Add single threaded mode back in

This is a slightly different implementation due to the use of promises
for both async and sync modes. The difference is whether the thread pool
is set to synchronous or not.

lib/graphiti/scope.rb

@@ -5,8 +5,17 @@ class Scope
 
     # TODO: this could be a Concurrent::Promises.delay
     GLOBAL_THREAD_POOL_EXECUTOR = Concurrent::Delay.new do
-      # concurrency = Graphiti.config.concurrency_max_threads || 4
-      Concurrent::ThreadPoolExecutor.new(max_threads: 0, synchronous: true, fallback_policy: :caller_runs)
+      if Graphiti.config.concurrency
+        concurrency = Graphiti.config.concurrency_max_threads || 4
+        Concurrent::ThreadPoolExecutor.new(
+          min_threads: 0,
+          max_threads: concurrency,
+          max_queue: concurrency * 4,
+          fallback_policy: :caller_runs
+        )
+      else
+        Concurrent::ThreadPoolExecutor.new(max_threads: 0, synchronous: true, fallback_policy: :caller_runs)
+      end
     end
     private_constant :GLOBAL_THREAD_POOL_EXECUTOR
 

spec/scope_spec.rb

@@ -132,7 +132,7 @@
             stub_const(
               "Graphiti::Scope::GLOBAL_THREAD_POOL_EXECUTOR",
               Concurrent::Delay.new do
-                Concurrent::ThreadPoolExecutor.new(max_threads: 1, fallback_policy: :caller_runs)
+                Concurrent::ThreadPoolExecutor.new(min_threads: 1, max_threads: 1, fallback_policy: :caller_runs)
               end
             )
 

spec/sideloading_spec.rb

@@ -632,7 +632,7 @@ def type
       stub_const(
         "Graphiti::Scope::GLOBAL_THREAD_POOL_EXECUTOR",
         Concurrent::Delay.new do
-          Concurrent::ThreadPoolExecutor.new(max_threads: 0, synchronous: true, fallback_policy: :caller_runs)
+          Concurrent::ThreadPoolExecutor.new(min_threads: 1, max_threads: 1, fallback_policy: :caller_runs)
         end
       )