Fix migration script not being able to run if it fails midway (#16312)

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

* Fix old migration script

* Fix old migration script

* Refactor CorruptionError

db/migrate/20190726175042_add_case_insensitive_index_to_tags.rb

@@ -15,7 +15,13 @@ class AddCaseInsensitiveIndexToTags < ActiveRecord::Migration[5.2]
       Tag.where(id: redundant_tag_ids).in_batches.delete_all
     end
 
+    begin
       safety_assured { execute 'CREATE UNIQUE INDEX CONCURRENTLY index_tags_on_name_lower ON tags (lower(name))' }
+    rescue ActiveRecord::StatementInvalid
+      remove_index :tags, name: 'index_tags_on_name_lower'
+      raise
+    end
+
     remove_index :tags, name: 'index_tags_on_name'
     remove_index :tags, name: 'hashtag_search_index'
   end

db/migrate/20200620164023_add_fixed_lowercase_index_to_accounts.rb

@@ -1,16 +1,10 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
 class AddFixedLowercaseIndexToAccounts < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
   disable_ddl_transaction!
 
-  class CorruptionError < StandardError
-    def cause
-      nil
-    end
-
-    def backtrace
-      []
-    end
-  end
-
   def up
     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'
@@ -21,7 +15,8 @@ class AddFixedLowercaseIndexToAccounts < ActiveRecord::Migration[5.2]
     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'
+      remove_index :accounts, name: 'index_accounts_on_username_and_domain_lower'
+      raise CorruptionError
     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')

lib/mastodon/migration_helpers.rb

@@ -41,6 +41,20 @@
 
 module Mastodon
   module MigrationHelpers
+    class CorruptionError < StandardError
+      def initialize(message = nil)
+        super(message.presence || 'Migration failed because of index corruption, see https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/#fixing')
+      end
+
+      def cause
+        nil
+      end
+
+      def backtrace
+        []
+      end
+    end
+
     # Stub for Database.postgresql? from GitLab
     def self.postgresql?
       ActiveRecord::Base.configurations[Rails.env]['adapter'].casecmp('postgresql').zero?