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.

CHANGELOG.md

@@ -9,6 +9,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 - Adds an optional path parameter to the Commits API to filter commits by path (Luis HGO)
 - Fix Markdown styling inside reference links (Jan Zdráhal)
 - Fix extra space on Build sidebar on Firefox !7060
+- Fail gracefully when creating merge request with non-existing branch (alexsanford)
 - Fix mobile layout issues in admin user overview page !7087
 - Fix HipChat notifications rendering (airatshigapov, eisnerd)
 - Refactor Jira service to use jira-ruby gem

app/services/merge_requests/build_service.rb

@@ -13,20 +13,8 @@ module MergeRequests
       merge_request.target_project ||= (project.forked_from_project || project)
       merge_request.target_branch ||= merge_request.target_project.default_branch
 
-      if merge_request.target_branch.blank? || merge_request.source_branch.blank?
-        message =
-          if params[:source_branch] || params[:target_branch]
-            "You must select source and target branch"
-          end
-
-        return build_failed(merge_request, message)
-      end
-
-      if merge_request.source_project == merge_request.target_project &&
-         merge_request.target_branch == merge_request.source_branch
-
-        return build_failed(merge_request, 'You must select different branches')
-      end
+      messages = validate_branches(merge_request)
+      return build_failed(merge_request, messages) unless messages.empty?
 
       compare = CompareService.new.execute(
         merge_request.source_project,
@@ -43,6 +31,34 @@ module MergeRequests
 
     private
 
+    def validate_branches(merge_request)
+      messages = []
+
+      if merge_request.target_branch.blank? || merge_request.source_branch.blank?
+        messages <<
+          if params[:source_branch] || params[:target_branch]
+            "You must select source and target branch"
+          end
+      end
+
+      if merge_request.source_project == merge_request.target_project &&
+         merge_request.target_branch == merge_request.source_branch
+
+        messages << 'You must select different branches'
+      end
+
+      # See if source and target branches exist
+      unless merge_request.source_project.commit(merge_request.source_branch)
+        messages << "Source branch \"#{merge_request.source_branch}\" does not exist"
+      end
+
+      unless merge_request.target_project.commit(merge_request.target_branch)
+        messages << "Target branch \"#{merge_request.target_branch}\" does not exist"
+      end
+
+      messages
+    end
+
     # When your branch name starts with an iid followed by a dash this pattern will be
     # interpreted as the user wants to close that issue on this project.
     #
@@ -91,8 +107,10 @@ module MergeRequests
       merge_request
     end
 
-    def build_failed(merge_request, message)
-      merge_request.errors.add(:base, message) unless message.nil?
+    def build_failed(merge_request, messages)
+      messages.compact.each do |message|
+        merge_request.errors.add(:base, message)
+      end
       merge_request.compare_commits = []
       merge_request.can_be_created = false
       merge_request

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

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