1 files changed, 91 insertions, 0 deletions
diff --git a/lib/gitlab/git/conflict/resolver.rb b/lib/gitlab/git/conflict/resolver.rb
new file mode 100644
index 00000000000..df509c5f4ce
--- /dev/null
+++ b/lib/gitlab/git/conflict/resolver.rb
@@ -0,0 +1,91 @@
+module Gitlab
+  module Git
+    module Conflict
+      class Resolver
+        ConflictSideMissing = Class.new(StandardError)
+        ResolutionError = Class.new(StandardError)
+
+        def initialize(repository, our_commit, target_repository, their_commit)
+          @repository = repository
+          @our_commit = our_commit.rugged_commit
+          @target_repository = target_repository
+          @their_commit = their_commit.rugged_commit
+        end
+
+        def conflicts
+          @conflicts ||= begin
+            target_index = @target_repository.rugged.merge_commits(@our_commit, @their_commit)
+
+            # We don't need to do `with_repo_branch_commit` here, because the target
+            # project always fetches source refs when creating merge request diffs.
+            target_index.conflicts.map do |conflict|
+              raise ConflictSideMissing unless conflict[:theirs] && conflict[:ours]
+
+              Gitlab::Git::Conflict::File.new(
+                @target_repository,
+                @our_commit.oid,
+                conflict,
+                target_index.merge_file(conflict[:ours][:path])[:data]
+              )
+            end
+          end
+        end
+
+        def resolve_conflicts(user, files, source_branch:, target_branch:, commit_message:)
+          @repository.with_repo_branch_commit(@target_repository, target_branch) do
+            files.each do |file_params|
+              conflict_file = conflict_for_path(file_params[:old_path], file_params[:new_path])
+
+              write_resolved_file_to_index(conflict_file, file_params)
+            end
+
+            unless index.conflicts.empty?
+              missing_files = index.conflicts.map { |file| file[:ours][:path] }
+
+              raise ResolutionError, "Missing resolutions for the following files: #{missing_files.join(', ')}"
+            end
+
+            commit_params = {
+              message: commit_message,
+              parents: [@our_commit, @their_commit].map(&:oid)
+            }
+
+            @repository.commit_index(user, source_branch, index, commit_params)
+          end
+        end
+
+        def conflict_for_path(old_path, new_path)
+          conflicts.find do |conflict|
+            conflict.their_path == old_path && conflict.our_path == new_path
+          end
+        end
+
+        private
+
+        # We can only write when getting the merge index from the source
+        # project, because we will write to that project. We don't use this all
+        # the time because this fetches a ref into the source project, which
+        # isn't needed for reading.
+        def index
+          @index ||= @repository.rugged.merge_commits(@our_commit, @their_commit)
+        end
+
+        def write_resolved_file_to_index(file, params)
+          if params[:sections]
+            resolved_lines = file.resolve_lines(params[:sections])
+            new_file = resolved_lines.map { |line| line[:full_line] }.join("\n")
+
+            new_file << "\n" if file.our_blob.data.ends_with?("\n")
+          elsif params[:content]
+            new_file = file.resolve_content(params[:content])
+          end
+
+          our_path = file.our_path
+
+          index.add(path: our_path, oid: @repository.rugged.write(new_file, :blob), mode: file.our_mode)
+          index.conflict_remove(our_path)
+        end
+      end
+    end
+  end
+end