Require loaders to be used within a GraphQL::Batch.batch block (#36)

This is needed to avoid global state leaking between requests/tests.

README.md

@@ -109,13 +109,19 @@ end
 
 ## Unit Testing
 
-Promise#sync can be used to wait for a promise to be resolved and return its result. This can be useful for debugging and unit testing loaders.
+Your loaders can be tested outside of a GraphQL query by doing the
+batch loads in a block passed to GraphQL::Batch.batch.  That method
+will set up thread-local state to store the loaders, batch load any
+promise returned from the block then clear the thread-local state
+to avoid leaking state between tests.
 
 ```ruby
   def test_single_query
     product = products(:snowboard)
-    query = RecordLoader.for(Product).load(args["id"]).then(&:title)
-    assert_equal product.title, query.sync
+    title = GraphQL::Batch.batch do
+      RecordLoader.for(Product).load(product.id).then(&:title)
+    end
+    assert_equal product.title, title
   end
 ```
 

lib/graphql/batch.rb

@@ -4,6 +4,17 @@
 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
+        Promise.sync(yield)
+      ensure
+        GraphQL::Batch::Executor.current = nil
+      end
+    end
   end
 end
 

lib/graphql/batch/execution_strategy.rb

@@ -1,15 +1,16 @@
 module GraphQL::Batch
   class ExecutionStrategy < GraphQL::Query::SerialExecution
     def execute(_, _, query)
-      deep_sync(super)
+      GraphQL::Batch.batch do
+        as_promise_unless_resolved(super)
+      end
     rescue GraphQL::InvalidNullError => err
       err.parent_error? || query.context.errors.push(err)
       nil
-    ensure
-      GraphQL::Batch::Executor.current.clear
     end
 
-    def deep_sync(result)
+    # Needed for MutationExecutionStrategy
+    def deep_sync(result) #:nodoc:
       Promise.sync(as_promise_unless_resolved(result))
     end
 

lib/graphql/batch/executor.rb

@@ -4,7 +4,11 @@ class Executor
     private_constant :THREAD_KEY
 
     def self.current
-      Thread.current[THREAD_KEY] ||= new
+      Thread.current[THREAD_KEY]
+    end
+
+    def self.current=(executor)
+      Thread.current[THREAD_KEY] = executor
     end
 
     attr_reader :loaders