Fix failed LDAP logins when nil user_id present

When a LDAP user signs in the for the first time and if there
is an `Identity` object with `user_id` of `nil`, new users
will not be able to be register until that entry is cleared
because of the way identities are created:

1. First, the User object is built but not saved, so it has no `id`.
2. Then, `user.identities.build(provider: 'ldapmain')` is called,
   but it does not have an associated `user_id` as a result.
3. `User#save` is called, but the `Identity` validation fails if an
   existing entry with `user_id` of `nil` already exists.

The uniqueness validation for `nil` values doesn't make any sense in
this case. We should be enforcing this at the database level with a
foreign key constraint. To work around the issue we can validate
against the user instead, which does the right thing even when
the user isn't saved yet.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56734

app/models/identity.rb

@@ -8,7 +8,7 @@ class Identity < ActiveRecord::Base
 
   validates :provider, presence: true
   validates :extern_uid, allow_blank: true, uniqueness: { scope: UniquenessScopes.scopes, case_sensitive: false }
-  validates :user_id, uniqueness: { scope: UniquenessScopes.scopes }
+  validates :user, uniqueness: { scope: UniquenessScopes.scopes }
 
   before_save :ensure_normalized_extern_uid, if: :extern_uid_changed?
   after_destroy :clear_user_synced_attributes, if: :user_synced_attributes_metadata_from_provider?

changelogs/unreleased/sh-disable-nil-user-id-identity-validation.yml

+---
+title: Fix failed LDAP logins when nil user_id present
+merge_request: 24749
+author:
+type: fixed

spec/models/identity_spec.rb

@@ -10,6 +10,40 @@ describe Identity do
     it { is_expected.to respond_to(:extern_uid) }
   end
 
+  describe 'validations' do
+    set(:user) { create(:user) }
+
+    context 'with existing user and provider' do
+      before do
+        create(:identity, provider: 'ldapmain', user_id: user.id)
+      end
+
+      it 'returns false for a duplicate entry' do
+        identity = user.identities.build(provider: 'ldapmain', user_id: user.id)
+
+        expect(identity.validate).to be_falsey
+      end
+
+      it 'returns true when a different provider is used' do
+        identity = user.identities.build(provider: 'gitlab', user_id: user.id)
+
+        expect(identity.validate).to be_truthy
+      end
+    end
+
+    context 'with newly-created user' do
+      before do
+        create(:identity, provider: 'ldapmain', user_id: nil)
+      end
+
+      it 'successfully validates even with a nil user_id' do
+        identity = user.identities.build(provider: 'ldapmain')
+
+        expect(identity.validate).to be_truthy
+      end
+    end
+  end
+
   describe '#is_ldap?' do
     let(:ldap_identity) { create(:identity, provider: 'ldapmain') }
     let(:other_identity) { create(:identity, provider: 'twitter') }