Allow nested execution

nested.patch

commit ca10b17221be21f6ec71138da25059a5611e3eb9
Author: Eric Walker <eric.walker@holacracyone.com>
Date:   Tue Nov 14 13:54:08 2017 -0500

    Allow nested execution.

diff --git a/lib/graphql/batch.rb b/lib/graphql/batch.rb
index 89b7359..2a3118d 100644
--- a/lib/graphql/batch.rb
+++ b/lib/graphql/batch.rb
@@ -10,12 +10,11 @@ module GraphQL
     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.push GraphQL::Batch::Executor.new
         Promise.sync(yield)
       ensure
-        GraphQL::Batch::Executor.current = nil
+        GraphQL::Batch::Executor.pop
       end
     end
 
diff --git a/lib/graphql/batch/executor.rb b/lib/graphql/batch/executor.rb
index e9ff2ff..d426130 100644
--- a/lib/graphql/batch/executor.rb
+++ b/lib/graphql/batch/executor.rb
@@ -4,11 +4,23 @@ module GraphQL::Batch
     private_constant :THREAD_KEY
 
     def self.current
-      Thread.current[THREAD_KEY]
+      stack.last
     end
 
     def self.current=(executor)
-      Thread.current[THREAD_KEY] = executor
+      push executor
+    end
+
+    def self.pop
+      stack.pop
+    end
+
+    def self.push(executor)
+      stack.push executor
+    end
+
+    def self.stack
+      Thread.current[THREAD_KEY] ||= []
     end
 
     attr_reader :loaders
diff --git a/lib/graphql/batch/setup.rb b/lib/graphql/batch/setup.rb
index 6ec3f50..1c7e269 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.push GraphQL::Batch::Executor.new
       end
 
       def end_batching
-        GraphQL::Batch::Executor.current = nil
+        GraphQL::Batch::Executor.pop
       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