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

In case the root project of a Fork-of-fork is deleted, the ForkNetwork
and the membership for that fork network is never created. In this
case we shouldn't try to create the membership, since the parent
membership will never be created.

This means that these fork networks will be lost.

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