1 files changed, 94 insertions, 0 deletions
diff --git a/rubocop/cop/migration/safer_boolean_column.rb b/rubocop/cop/migration/safer_boolean_column.rb
new file mode 100644
index 00000000000..0335c25d85d
--- /dev/null
+++ b/rubocop/cop/migration/safer_boolean_column.rb
@@ -0,0 +1,94 @@
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # This cop requires a default value and disallows nulls for boolean
+      # columns on small tables.
+      #
+      # In general, this prevents 3-state-booleans.
+      # https://robots.thoughtbot.com/avoid-the-threestate-boolean-problem
+      #
+      # In particular, for the `application_settings` table, this ensures that
+      # upgraded installations get a proper default for the new boolean setting.
+      # A developer might otherwise mistakenly assume that a value in
+      # `ApplicationSetting.defaults` is sufficient.
+      #
+      # See https://gitlab.com/gitlab-org/gitlab-ee/issues/2750 for more
+      # information.
+      class SaferBooleanColumn < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        DEFAULT_OFFENSE = 'Boolean columns on the `%s` table should have a default. You may wish to use `add_column_with_default`.'.freeze
+        NULL_OFFENSE = 'Boolean columns on the `%s` table should disallow nulls.'.freeze
+        DEFAULT_AND_NULL_OFFENSE = 'Boolean columns on the `%s` table should have a default and should disallow nulls. You may wish to use `add_column_with_default`.'.freeze
+
+        SMALL_TABLES = %i[
+          application_settings
+        ].freeze
+
+        def_node_matcher :add_column?, <<~PATTERN
+          (send nil :add_column $...)
+        PATTERN
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          matched = add_column?(node)
+
+          return unless matched
+
+          table, _, type = matched.to_a.take(3).map(&:children).map(&:first)
+          opts = matched[3]
+
+          return unless SMALL_TABLES.include?(table) && type == :boolean
+
+          no_default = no_default?(opts)
+          nulls_allowed = nulls_allowed?(opts)
+
+          offense = if no_default && nulls_allowed
+                      DEFAULT_AND_NULL_OFFENSE
+                    elsif no_default
+                      DEFAULT_OFFENSE
+                    elsif nulls_allowed
+                      NULL_OFFENSE
+                    end
+
+          add_offense(node, :expression, format(offense, table)) if offense
+        end
+
+        def no_default?(opts)
+          return true unless opts
+
+          each_hash_node_pair(opts) do |key, value|
+            return value == 'nil' if key == :default
+          end
+        end
+
+        def nulls_allowed?(opts)
+          return true unless opts
+
+          each_hash_node_pair(opts) do |key, value|
+            return value != 'false' if key == :null
+          end
+        end
+
+        def each_hash_node_pair(hash_node, &block)
+          hash_node.each_node(:pair) do |pair|
+            key = hash_pair_key(pair)
+            value = hash_pair_value(pair)
+            yield(key, value)
+          end
+        end
+
+        def hash_pair_key(pair)
+          pair.children[0].children[0]
+        end
+
+        def hash_pair_value(pair)
+          pair.children[1].source
+        end
+      end
+    end
+  end
+end