Fetch and map refs/pull to refs/merge-requests in the GH import

update MR diff

app/models/concerns/repository_mirroring.rb

 module RepositoryMirroring
-  def set_remote_as_mirror(name)
-    config = raw_repository.rugged.config
+  IMPORT_REFS = %w[+refs/pull/*/head:refs/merge-requests/*/head +refs/heads/*:refs/heads/* +refs/tags/*:refs/tags/*].freeze
+  def set_remote_as_mirror(name)
     # This is used to define repository as equivalent as "git clone --mirror"
     config["remote.#{name}.fetch"] = 'refs/*:refs/*'
     config["remote.#{name}.mirror"] = true
     config["remote.#{name}.prune"] = true
   end
+  def set_import_remote_as_mirror(name)
+    # Add first fetch with Rugged so it does not create its own.
+    config["remote.#{name}.fetch"] = IMPORT_REFS.first
+
+    IMPORT_REFS.drop(1).each do |ref|
+      run_git(%W[config --add remote.#{name}.fetch #{ref}])
+    end
+
+    config["remote.#{name}.mirror"] = true
+    config["remote.#{name}.prune"] = true
+  rescue Rugged::ConfigError
+    # Ignore multivar errors when the config already exist
+    # TODO: refactor/fix this
+  end
+
   def fetch_mirror(remote, url)
     add_remote(remote, url)
     set_remote_as_mirror(remote)
     fetch_remote(remote, forced: true)
     remove_remote(remote)
   end
+
+  def config
+    raw_repository.rugged.config
+  end
 end

app/models/merge_request_diff.rb

@@ -55,7 +55,8 @@ class MergeRequestDiff < ActiveRecord::Base
   end
   def ensure_commit_shas
-    merge_request.fetch_ref
+    merge_request.fetch_ref unless merge_request.source_branch_head
+
     self.start_commit_sha ||= merge_request.target_branch_sha
     self.head_commit_sha  ||= merge_request.source_branch_sha
     self.base_commit_sha  ||= find_base_sha

lib/github/import.rb

@@ -56,7 +56,7 @@ module Github
       begin
         project.ensure_repository
         project.repository.add_remote('github', repo_url)
-        project.repository.set_remote_as_mirror('github')
+        project.repository.set_import_remote_as_mirror('github')
         project.repository.fetch_remote('github', forced: true)
       rescue Gitlab::Git::Repository::NoRepository, Gitlab::Shell::Error => e
         error(:project, repo_url, e.message)
@@ -140,7 +140,7 @@ module Github
         response.body.each do |raw|
           pull_request  = Github::Representation::PullRequest.new(raw, options.merge(project: project))
           merge_request = MergeRequest.find_or_initialize_by(iid: pull_request.iid, source_project_id: project.id)
-          next unless merge_request.new_record? && pull_request.valid?
+          next unless merge_request.new_record? && pull_request.valid? && merge_request.source_branch_head
           begin
             pull_request.restore_branches!
@@ -173,8 +173,6 @@ module Github
             fetch_comments(merge_request, :review_comment, review_comments_url, LegacyDiffNote)
           rescue => e
             error(:pull_request, pull_request.url, e.message)
-          ensure
-            pull_request.remove_restored_branches!
           end
         end

lib/github/representation/pull_request.rb

@@ -10,7 +10,9 @@ module Github
       def source_branch_name
         @source_branch_name ||=
-          if cross_project? || !source_branch_exists?
+          if opened? && project.repository.branch_exists?(source_branch.ref)
+            source_branch.ref
+          elsif cross_project? && opened?
             source_branch_name_prefixed
           else
             source_branch_ref
@@ -20,7 +22,7 @@ module Github
       def source_branch_exists?
         return @source_branch_exists if defined?(@source_branch_exists)
-        @source_branch_exists = !cross_project? && source_branch.exists?
+        @source_branch_exists = project.repository.branch_exists?(source_branch_name)
       end
       def target_project
@@ -55,13 +57,6 @@ module Github
         restore_target_branch!
       end
-      def remove_restored_branches!
-        return if opened?
-
-        remove_source_branch!
-        remove_target_branch!
-      end
-
       private
       def project
@@ -72,8 +67,12 @@ module Github
         @source_branch ||= Representation::Branch.new(raw['head'], repository: project.repository)
       end
+      def source_branch_ref
+        "refs/merge-requests/#{iid}/head"
+      end
+
       def source_branch_name_prefixed
-        "gh-#{target_branch_short_sha}/#{iid}/#{source_branch_user}/#{source_branch_ref}"
+        "gh-#{target_branch_short_sha}/#{iid}/#{source_branch_user}/#{source_branch.ref}"
       end
       def target_branch
@@ -101,20 +100,6 @@ module Github
         target_branch.restore!(target_branch_name)
       end
-
-      def remove_source_branch!
-        # We should remove the source/target branches only if they were
-        # restored. Otherwise, we'll remove branches like 'master' that
-        # target_branch_exists? returns true. In other words, we need
-        # to clean up only the restored branches that (source|target)_branch_exists?
-        # returns false for the first time it has been called, because of
-        # this that is important to memoize these values.
-        source_branch.remove!(source_branch_name) unless source_branch_exists?
-      end
-
-      def remove_target_branch!
-        target_branch.remove!(target_branch_name) unless target_branch_exists?
-      end
     end
   end
 end