Fix GitHub importer for PRs of deleted forked repositories

lib/gitlab/github_import/branch_formatter.rb

@@ -11,6 +11,10 @@ module Gitlab
         sha.present? && ref.present?
       end
 
+      def user
+        raw_data.user.login
+      end
+
       private
 
       def branch_exists?

lib/gitlab/github_import/pull_request_formatter.rb

 module Gitlab
   module GithubImport
     class PullRequestFormatter < IssuableFormatter
-      delegate :exists?, :project, :ref, :repo, :sha, to: :source_branch, prefix: true
-      delegate :exists?, :project, :ref, :repo, :sha, to: :target_branch, prefix: true
+      delegate :user, :exists?, :project, :ref, :repo, :sha, to: :source_branch, prefix: true
+      delegate :user, :exists?, :project, :ref, :repo, :sha, to: :target_branch, prefix: true
 
       def attributes
         {
@@ -39,13 +39,23 @@ module Gitlab
       def source_branch_name
         @source_branch_name ||= begin
           if cross_project?
+            if source_branch_repo
               "pull/#{number}/#{source_branch_repo.full_name}/#{source_branch_ref}"
+            else
+              "pull/#{number}/#{source_branch_user}/#{source_branch_ref}"
+            end
           else
             source_branch_exists? ? source_branch_ref : "pull/#{number}/#{source_branch_ref}"
           end
         end
       end
 
+      def source_branch_exists?
+        return false if cross_project?
+
+        source_branch.exists?
+      end
+
       def target_branch
         @target_branch ||= BranchFormatter.new(project, raw_data.base)
       end
@@ -57,7 +67,9 @@ module Gitlab
       end
 
       def cross_project?
-        source_branch.repo.id != target_branch.repo.id
+        return true if source_branch_repo.nil?
+
+        source_branch_repo.id != target_branch_repo.id
       end
 
       def opened?

spec/lib/gitlab/github_import/pull_request_formatter_spec.rb

@@ -13,6 +13,7 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
   let(:target_branch) { double(ref: 'master', repo: target_repo, sha: target_sha) }
   let(:removed_branch) { double(ref: 'removed-branch', repo: source_repo, sha: '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b') }
   let(:forked_branch) { double(ref: 'master', repo: forked_source_repo, sha: '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b') }
+  let(:branch_deleted_repo) { double(ref: 'master', repo: nil, sha: '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b', user: octocat) }
   let(:octocat) { double(id: 123456, login: 'octocat', email: 'octocat@example.com') }
   let(:created_at) { DateTime.strptime('2011-01-26T19:01:12Z') }
   let(:updated_at) { DateTime.strptime('2011-01-27T19:01:12Z') }
@@ -215,6 +216,14 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
         expect(pull_request.source_branch_name).to eq 'pull/1347/company/otherproject/master'
       end
     end
+
+    context 'when source branch is from a deleted fork' do
+      let(:raw_data) { double(base_data.merge(head: branch_deleted_repo)) }
+
+      it 'prefixes branch name with pull request number and user login to avoid collision' do
+        expect(pull_request.source_branch_name).to eq 'pull/1347/octocat/master'
+      end
+    end
   end
 
   shared_examples 'Gitlab::GithubImport::PullRequestFormatter#target_branch_name' do
@@ -290,6 +299,14 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
       end
     end
 
+    context 'when source repository does not exist anymore' do
+      let(:raw_data) { double(base_data.merge(head: branch_deleted_repo)) }
+
+      it 'returns true' do
+        expect(pull_request.cross_project?).to eq true
+      end
+    end
+
     context 'when source and target repositories are the same' do
       let(:raw_data) { double(base_data.merge(head: source_branch)) }
 
@@ -299,6 +316,14 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
     end
   end
 
+  describe '#source_branch_exists?' do
+    let(:raw_data) { double(base_data.merge(head: forked_branch)) }
+
+    it 'returns false when is a cross_project' do
+      expect(pull_request.source_branch_exists?).to eq false
+    end
+  end
+
   describe '#url' do
     let(:raw_data) { double(base_data) }