Disable PostgreSQL statement timeout during migrations

Long-running migrations may take more than the timeout allowed by
the database. Disable the session's statement timeout to ensure
migrations don't get killed prematurely.

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.10.0 (unreleased)
   - Expose {should,force}_remove_source_branch (Ben Boeckel)
+  - Disable PostgreSQL statement timeout during migrations
   - Fix projects dropdown loading performance with a simplified api cal. !5113 (tiagonbotelho)
   - Fix commit builds API, return all builds for all pipelines for given commit. !4849
   - Replace Haml with Hamlit to make view rendering faster. !3666

lib/gitlab/database/migration_helpers.rb

@@ -25,6 +25,13 @@ module Gitlab
         add_index(table_name, column_name, options)
       end
 
+      # Long-running migrations may take more than the timeout allowed by
+      # the database. Disable the session's statement timeout to ensure
+      # migrations don't get killed prematurely. (PostgreSQL only)
+      def disable_statement_timeout
+        ActiveRecord::Base.connection.execute('SET statement_timeout TO 0') if Database.postgresql?
+      end
+
       # Updates the value of a column in batches.
       #
       # This method updates the table in batches of 5% of the total row count.
@@ -134,6 +141,7 @@ module Gitlab
         end
 
         transaction do
+          disable_statement_timeout
           add_column(table, column, type, default: nil)
 
           # Changing the default before the update ensures any newly inserted