Add validation errors to Merge Request form

If source branch or target branch doesn't exist in the project, display
form validation errors. Previously, this caused a 500 error code.

2 files changed, 57 insertions, 0 deletions

diff --git a/spec/features/merge_requests/create_new_mr_spec.rb b/spec/features/merge_requests/create_new_mr_spec.rb
index b963d1305b5..c68e1ea4af9 100644
--- a/spec/features/merge_requests/create_new_mr_spec.rb
+++ b/spec/features/merge_requests/create_new_mr_spec.rb
@@ -59,4 +59,12 @@ feature 'Create New Merge Request', feature: true, js: true do
       expect(page).to have_css('a.btn.active', text: 'Side-by-side')
     end
   end
+
+  it 'does not allow non-existing branches' do
+    visit new_namespace_project_merge_request_path(project.namespace, project, merge_request: { target_branch: 'non-exist-target', source_branch: 'non-exist-source' })
+
+    expect(page).to have_content('The form contains the following errors')
+    expect(page).to have_content('Source branch "non-exist-source" does not exist')
+    expect(page).to have_content('Target branch "non-exist-target" does not exist')
+  end
 end
diff --git a/spec/services/merge_requests/build_service_spec.rb b/spec/services/merge_requests/build_service_spec.rb
index 3a3f07ddcb9..3f5df049ea2 100644
--- a/spec/services/merge_requests/build_service_spec.rb
+++ b/spec/services/merge_requests/build_service_spec.rb
@@ -25,6 +25,8 @@ describe MergeRequests::BuildService, services: true do
 
   before do
     allow(CompareService).to receive_message_chain(:new, :execute).and_return(compare)
+    allow(project).to receive(:commit).and_return(commit_1)
+    allow(project).to receive(:commit).and_return(commit_2)
   end
 
   describe 'execute' do
@@ -193,5 +195,52 @@ describe MergeRequests::BuildService, services: true do
         end
       end
     end
+
+    context 'source branch does not exist' do
+      before do
+        allow(project).to receive(:commit).with(source_branch).and_return(nil)
+        allow(project).to receive(:commit).with(target_branch).and_return(commit_1)
+      end
+
+      it 'forbids the merge request from being created' do
+        expect(merge_request.can_be_created).to eq(false)
+      end
+
+      it 'adds an error message to the merge request' do
+        expect(merge_request.errors).to contain_exactly('Source branch "feature-branch" does not exist')
+      end
+    end
+
+    context 'target branch does not exist' do
+      before do
+        allow(project).to receive(:commit).with(source_branch).and_return(commit_1)
+        allow(project).to receive(:commit).with(target_branch).and_return(nil)
+      end
+
+      it 'forbids the merge request from being created' do
+        expect(merge_request.can_be_created).to eq(false)
+      end
+
+      it 'adds an error message to the merge request' do
+        expect(merge_request.errors).to contain_exactly('Target branch "master" does not exist')
+      end
+    end
+
+    context 'both source and target branches do not exist' do
+      before do
+        allow(project).to receive(:commit).and_return(nil)
+      end
+
+      it 'forbids the merge request from being created' do
+        expect(merge_request.can_be_created).to eq(false)
+      end
+
+      it 'adds both error messages to the merge request' do
+        expect(merge_request.errors).to contain_exactly(
+          'Source branch "feature-branch" does not exist',
+          'Target branch "master" does not exist'
+        )
+      end
+    end
   end
 end