Allow nested queries

nesting.patch

commit 39203ff494701c0445cec1139dbebf981c0b5a38
Author: Eric Walker <eric.walker@holacracyone.com>
Date:   Thu Nov 16 15:10:08 2017 -0500

    Add GraphQL::Batch::Executor.setup, .teardown.

diff --git a/lib/graphql/batch.rb b/lib/graphql/batch.rb
index 89b7359..ee70f75 100644
--- a/lib/graphql/batch.rb
+++ b/lib/graphql/batch.rb
@@ -7,15 +7,13 @@ require "promise.rb"
 module GraphQL
   module Batch
     BrokenPromiseError = ::Promise::BrokenError
-    class NestedError < StandardError; end
 
     def self.batch
-      raise NestedError if GraphQL::Batch::Executor.current
       begin
-        GraphQL::Batch::Executor.current = GraphQL::Batch::Executor.new
+        GraphQL::Batch::Executor.setup
         Promise.sync(yield)
       ensure
-        GraphQL::Batch::Executor.current = nil
+        GraphQL::Batch::Executor.teardown
       end
     end
 
diff --git a/lib/graphql/batch/executor.rb b/lib/graphql/batch/executor.rb
index e9ff2ff..45db827 100644
--- a/lib/graphql/batch/executor.rb
+++ b/lib/graphql/batch/executor.rb
@@ -3,12 +3,39 @@ module GraphQL::Batch
     THREAD_KEY = :"#{name}.batched_queries"
     private_constant :THREAD_KEY
 
-    def self.current
-      Thread.current[THREAD_KEY]
-    end
+    NESTING_LEVEL_KEY = :"#{THREAD_KEY}_nesting_level"
+    private_constant :NESTING_LEVEL_KEY
+
+    class << self
+      def setup
+        increment_level!
+        Thread.current[THREAD_KEY] ||= GraphQL::Batch::Executor.new
+      end
+
+      def teardown
+        if decrement_level! < 1
+          Thread.current[THREAD_KEY] = nil
+        end
+      end
+
+      def current
+        Thread.current[THREAD_KEY]
+      end
+
+      def current=(executor)
+        Thread.current[NESTING_LEVEL_KEY] = executor ? 1 : 0
+        Thread.current[THREAD_KEY] = executor
+      end
 
-    def self.current=(executor)
-      Thread.current[THREAD_KEY] = executor
+      def increment_level!
+        level = Thread.current[NESTING_LEVEL_KEY] || 0
+        Thread.current[NESTING_LEVEL_KEY] = level + 1
+      end
+
+      def decrement_level!
+        level = Thread.current[NESTING_LEVEL_KEY] || 1
+        Thread.current[NESTING_LEVEL_KEY] = level - 1
+      end
     end
 
     attr_reader :loaders
diff --git a/lib/graphql/batch/setup.rb b/lib/graphql/batch/setup.rb
index 6ec3f50..c70468e 100644
--- a/lib/graphql/batch/setup.rb
+++ b/lib/graphql/batch/setup.rb
@@ -2,12 +2,11 @@ module GraphQL::Batch
   class Setup
     class << self
       def start_batching
-        raise NestedError if GraphQL::Batch::Executor.current
-        GraphQL::Batch::Executor.current = GraphQL::Batch::Executor.new
+        GraphQL::Batch::Executor.setup
       end
 
       def end_batching
-        GraphQL::Batch::Executor.current = nil
+        GraphQL::Batch::Executor.teardown
       end
 
       def instrument_field(schema, type, field)
diff --git a/test/batch_test.rb b/test/batch_test.rb
index 06f0db9..4e75cc0 100644
--- a/test/batch_test.rb
+++ b/test/batch_test.rb
@@ -10,9 +10,8 @@ class GraphQL::BatchTest < Minitest::Test
 
   def test_nested_batch
     GraphQL::Batch.batch do
-      assert_raises(GraphQL::Batch::NestedError) do
-        GraphQL::Batch.batch {}
-      end
+      # It does not raise
+      GraphQL::Batch.batch {}
     end
   end
 end