +87,7 @@ RSpec.describe MergeRequests::MergeOrchestrationService do
 
     context 'when merge request is not mergeable' do
       before do
-        allow(merge_request).to receive(:mergeable_state?) { false }
+        allow(merge_request).to receive(:mergeable?) { false }
       end
 
       it { is_expected.to eq(false) }