Add rake task `db:obsolete_ignored_columns`

Show a list of obsolete `ignored_columns`

2 files changed, 59 insertions, 0 deletions

diff --git a/lib/gitlab/database/obsolete_ignored_columns.rb b/lib/gitlab/database/obsolete_ignored_columns.rb
new file mode 100644
index 00000000000..6266b6a4b65
--- /dev/null
+++ b/lib/gitlab/database/obsolete_ignored_columns.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    # Checks which `ignored_columns` can be safely removed by scanning
+    # the current schema for all `ApplicationRecord` descendants.
+    class ObsoleteIgnoredColumns
+      def initialize(base = ApplicationRecord)
+        @base = base
+      end
+
+      def execute
+        @base.descendants.map do |klass|
+          next if klass.abstract_class?
+
+          safe_to_remove = ignored_columns_safe_to_remove_for(klass)
+          next if safe_to_remove.empty?
+
+          [klass.name, safe_to_remove]
+        end.compact.sort_by(&:first)
+      end
+
+      private
+
+      def ignored_columns_safe_to_remove_for(klass)
+        ignored = klass.ignored_columns.map(&:to_s)
+
+        return [] if ignored.empty?
+
+        schema = klass.connection.schema_cache.columns_hash(klass.table_name)
+        existing = schema.values.map(&:name)
+
+        used = ignored & existing
+        ignored - used
+      end
+    end
+  end
+end
diff --git a/lib/tasks/db_obsolete_ignored_columns.rake b/lib/tasks/db_obsolete_ignored_columns.rake
new file mode 100644
index 00000000000..184e407f28c
--- /dev/null
+++ b/lib/tasks/db_obsolete_ignored_columns.rake
@@ -0,0 +1,21 @@
+desc 'Show a list of obsolete `ignored_columns`'
+task 'db:obsolete_ignored_columns' => :environment do
+  list = Gitlab::Database::ObsoleteIgnoredColumns.new.execute
+
+  if list.empty?
+    puts 'No obsolete `ignored_columns` found.'
+  else
+    puts 'The following `ignored_columns` are obsolete and can be removed:'
+
+    list.each do |name, ignored_columns|
+      puts "- #{name}: #{ignored_columns.join(', ')}"
+    end
+
+    puts <<~TEXT
+
+      WARNING: Removing columns is tricky because running GitLab processes may still be using the columns.
+
+      See also https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns
+    TEXT
+  end
+end