Clean up LFS objects when creating fork relation

A forked project stores its LFS objects in the `forked_from_project`.
So the LFS objects become inaccessible, and therefore delete them from
the database so they'll get cleaned up.

To be refactored when implementing
https://gitlab.com/gitlab-org/gitlab-ce/issues/39769

app/services/projects/fork_service.rb

@@ -17,6 +17,14 @@ module Projects
       link_fork_network(fork_to_project)
+      # A forked project stores its LFS objects in the `forked_from_project`.
+      # So the LFS objects become inaccessible, and therefore delete them from
+      # the database so they'll get cleaned up.
+      #
+      # TODO: refactor this to get the correct lfs objects when implementing
+      #       https://gitlab.com/gitlab-org/gitlab-ce/issues/39769
+      fork_to_project.lfs_objects_projects.delete_all
+
       fork_to_project
     end

doc/api/projects.md

@@ -1419,6 +1419,11 @@ Allows modification of the forked relationship between existing projects. Availa
 ### Create a forked from/to relation between existing projects
+CAUTION: **Warning:**
+This will destroy the LFS objects stored in the fork.
+So to retain the LFS objects, make sure you've pulled them **before** creating the fork relation,
+and push them again **after** creating the fork relation.
+
 ```
 POST /projects/:id/fork/:forked_from_id
 ```

spec/services/projects/fork_service_spec.rb

@@ -264,6 +264,14 @@ describe Projects::ForkService do
         expect(fork_from_project.forks_count).to eq(1)
       end
+
+      it 'leaves no LFS objects dangling' do
+        create(:lfs_objects_project, project: fork_to_project)
+
+        expect { subject.execute(fork_to_project) }
+          .to change { fork_to_project.lfs_objects_projects.count }
+          .to(0)
+      end
     end
   end
 end