Implement final review comments from @DouweM and @rymai

- Have `Uniquify` take a block instead of a Proc/function. This is more
  idiomatic than passing around a function in Ruby.

- Block a user before moving their issues to the ghost user. This avoids a data
  race where an issue is created after the issues are migrated to the ghost user,
  and before the destroy takes place.

- No need to migrate issues (to the ghost user) in a transaction, because
  we're using `update_all`

- Other minor changes

1 files changed, 4 insertions, 0 deletions

diff --git a/spec/services/users/destroy_spec.rb b/spec/services/users/destroy_spec.rb
index 4f0834064e5..922e82445d0 100644
--- a/spec/services/users/destroy_spec.rb
+++ b/spec/services/users/destroy_spec.rb
@@ -47,6 +47,10 @@ describe Users::DestroyService, services: true do
 
           expect(migrated_issue.author).to eq(User.ghost)
         end
+
+        it 'blocks the user before migrating issues to the "Ghost User' do
+          expect(user).to be_blocked
+        end
       end
 
       context "for an issue the user was assigned to" do