enable automatic parallel manager detection

This enables the use of `#in_parallel` without manually passing in a
parallel manager instance.

If the adapter you're using has a parallel manager, it will work as
expected. If it doesn't, it'll work as normal sequential requests.

lib/faraday/adapter.rb

@@ -1,5 +1,16 @@
 module Faraday
   class Adapter < Middleware
+    class << self
+      attr_accessor :supports_parallel_requests
+      alias supports_parallel_requests? supports_parallel_requests
+
+      # valid parallel managers should respond to #run with no parameters.
+      # otherwise, return a short wrapper around it.
+      def setup_parallel_manager(options = {})
+        nil
+      end
+    end
+
     CONTENT_LENGTH = 'Content-Length'.freeze
 
     extend AutoloadHelper

lib/faraday/adapter/em_synchrony.rb

@@ -10,6 +10,8 @@ module Faraday
         require 'fiber'
       end
 
+      self.supports_parallel_requests = true
+
       def self.setup_parallel_manager(options = {})
         ParallelManager.new
       end

lib/faraday/connection.rb

@@ -11,8 +11,8 @@ module Faraday
     METHODS = Set.new [:get, :post, :put, :delete, :head, :patch, :options]
     METHODS_WITH_BODIES = Set.new [:post, :put, :patch, :options]
 
-    attr_accessor :parallel_manager
+    attr_reader   :params, :headers, :url_prefix, :builder, :options, :ssl, :parallel_manager
-    attr_reader   :params, :headers, :url_prefix, :builder, :options, :ssl
+    attr_writer   :default_parallel_manager
 
     # :url
     # :params
@@ -28,7 +28,7 @@ module Faraday
       @params  = Utils::ParamsHash.new
       @options = options[:request] || {}
       @ssl     = options[:ssl]     || {}
-      @parallel_manager = options[:parallel]
+      @default_parallel_manager = options[:parallel_manager]
 
       @builder = options[:builder] || begin
         # pass an empty block to Builder so it doesn't assume default middleware
@@ -108,12 +108,24 @@ module Faraday
       @builder.insert(0, Faraday::Request::TokenAuthentication, token, options)
     end
 
+    def default_parallel_manager
+      return @default_parallel_manager if @default_parallel_manager
+
+      adapter = @builder.handlers.select { |h|
+        h.klass.respond_to?(:setup_parallel_manager)
+      }.first
+
+      if adapter
+        @default_parallel_manager = adapter.klass.setup_parallel_manager
+      end
+    end
+
     def in_parallel?
       !!@parallel_manager
     end
 
-    def in_parallel(manager)
+    def in_parallel(manager = nil)
-      @parallel_manager = manager
+      @parallel_manager = manager || default_parallel_manager
       yield
       @parallel_manager && @parallel_manager.run
     ensure

lib/faraday/middleware.rb

@@ -3,15 +3,8 @@ module Faraday
     extend MiddlewareRegistry
 
     class << self
-      attr_accessor :load_error, :supports_parallel_requests
+      attr_accessor :load_error
       private :load_error=
-      alias supports_parallel_requests? supports_parallel_requests
-
-      # valid parallel managers should respond to #run with no parameters.
-      # otherwise, return a short wrapper around it.
-      def setup_parallel_manager(options = {})
-        nil
-      end
     end
 
     self.load_error = nil

test/adapters/live_test.rb

@@ -159,7 +159,7 @@ else
           connection = create_connection(adapter)
           adapter    = real_adapter_for(adapter)
 
-          connection.in_parallel(adapter.setup_parallel_manager) do
+          connection.in_parallel do
             resp1 = connection.get('json')
             resp2 = connection.get('json')
             if adapter.supports_parallel_requests?