Fix conflict resolution from corrupted upstream

I don't know why this happens exactly, but given an upstream and fork repository
from a customer, both of which required GC, resolving conflicts would corrupt
the fork so badly that it couldn't be cloned.

This isn't a perfect fix for that case, because the MR may still need to be
merged manually, but it does ensure that the repository is at least usable.

My best guess is that when we generate the index for the conflict
resolution (which we previously did in the target project), we obtain a
reference to an OID that doesn't exist in the source, even though we already
fetch the refs from the target into the source.

Explicitly setting the source project as the place to get the merge index from
seems to prevent repository corruption in this way.

2 files changed, 295 insertions, 0 deletions

diff --git a/spec/services/merge_requests/conflicts/list_service_spec.rb b/spec/services/merge_requests/conflicts/list_service_spec.rb
new file mode 100644
index 00000000000..e8a305d6130
--- /dev/null
+++ b/spec/services/merge_requests/conflicts/list_service_spec.rb
@@ -0,0 +1,73 @@
+require 'spec_helper'
+
+describe MergeRequests::Conflicts::ListService do
+  describe '#can_be_resolved_in_ui?' do
+    def create_merge_request(source_branch)
+      create(:merge_request, source_branch: source_branch, target_branch: 'conflict-start') do |mr|
+        mr.mark_as_unmergeable
+      end
+    end
+
+    def conflicts_service(merge_request)
+      described_class.new(merge_request)
+    end
+
+    it 'returns a falsey value when the MR can be merged without conflicts' do
+      merge_request = create_merge_request('master')
+      merge_request.mark_as_mergeable
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the MR is marked as having conflicts, but has none' do
+      merge_request = create_merge_request('master')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the MR has a missing ref after a force push' do
+      merge_request = create_merge_request('conflict-resolvable')
+      service = conflicts_service(merge_request)
+      allow(service.conflicts).to receive(:merge_index).and_raise(Rugged::OdbError)
+
+      expect(service.can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the MR does not support new diff notes' do
+      merge_request = create_merge_request('conflict-resolvable')
+      merge_request.merge_request_diff.update_attributes(start_commit_sha: nil)
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the conflicts contain a large file' do
+      merge_request = create_merge_request('conflict-too-large')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the conflicts contain a binary file' do
+      merge_request = create_merge_request('conflict-binary-file')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a falsey value when the conflicts contain a file edited in one branch and deleted in another' do
+      merge_request = create_merge_request('conflict-missing-side')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
+    end
+
+    it 'returns a truthy value when the conflicts are resolvable in the UI' do
+      merge_request = create_merge_request('conflict-resolvable')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_truthy
+    end
+
+    it 'returns a truthy value when the conflicts have to be resolved in an editor' do
+      merge_request = create_merge_request('conflict-contains-conflict-markers')
+
+      expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_truthy
+    end
+  end
+end
diff --git a/spec/services/merge_requests/conflicts/resolve_service_spec.rb b/spec/services/merge_requests/conflicts/resolve_service_spec.rb
new file mode 100644
index 00000000000..19e8d5cc5f1
--- /dev/null
+++ b/spec/services/merge_requests/conflicts/resolve_service_spec.rb
@@ -0,0 +1,222 @@
+require 'spec_helper'
+
+describe MergeRequests::Conflicts::ResolveService do
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :repository) }
+
+  let(:fork_project) do
+    create(:forked_project_with_submodules) do |fork_project|
+      fork_project.build_forked_project_link(forked_to_project_id: fork_project.id, forked_from_project_id: project.id)
+      fork_project.save
+    end
+  end
+
+  let(:merge_request) do
+    create(:merge_request,
+           source_branch: 'conflict-resolvable', source_project: project,
+           target_branch: 'conflict-start')
+  end
+
+  let(:merge_request_from_fork) do
+    create(:merge_request,
+           source_branch: 'conflict-resolvable-fork', source_project: fork_project,
+           target_branch: 'conflict-start', target_project: project)
+  end
+
+  describe '#execute' do
+    let(:service) { described_class.new(merge_request) }
+
+    context 'with section params' do
+      let(:params) do
+        {
+          files: [
+            {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb',
+              sections: {
+                '2f6fcd96b88b36ce98c38da085c795a27d92a3dd_14_14' => 'head'
+              }
+            }, {
+              old_path: 'files/ruby/regex.rb',
+              new_path: 'files/ruby/regex.rb',
+              sections: {
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_9_9' => 'head',
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_21_21' => 'origin',
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_49_49' => 'origin'
+              }
+            }
+          ],
+          commit_message: 'This is a commit message!'
+        }
+      end
+
+      context 'when the source and target project are the same' do
+        before do
+          service.execute(user, params)
+        end
+
+        it 'creates a commit with the message' do
+          expect(merge_request.source_branch_head.message).to eq(params[:commit_message])
+        end
+
+        it 'creates a commit with the correct parents' do
+          expect(merge_request.source_branch_head.parents.map(&:id)).
+            to eq(%w(1450cd639e0bc6721eb02800169e464f212cde06
+                     824be604a34828eb682305f0d963056cfac87b2d))
+        end
+      end
+
+      context 'when the source project is a fork and does not contain the HEAD of the target branch' do
+        let!(:target_head) do
+          project.repository.create_file(
+            user,
+            'new-file-in-target',
+            '',
+            message: 'Add new file in target',
+            branch_name: 'conflict-start')
+        end
+
+        def resolve_conflicts
+          described_class.new(merge_request_from_fork).execute(user, params)
+        end
+
+        it 'gets conflicts from the source project' do
+          expect(fork_project.repository.rugged).to receive(:merge_commits).and_call_original
+          expect(project.repository.rugged).not_to receive(:merge_commits)
+
+          resolve_conflicts
+        end
+
+        it 'creates a commit with the message' do
+          resolve_conflicts
+
+          expect(merge_request_from_fork.source_branch_head.message).to eq(params[:commit_message])
+        end
+
+        it 'creates a commit with the correct parents' do
+          resolve_conflicts
+
+          expect(merge_request_from_fork.source_branch_head.parents.map(&:id)).
+            to eq(['404fa3fc7c2c9b5dacff102f353bdf55b1be2813',
+                   target_head])
+        end
+      end
+    end
+
+    context 'with content and sections params' do
+      let(:popen_content) { "class Popen\nend" }
+
+      let(:params) do
+        {
+          files: [
+            {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb',
+              content: popen_content
+            }, {
+              old_path: 'files/ruby/regex.rb',
+              new_path: 'files/ruby/regex.rb',
+              sections: {
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_9_9' => 'head',
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_21_21' => 'origin',
+                '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_49_49' => 'origin'
+              }
+            }
+          ],
+          commit_message: 'This is a commit message!'
+        }
+      end
+
+      before do
+        service.execute(user, params)
+      end
+
+      it 'creates a commit with the message' do
+        expect(merge_request.source_branch_head.message).to eq(params[:commit_message])
+      end
+
+      it 'creates a commit with the correct parents' do
+        expect(merge_request.source_branch_head.parents.map(&:id)).
+          to eq(%w(1450cd639e0bc6721eb02800169e464f212cde06
+                   824be604a34828eb682305f0d963056cfac87b2d))
+      end
+
+      it 'sets the content to the content given' do
+        blob = merge_request.source_project.repository.blob_at(merge_request.source_branch_head.sha,
+                                                               'files/ruby/popen.rb')
+
+        expect(blob.data).to eq(popen_content)
+      end
+    end
+
+    context 'when a resolution section is missing' do
+      let(:invalid_params) do
+        {
+          files: [
+            {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb',
+              content: ''
+            }, {
+              old_path: 'files/ruby/regex.rb',
+              new_path: 'files/ruby/regex.rb',
+              sections: { '6eb14e00385d2fb284765eb1cd8d420d33d63fc9_9_9' => 'head' }
+            }
+          ],
+          commit_message: 'This is a commit message!'
+        }
+      end
+
+      it 'raises a MissingResolution error' do
+        expect { service.execute(user, invalid_params) }.
+          to raise_error(Gitlab::Conflict::File::MissingResolution)
+      end
+    end
+
+    context 'when the content of a file is unchanged' do
+      let(:list_service) { MergeRequests::Conflicts::ListService.new(merge_request) }
+
+      let(:invalid_params) do
+        {
+          files: [
+            {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb',
+              content: ''
+            }, {
+              old_path: 'files/ruby/regex.rb',
+              new_path: 'files/ruby/regex.rb',
+              content: list_service.conflicts.file_for_path('files/ruby/regex.rb', 'files/ruby/regex.rb').content
+            }
+          ],
+          commit_message: 'This is a commit message!'
+        }
+      end
+
+      it 'raises a MissingResolution error' do
+        expect { service.execute(user, invalid_params) }.
+          to raise_error(Gitlab::Conflict::File::MissingResolution)
+      end
+    end
+
+    context 'when a file is missing' do
+      let(:invalid_params) do
+        {
+          files: [
+            {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb',
+              content: ''
+            }
+          ],
+          commit_message: 'This is a commit message!'
+        }
+      end
+
+      it 'raises a MissingFiles error' do
+        expect { service.execute(user, invalid_params) }.
+          to raise_error(described_class::MissingFiles)
+      end
+    end
+  end
+end