Add test for a race-condition

lib/batch_loader.rb

@@ -18,8 +18,9 @@ def self.for(item)
     new(item: item)
   end
 
-  def initialize(item:)
+  def initialize(item:, executor_proxy: nil)
     @item = item
+    @__executor_proxy = executor_proxy
   end
 
   def batch(default_value: nil, cache: true, &batch_block)
@@ -76,7 +77,7 @@ def __ensure_batched
     return if __executor_proxy.value_loaded?(item: @item)
 
     items = __executor_proxy.list_items
-    loader =  -> (item, value = (no_value = true; nil), &block) {
+    loader =  -> (item, value = (no_value = true; nil), &block) do
       if no_value && !block
         raise ArgumentError, "Please pass a value or a block"
       elsif block && !no_value
@@ -85,7 +86,7 @@ def __ensure_batched
 
       next_value = block ? block.call(__executor_proxy.loaded_value(item: item)) : value
       __executor_proxy.load(item: item, value: next_value)
-    }
+    end
 
     @batch_block.call(items, loader)
     items.each do |item|

spec/batch_loader_spec.rb

@@ -99,6 +99,20 @@
       expect(lazy).to eq(2)
     end
 
+    it 'is thread-safe' do
+      batch_block = Proc.new do |ids, loader|
+        ids.each do |id|
+          thread = Thread.new { loader.call(id) { |value| value << id } }
+          loader.call(id) { |value| value << id + 1 }
+          thread.join
+        end
+      end
+      slow_executor_proxy = SlowExecutorProxy.new([], &batch_block)
+      lazy = BatchLoader.new(item: 1, executor_proxy: slow_executor_proxy).batch(default_value: [], &batch_block)
+
+      expect(lazy).to eq([1, 2])
+    end
+
     it 'supports alternative default values' do
       lazy = BatchLoader.for(1).batch(default_value: 123) do |nums, loader|
         # No-op, so default is returned

spec/spec_helper.rb

@@ -11,6 +11,14 @@
 require_relative "./fixtures/models"
 require_relative "./fixtures/graphql_schema"
 
+class SlowExecutorProxy < BatchLoader::ExecutorProxy
+  def value_loaded?(item:)
+    result = loaded.key?(item)
+    sleep 0.5
+    result
+  end
+end
+
 RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
   config.example_status_persistence_file_path = ".rspec_status"