Merge branch '21459-resolving-merge-conflicts-from-a-forked-repo-through-ui' into 'master'

Fix resolving conflicts on forks

## What does this MR do?

When we resolve conflicts, we create a merge commit in the source branch with parents `[source_branch_head, target_branch_head]`. But when the MR is from a fork, `target_branch_head` might not exist in the source repo at all, so we need to fetch it if it isn't there. We can do this locally so it should be fast.

## Are there points in the code the reviewer needs to double check?

The `TestEnv` changes are needed to reset the branch refs if we're reusing a git directory locally - otherwise, there might not be conflicts!

## Why was this MR needed?

It's a bug in a new feature!

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21459.

See merge request !6082

app/services/merge_requests/resolve_service.rb

 module MergeRequests
   class ResolveService < MergeRequests::BaseService
-    attr_accessor :conflicts, :rugged, :merge_index
+    attr_accessor :conflicts, :rugged, :merge_index, :merge_request
 
     def execute(merge_request)
       @conflicts = merge_request.conflicts
       @rugged = project.repository.rugged
       @merge_index = conflicts.merge_index
+      @merge_request = merge_request
+
+      fetch_their_commit!
 
       conflicts.files.each do |file|
         write_resolved_file_to_index(file, params[:sections])
@@ -27,5 +30,21 @@ module MergeRequests
       merge_index.add(path: our_path, oid: rugged.write(new_file, :blob), mode: file.our_mode)
       merge_index.conflict_remove(our_path)
     end
+
+    # If their commit (in the target project) doesn't exist in the source project, it
+    # can't be a parent for the merge commit we're about to create. If that's the case,
+    # fetch the target branch ref into the source project so the commit exists in both.
+    #
+    def fetch_their_commit!
+      return if rugged.include?(conflicts.their_commit.oid)
+
+      random_string = SecureRandom.hex
+
+      project.repository.fetch_ref(
+        merge_request.target_project.repository.path_to_repo,
+        "refs/heads/#{merge_request.target_branch}",
+        "refs/tmp/#{random_string}/head"
+      )
+    end
   end
 end

spec/services/merge_requests/resolve_service_spec.rb

+require 'spec_helper'
+
+describe MergeRequests::ResolveService do
+  let(:user) { create(:user) }
+  let(:project) { create(:project) }
+
+  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
+    context 'with valid params' do
+      let(:params) do
+        {
+          sections: {
+            '2f6fcd96b88b36ce98c38da085c795a27d92a3dd_14_14' => 'head',
+            '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
+          MergeRequests::ResolveService.new(project, user, params).execute(merge_request)
+        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(['1450cd639e0bc6721eb02800169e464f212cde06',
+                   '75284c70dd26c87f2a3fb65fd5a1f0b0138d3a6b'])
+        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.commit_file(user, 'new-file-in-target', '', 'Add new file in target', 'conflict-start', false)
+        end
+
+        before do
+          MergeRequests::ResolveService.new(fork_project, user, params).execute(merge_request_from_fork)
+        end
+
+        it 'creates a commit with the message' do
+          expect(merge_request_from_fork.source_branch_head.message).to eq(params[:commit_message])
+        end
+
+        it 'creates a commit with the correct parents' do
+          expect(merge_request_from_fork.source_branch_head.parents.map(&:id)).
+            to eq(['404fa3fc7c2c9b5dacff102f353bdf55b1be2813',
+                   target_head])
+        end
+      end
+    end
+
+    context 'when a resolution is missing' do
+      let(:invalid_params) { { sections: { '2f6fcd96b88b36ce98c38da085c795a27d92a3dd_14_14' => 'head' } } }
+      let(:service) { MergeRequests::ResolveService.new(project, user, invalid_params) }
+
+      it 'raises a MissingResolution error' do
+        expect { service.execute(merge_request) }.
+          to raise_error(Gitlab::Conflict::File::MissingResolution)
+      end
+    end
+  end
+end

spec/support/test_env.rb

@@ -6,7 +6,7 @@ module TestEnv
   # When developing the seed repository, comment out the branch you will modify.
   BRANCH_SHA = {
     'empty-branch'                       => '7efb185',
-    'ends-with.json'                     => '98b0d8b3',
+    'ends-with.json'                     => '98b0d8b',
     'flatten-dir'                        => 'e56497b',
     'feature'                            => '0b4bc9a',
     'feature_conflict'                   => 'bb5206f',
@@ -37,9 +37,10 @@ module TestEnv
   # need to keep all the branches in sync.
   # We currently only need a subset of the branches
   FORKED_BRANCH_SHA = {
-    'add-submodule-version-bump' => '3f547c08',
-    'master' => '5937ac0',
-    'remove-submodule' => '2a33e0c0'
+    'add-submodule-version-bump' => '3f547c0',
+    'master'                     => '5937ac0',
+    'remove-submodule'           => '2a33e0c',
+    'conflict-resolvable-fork'   => '404fa3f'
   }
 
   # Test environment
@@ -117,22 +118,7 @@ module TestEnv
       system(*%W(#{Gitlab.config.git.bin_path} clone -q #{clone_url} #{repo_path}))
     end
 
-    Dir.chdir(repo_path) do
-      branch_sha.each do |branch, sha|
-        # Try to reset without fetching to avoid using the network.
-        reset = %W(#{Gitlab.config.git.bin_path} update-ref refs/heads/#{branch} #{sha})
-        unless system(*reset)
-          if system(*%W(#{Gitlab.config.git.bin_path} fetch origin))
-            unless system(*reset)
-              raise 'The fetched test seed '\
-              'does not contain the required revision.'
-            end
-          else
-            raise 'Could not fetch test seed repository.'
-          end
-        end
-      end
-    end
+    set_repo_refs(repo_path, branch_sha)
 
     # We must copy bare repositories because we will push to them.
     system(git_env, *%W(#{Gitlab.config.git.bin_path} clone -q --bare #{repo_path} #{repo_path_bare}))
@@ -144,6 +130,7 @@ module TestEnv
     FileUtils.mkdir_p(target_repo_path)
     FileUtils.cp_r("#{base_repo_path}/.", target_repo_path)
     FileUtils.chmod_R 0755, target_repo_path
+    set_repo_refs(target_repo_path, BRANCH_SHA)
   end
 
   def repos_path
@@ -160,6 +147,7 @@ module TestEnv
     FileUtils.mkdir_p(target_repo_path)
     FileUtils.cp_r("#{base_repo_path}/.", target_repo_path)
     FileUtils.chmod_R 0755, target_repo_path
+    set_repo_refs(target_repo_path, FORKED_BRANCH_SHA)
   end
 
   # When no cached assets exist, manually hit the root path to create them
@@ -209,4 +197,23 @@ module TestEnv
   def git_env
     { 'GIT_TEMPLATE_DIR' => '' }
   end
+
+  def set_repo_refs(repo_path, branch_sha)
+    Dir.chdir(repo_path) do
+      branch_sha.each do |branch, sha|
+        # Try to reset without fetching to avoid using the network.
+        reset = %W(#{Gitlab.config.git.bin_path} update-ref refs/heads/#{branch} #{sha})
+        unless system(*reset)
+          if system(*%W(#{Gitlab.config.git.bin_path} fetch origin))
+            unless system(*reset)
+              raise 'The fetched test seed '\
+              'does not contain the required revision.'
+            end
+          else
+            raise 'Could not fetch test seed repository.'
+          end
+        end
+      end
+    end
+  end
 end