Treewalker: Yield for each array element itself, not just children

Due to an oversight in how the treewalker was implemented, we would
never pass the direct array elements to the walk! block, only any
children the elements have.

In practice this is usually not an issue, since the array elements
are typically the generic PgQuery::Node object, and we would call the
walk! function successfully on its children. However, working with those
node objects directly is necessary when wanting to replace the node.

This may be a backwards incompatible change if one relies on the
parent_field to always be a symbol (it can now be an integer as well).

lib/pg_query/treewalker.rb

@@ -20,13 +20,18 @@ def treewalker!(tree) # rubocop:disable Metrics/CyclomaticComplexity
             node = parent_node[parent_field.to_s]
             next if node.nil?
             location = parent_location + [parent_field]
-
             yield(parent_node, parent_field, node, location) if node.is_a?(Google::Protobuf::MessageExts) || node.is_a?(Google::Protobuf::RepeatedField)
 
             nodes << [node, location] unless node.nil?
           end
         when Google::Protobuf::RepeatedField
-          nodes += parent_node.map.with_index { |e, idx| [e, parent_location + [idx]] }
+          parent_node.each_with_index do |node, parent_field|
+            next if node.nil?
+            location = parent_location + [parent_field]
+            yield(parent_node, parent_field, node, location) if node.is_a?(Google::Protobuf::MessageExts) || node.is_a?(Google::Protobuf::RepeatedField)
+
+            nodes << [node, location] unless node.nil?
+          end
         end
 
         break if nodes.empty?

spec/lib/treewalker_spec.rb

@@ -0,0 +1,36 @@
+require 'spec_helper'
+
+describe PgQuery, '.treewalker' do
+  it 'walks nodes contained in repeated fields' do
+    locations = []
+    described_class.parse("SELECT to_timestamp($1)").walk! do |_, _, _, location|
+      locations << location
+    end
+    expect(locations).to match_array [
+      [:stmts],
+      [:stmts, 0],
+      [:stmts, 0, :stmt],
+      [:stmts, 0, :stmt, :select_stmt],
+      [:stmts, 0, :stmt, :select_stmt, :distinct_clause],
+      [:stmts, 0, :stmt, :select_stmt, :target_list],
+      [:stmts, 0, :stmt, :select_stmt, :from_clause],
+      [:stmts, 0, :stmt, :select_stmt, :group_clause],
+      [:stmts, 0, :stmt, :select_stmt, :window_clause],
+      [:stmts, 0, :stmt, :select_stmt, :values_lists],
+      [:stmts, 0, :stmt, :select_stmt, :sort_clause],
+      [:stmts, 0, :stmt, :select_stmt, :locking_clause],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :indirection],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :funcname],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :args],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :agg_order],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :funcname, 0],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :args, 0],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :funcname, 0, :string],
+      [:stmts, 0, :stmt, :select_stmt, :target_list, 0, :res_target, :val, :func_call, :args, 0, :param_ref]
+    ]
+  end
+end