Spec forking a project after source moved shard

This specs forking a project that was already the source of a pool
repository after the source project was moved to a different shard.

In this case the old pool repository will continue to exist, containing
all the deduplicated objects from the forks on the first shard. But
we'll create a new PoolRepository for the shart the source project was
moved to.

Then, new forks will be deduplicated using the new PoolRepository on
the new shard.

changelogs/unreleased/bvl-allow-moving-forked-projects-across-shards.yml

+---
+title: Fix forking a deduplicated project after it was moved to a different shard
+merge_request: 21339
+author:
+type: fixed

db/migrate/20191206122926_update_index_for_pool_repositories.rb

@@ -15,7 +15,10 @@ class UpdateIndexForPoolRepositories < ActiveRecord::Migration[5.2]
   end
   def down
-    remove_concurrent_index :pool_repositories, [:source_project_id, :shard_id], unique: true
+    # Not adding this index as a unique one, since while the new index existed
+    # we could have created multiple pool repositories for a project. In that
+    # case this rollback would fail.
     add_concurrent_index :pool_repositories, :source_project_id
+    remove_concurrent_index :pool_repositories, [:source_project_id, :shard_id], unique: true
   end
 end

ee/spec/services/projects/fork_service_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+# This spec lives in `ee/` since moving shards is an EE-only feature.
+describe Projects::ForkService do
+  include ProjectForksHelper
+
+  context 'when a project is already forked' do
+    it 'creates a new poolresository after the project is moved to a new shard' do
+      project = create(:project, :public, :repository)
+      fork_before_move = fork_project(project)
+
+      # Stub everything required to move a project to a Gitaly shard that does not exist
+      allow(Gitlab.config.repositories.storages).to receive(:keys).and_return(%w(default test_second_storage))
+      allow_any_instance_of(Gitlab::Git::Repository).to receive(:fetch_repository_as_mirror).and_return(true)
+
+      Projects::UpdateRepositoryStorageService.new(project).execute('test_second_storage')
+      fork_after_move = fork_project(project)
+      pool_repository_before_move = PoolRepository.joins(:shard)
+                                      .where(source_project: project, shards: { name: 'default' }).first
+      pool_repository_after_move = PoolRepository.joins(:shard)
+                                     .where(source_project: project, shards: { name: 'test_second_storage' }).first
+
+      expect(fork_before_move.pool_repository).to eq(pool_repository_before_move)
+      expect(fork_after_move.pool_repository).to eq(pool_repository_after_move)
+    end
+  end
+end