Apply patches when creating MR via email

This allows users to add patches as attachments to merge request
created via email.

When an email to create a merge request is sent, all the attachments
ending in `.patch` will be applied to the branch specified in the
subject of the email. If the branch did not exist, it will be created
from the HEAD of the repository.

When the patches could not be applied, the error message will be
replied to the user.

The patches can have a maximum combined size of 2MB for now.

1 files changed, 69 insertions, 0 deletions

diff --git a/spec/lib/gitlab/email/handler/create_merge_request_handler_spec.rb b/spec/lib/gitlab/email/handler/create_merge_request_handler_spec.rb
index ace3104f36f..f276f1a8ddf 100644
--- a/spec/lib/gitlab/email/handler/create_merge_request_handler_spec.rb
+++ b/spec/lib/gitlab/email/handler/create_merge_request_handler_spec.rb
@@ -93,5 +93,74 @@ describe Gitlab::Email::Handler::CreateMergeRequestHandler do
         end
       end
     end
+
+    context 'when the email contains patch attachments' do
+      let(:email_raw) { fixture_file("emails/valid_merge_request_with_patch.eml") }
+
+      it 'creates the source branch and applies the patches' do
+        receiver.execute
+
+        branch = project.repository.find_branch('new-branch-with-a-patch')
+
+        expect(branch).not_to be_nil
+        expect(branch.dereferenced_target.message).to include('A commit from a patch')
+      end
+
+      it 'creates the merge request' do
+        expect { receiver.execute }
+          .to change { project.merge_requests.where(source_branch: 'new-branch-with-a-patch').size }.by(1)
+      end
+
+      it 'does not mention the patches in the created merge request' do
+        receiver.execute
+
+        merge_request = project.merge_requests.find_by!(source_branch: 'new-branch-with-a-patch')
+
+        expect(merge_request.description).not_to include('0001-A-commit-from-a-patch.patch')
+      end
+
+      context 'when the patch could not be applied' do
+        let(:email_raw) { fixture_file("emails/merge_request_with_conflicting_patch.eml") }
+
+        it 'raises an error' do
+          expect { receiver.execute }.to raise_error(Gitlab::Email::InvalidAttachment)
+        end
+      end
+
+      context 'when specifying the target branch using quick actions' do
+        let(:email_raw) { fixture_file('emails/merge_request_with_patch_and_target_branch.eml') }
+
+        it 'creates the merge request with the correct target branch' do
+          receiver.execute
+
+          merge_request = project.merge_requests.find_by!(source_branch: 'new-branch-with-a-patch')
+
+          expect(merge_request.target_branch).to eq('with-codeowners')
+        end
+
+        it 'based the merge request of the target_branch' do
+          receiver.execute
+
+          merge_request = project.merge_requests.find_by!(source_branch: 'new-branch-with-a-patch')
+
+          expect(merge_request.diff_base_commit).to eq(project.repository.commit('with-codeowners'))
+        end
+      end
+    end
+  end
+
+  describe '#patch_attachments' do
+    let(:email_raw) { fixture_file('emails/merge_request_multiple_patches.eml') }
+    let(:mail) { Mail::Message.new(email_raw) }
+    subject(:handler) { described_class.new(mail, mail_key) }
+
+    it 'orders attachments ending in `.patch` by name' do
+      expected_filenames = ["0001-A-commit-from-a-patch.patch",
+                            "0002-This-does-not-apply-to-the-feature-branch.patch"]
+
+      attachments = handler.__send__(:patch_attachments).map(&:filename)
+
+      expect(attachments).to eq(expected_filenames)
+    end
   end
 end