WHAT

faraday.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.3'
 
+  spec.add_dependency 'connection_pool', '~> 2.2'
   spec.add_dependency 'multipart-post', '>= 1.2', '< 3'
 
   spec.require_paths = %w[lib spec/external_adapters]

lib/faraday/adapter.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'faraday/adapter/concerns/connection_pooling'
+
 module Faraday
   # Base class for all Faraday adapters. Adapters are
   # responsible for fulfilling a Faraday request.
@@ -40,10 +42,14 @@ def inherited(subclass)
     extend Parallelism
     self.supports_parallel = false
 
+    include ConnectionPooling
+    self.supports_pooling = false
+
     def initialize(_app = nil, opts = {}, &block)
       @app = ->(env) { env.response }
       @connection_options = opts
       @config_block = block
+      initialize_pool(opts[:pool] || {}) if self.class.supports_pooling
     end
 
     def call(env)

lib/faraday/adapter/concerns/connection_pooling.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+# This module marks an Adapter as supporting connection pooling.
+module ConnectionPooling
+  def self.included(base)
+    base.extend(ClassMethods)
+  end
+
+  # Class methods injected into the class including this module.
+  module ClassMethods
+    attr_accessor :supports_pooling
+
+    def inherited(subclass)
+      super
+      subclass.supports_pooling = supports_pooling
+    end
+  end
+
+  attr_reader :pool
+
+  MISSING_CONNECTION_ERROR = 'You need to define a `connection` method' \
+    'in order to support connection pooling!'
+
+  # Initializes the connection pool.
+  #
+  # @param opts [Hash] the options to pass to `ConnectionPool` initializer.
+  def initialize_pool(opts = {})
+    ensure_connection_defined!
+    @pool = ConnectionPool.new(opts, &method(:connection))
+  end
+
+  # Checks if `connection` method exists and raises an error otherwise.
+  def ensure_connection_defined!
+    return if self.class.method_defined?(:connection)
+
+    raise NoMethodError, MISSING_CONNECTION_ERROR
+  end
+end

lib/faraday/adapter/patron.rb

@@ -6,39 +6,19 @@ class Adapter
     class Patron < Faraday::Adapter
       dependency 'patron'
 
+      self.supports_pooling = true
+      def connection
+        ::Patron::Session.new
+      end
+
       def call(env)
         super
         # TODO: support streaming requests
         env[:body] = env[:body].read if env[:body].respond_to? :read
 
-        session = ::Patron::Session.new
-        @config_block&.call(session)
-        if (env[:url].scheme == 'https') && env[:ssl]
-          configure_ssl(session, env[:ssl])
-        end
-
-        if (req = env[:request])
-          if req[:timeout]
-            session.timeout = session.connect_timeout = req[:timeout]
-          end
-          session.connect_timeout = req[:open_timeout] if req[:open_timeout]
-
-          if (proxy = req[:proxy])
-            proxy_uri = proxy[:uri].dup
-            proxy_uri.user = proxy[:user] &&
-                             Utils.escape(proxy[:user]).gsub('+', '%20')
-            proxy_uri.password = proxy[:password] &&
-                                 Utils.escape(proxy[:password]).gsub('+', '%20')
-            session.proxy = proxy_uri.to_s
-          end
-        end
-
-        response = begin
-          data = env[:body] ? env[:body].to_s : nil
-          session.request(env[:method], env[:url].to_s,
-                          env[:request_headers], data: data)
-                   rescue Errno::ECONNREFUSED, ::Patron::ConnectionFailed
-                     raise Faraday::ConnectionFailed, $ERROR_INFO
+        response = pool.with do |session|
+          @config_block&.call(session)
+          perform_request(session, env)
         end
 
         if (req = env[:request]).stream_response?
@@ -83,6 +63,37 @@ def call(env)
         end
       end
 
+      def perform_request(session, env)
+        if (env[:url].scheme == 'https') && env[:ssl]
+          configure_ssl(session, env[:ssl])
+        end
+
+        if (req = env[:request])
+          if req[:timeout]
+            session.timeout = session.connect_timeout = req[:timeout]
+          end
+          session.connect_timeout = req[:open_timeout] if req[:open_timeout]
+
+          if (proxy = req[:proxy])
+            proxy_uri = proxy[:uri].dup
+            proxy_uri.user = proxy[:user] &&
+                             Utils.escape(proxy[:user]).gsub('+', '%20')
+            proxy_uri.password = proxy[:password] &&
+                                 Utils.escape(proxy[:password]).gsub('+', '%20')
+            session.proxy = proxy_uri.to_s
+          end
+        end
+
+        begin
+          data = env[:body] ? env[:body].to_s : nil
+          puts "start request... #{pool.available}"
+          session.request(env[:method], env[:url].to_s,
+                          env[:request_headers], data: data)
+        rescue Errno::ECONNREFUSED, ::Patron::ConnectionFailed
+          raise Faraday::ConnectionFailed, $ERROR_INFO
+        end
+      end
+
       def configure_ssl(session, ssl)
         if ssl.fetch(:verify, true)
           session.cacert = ssl[:ca_file]

spec/faraday/adapter/patron_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe Faraday::Adapter::Patron do
-  features :reason_phrase_parse
+  features :reason_phrase_parse, :pooling
 
   it_behaves_like 'an adapter'
 

spec/support/shared_examples/request_method.rb

@@ -231,4 +231,23 @@
 
     expect { conn.public_send(http_method, '/') }.to raise_error(Faraday::ProxyAuthError)
   end
+
+  on_feature :pooling do
+    it 'uses a connection_pool internally' do
+      pool = nil
+      allow_any_instance_of(described_class).to receive(:pool).and_wrap_original do |m, *args|
+        pool ||= m.call(*args)
+      end
+
+      request_stub.to_return do |req|
+        nested = (req.headers['Nested'] || 1).to_i
+        return if nested >= pool.size
+        expect(pool.available).to eq(pool.size - nested)
+        conn.public_send(http_method, '/', nil, nested: nested + 1)
+      end
+
+      conn.public_send(http_method, '/')
+      request_stub.disable
+    end
+  end
 end