Fix old migration script not being able to run if it fails midway (#15361)

* Fix old migration script not being able to run if it fails midway

Improve the robustness of a migration script likely to fail because of database
corruption so it can run again once database corruptions are fixed.

* Display a specific error message in case of index corruption

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>
Co-authored-by: Claire <claire.github-309c@sitedethib.com>

db/migrate/20200620164023_add_fixed_lowercase_index_to_accounts.rb

@@ -1,10 +1,30 @@
 class AddFixedLowercaseIndexToAccounts < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
+  class CorruptionError < StandardError
+    def cause
+      nil
+    end
+
+    def backtrace
+      []
+    end
+  end
+
   def up
-    rename_index :accounts, 'index_accounts_on_username_and_domain_lower', 'old_index_accounts_on_username_and_domain_lower' unless index_name_exists?(:accounts, 'old_index_accounts_on_username_and_domain_lower')
-    add_index :accounts, "lower (username), COALESCE(lower(domain), '')", name: 'index_accounts_on_username_and_domain_lower', unique: true, algorithm: :concurrently
-    remove_index :accounts, name: 'old_index_accounts_on_username_and_domain_lower'
+    if index_name_exists?(:accounts, 'old_index_accounts_on_username_and_domain_lower') && index_name_exists?(:accounts, 'index_accounts_on_username_and_domain_lower')
+      remove_index :accounts, name: 'index_accounts_on_username_and_domain_lower'
+    elsif index_name_exists?(:accounts, 'index_accounts_on_username_and_domain_lower')
+      rename_index :accounts, 'index_accounts_on_username_and_domain_lower', 'old_index_accounts_on_username_and_domain_lower'
+    end
+
+    begin
+      add_index :accounts, "lower (username), COALESCE(lower(domain), '')", name: 'index_accounts_on_username_and_domain_lower', unique: true, algorithm: :concurrently
+    rescue ActiveRecord::RecordNotUnique
+      raise CorruptionError, 'Migration failed because of index corruption, see https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/#fixing'
+    end
+
+    remove_index :accounts, name: 'old_index_accounts_on_username_and_domain_lower' if index_name_exists?(:accounts, 'old_index_accounts_on_username_and_domain_lower')
   end
 
   def down