Don't create fork networks for root projects that are deleted

lib/gitlab/background_migration/create_fork_network_memberships_range.rb

@@ -46,6 +46,11 @@ module Gitlab
             FROM fork_network_members
             WHERE fork_network_members.project_id = forked_project_links.forked_to_project_id
           )
+          AND EXISTS (
+            SELECT true
+            FROM projects
+            WHERE forked_project_links.forked_from_project_id = projects.id
+          )
           AND forked_project_links.id BETWEEN #{start_id} AND #{end_id}
         MISSING_MEMBERS
 

lib/gitlab/background_migration/populate_fork_networks_range.rb

@@ -20,6 +20,11 @@ module Gitlab
             FROM fork_networks
             WHERE forked_project_links.forked_from_project_id = fork_networks.root_project_id
           )
+          AND EXISTS (
+            SELECT true
+            FROM projects
+            WHERE projects.id = forked_project_links.forked_from_project_id
+          )
           AND forked_project_links.id BETWEEN #{start_id} AND #{end_id}
         INSERT_NETWORKS
 

spec/lib/gitlab/background_migration/populate_fork_networks_range_spec.rb

@@ -62,6 +62,14 @@ describe Gitlab::BackgroundMigration::PopulateForkNetworksRange, :migration, sch
     expect(base2_membership).not_to be_nil
   end
 
+  it 'skips links that had their source project deleted' do
+    forked_project_links.create(id: 6, forked_from_project_id: 99999, forked_to_project_id: create(:project).id)
+
+    migration.perform(5, 8)
+
+    expect(fork_networks.find_by(root_project_id: 99999)).to be_nil
+  end
+
   it 'schedules a job for inserting memberships for forks-of-forks' do
     delay = Gitlab::BackgroundMigration::CreateForkNetworkMembershipsRange::RESCHEDULE_DELAY