Don't try to create fork network memberships for forks of forks

changelogs/unreleased/bvl-fork-network-memberships-for-deleted-source.yml

+---
+title: Don't try to create fork network memberships for forks with a missing source
+merge_request: 15366
+author:
+type: fixed

lib/gitlab/background_migration/create_fork_network_memberships_range.rb

@@ -51,10 +51,20 @@ module Gitlab
             FROM projects
             WHERE forked_project_links.forked_from_project_id = projects.id
           )
+          AND NOT EXISTS (
+            SELECT true
+            FROM forked_project_links AS parent_links
+            WHERE parent_links.forked_to_project_id = forked_project_links.forked_from_project_id
+            AND NOT EXISTS (
+              SELECT true
+              FROM projects
+              WHERE parent_links.forked_from_project_id = projects.id
+            )
+          )
           AND forked_project_links.id BETWEEN #{start_id} AND #{end_id}
         MISSING_MEMBERS
 
-        ForkNetworkMember.count_by_sql(count_sql) > 0
+        ForkedProjectLink.count_by_sql(count_sql) > 0
       end
 
       def log(message)

spec/lib/gitlab/background_migration/create_fork_network_memberships_range_spec.rb

@@ -93,7 +93,14 @@ describe Gitlab::BackgroundMigration::CreateForkNetworkMembershipsRange, :migrat
   end
 
   it 'knows it is finished for this range' do
-    expect(migration.missing_members?(1, 7)).to be_falsy
+    expect(migration.missing_members?(1, 8)).to be_falsy
+  end
+
+  it 'does not miss members for forks of forks for which the root was deleted' do
+    forked_project_links.create(id: 9, forked_from_project_id: base1_fork1.id, forked_to_project_id: create(:project).id)
+    base1.destroy
+
+    expect(migration.missing_members?(7, 10)).to be_falsy
   end
 
   context 'with more forks' do