Prevent EE backport migrations from running if CE is not migrated

If a user upgraded to any GitLab 11.x EE version but switched
back to CE, it's possible the state of the EE tables are not
in the right state for the EE backport migration to work properly.
In particular, there were three tables that had trouble:

* epics
* geo_event_log
* vulnerability_feedback

The EE backport migration would fail while trying to add foreign key
constraints because a key didn't exist in the table. This happens
because any EE migration that add or removed columns between v11.0.0 and
v11.11.3 are not guaranteed to be applied in an CE installation. The EE
backport schema does not individually backport these migrations.

We now check if certain columns are present to determine whether
the backport migration is in the proper state. CE users are required
to upgrade to v11.11.3 EE if they ever installed EE previously before
they can go back to v12.x CE.

Tested via:

```
git checkout -f v11.0.0-ee
bundle exec rake db:reset
git checkout .; git checkout -f v11.11.3
bundle exec rake db:migrate
git checkout .; git checkout -f v12.0.0
bundle exec rake db:migrate
<failure happens>
```

changelogs/unreleased/sh-recover-ee-schema-backport-migration-failure.yml

+---
+title: Prevent EE backport migrations from running if CE is not migrated
+merge_request: 30002
+author:
+type: fixed

db/migrate/20190402150158_backport_enterprise_schema.rb

@@ -117,6 +117,8 @@ class BackportEnterpriseSchema < ActiveRecord::Migration[5.0]
   end
 
   def up
+    check_schema!
+
     create_missing_tables
 
     update_appearances
@@ -868,6 +870,52 @@ class BackportEnterpriseSchema < ActiveRecord::Migration[5.0]
     remove_column_if_exists(:geo_nodes, :internal_url)
   end
 
+  # Some users may have upgraded to EE at some point but downgraded to
+  # CE v11.11.3.  As a result, their EE tables may not be in the right
+  # state. Here we check for these such cases and attempt to guide the
+  # user into recovering from this state by upgrading to v11.11.3 EE
+  # before installing v12.0.0 CE.
+  def check_schema!
+    # The following cases will fail later when this migration attempts
+    # to add a foreign key for non-existent columns.
+    columns_to_check = [
+      [:epics, :parent_id], # Added in GitLab 11.7
+      [:geo_event_log, :cache_invalidation_event_id], # Added in GitLab 11.4
+      [:vulnerability_feedback, :merge_request_id] # Added in GitLab 11.9
+    ].freeze
+
+    columns_to_check.each do |table, column|
+      check_ee_columns!(table, column)
+    end
+  end
+
+  def check_ee_columns!(table, column)
+    return unless table_exists?(table)
+    return if column_exists?(table, column)
+
+    raise_ee_migration_error!(table, column)
+  end
+
+  def raise_ee_migration_error!(table, column)
+    message = "Your database is missing the '#{column}' column from the '#{table}' table that is present for GitLab EE."
+
+    message +=
+      if ::Gitlab.ee?
+        "\nUpgrade your GitLab instance to 11.11.3 EE first!"
+      else
+        <<~MSG
+
+          Even though it looks like you're running a CE installation, it appears
+          you may have installed GitLab EE at some point. To migrate to GitLab 12.0:
+
+          1. Install GitLab 11.11.3 EE
+          2. Install GitLab 12.0.x CE
+        MSG
+      end
+
+    raise Exception.new(message)
+  end
+
   def create_missing_tables
     create_table_if_not_exists "approval_merge_request_rule_sources", id: :bigserial do |t|
       t.bigint "approval_merge_request_rule_id", null: false

spec/migrations/backport_enterprise_schema_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require Rails.root.join('db', 'migrate', '20190402150158_backport_enterprise_schema.rb')
+
+describe BackportEnterpriseSchema, :migration, schema: 20190329085614 do
+  include MigrationsHelpers
+
+  def drop_if_exists(table)
+    active_record_base.connection.drop_table(table) if active_record_base.connection.table_exists?(table)
+  end
+
+  describe '#up' do
+    it 'creates new EE tables' do
+      migrate!
+
+      expect(active_record_base.connection.table_exists?(:epics)).to be true
+      expect(active_record_base.connection.table_exists?(:geo_nodes)).to be true
+    end
+
+    context 'missing EE columns' do
+      before do
+        drop_if_exists(:epics)
+
+        active_record_base.connection.create_table "epics" do |t|
+          t.integer :group_id, null: false, index: true
+          t.integer :author_id, null: false, index: true
+        end
+      end
+
+      after do
+        drop_if_exists(:epics)
+      end
+
+      it 'flags an error' do
+        expect { migrate! }.to raise_error(/Your database is missing.*that is present for GitLab EE/)
+      end
+    end
+  end
+end