Reuse Patron session with mutex

Reuse Patron session to support keep-alive/connection reuse and make
them thread-safe using a mutex.

fixes #1001

docs/adapters/patron.md

@@ -18,6 +18,12 @@ conn = Faraday.new(...) do |f|
 end
 ```
 
+## Multithreading
+
+This adapter use a mutex around the patron session to be thread-safe.
+A [connection_pool](https://rubygems.org/gems/connection_pool) can be
+used to share multiple connections between threads.
+
 ## Links
 
 * [Gem RDoc][rdoc]

lib/faraday/adapter/patron.rb

@@ -6,13 +6,25 @@ class Adapter
     class Patron < Faraday::Adapter
       dependency 'patron'
 
+      def initialize(app = nil, opts = {}, &block)
+        @mutex = Mutex.new
+        super(app, opts, &block)
+      end
+
       def call(env)
-        super
+        # Patron::Session is not thread-safe, use a mutex to be safe
+        @mutex.synchronize do
+          super
+          perform_request env
+        end
+      end
+
+      def perform_request(env)
         # TODO: support streaming requests
         env[:body] = env[:body].read if env[:body].respond_to? :read
 
-        session = ::Patron::Session.new
-        @config_block&.call(session)
+        session = @session ||= create_session
+
         if (env[:url].scheme == 'https') && env[:ssl]
           configure_ssl(session, env[:ssl])
         end
@@ -83,6 +95,12 @@ def call(env)
         end
       end
 
+      def create_session
+        session = ::Patron::Session.new
+        @config_block&.call(session)
+        session
+      end
+
       def configure_ssl(session, ssl)
         if ssl.fetch(:verify, true)
           session.cacert = ssl[:ca_file]

spec/faraday/adapter/patron_spec.rb

@@ -15,4 +15,21 @@
 
     expect { conn.get('/') }.to raise_error(RuntimeError, 'Configuration block called')
   end
+
+  it 'reuses the patron session for keep-alive' do
+    last_session = nil
+    conn = Faraday.new do |f|
+      f.adapter :patron do |session|
+        if last_session
+          expect(session).to eq last_session
+        else
+          last_session = session
+        end
+      end
+    end
+
+    stub_request(:get, 'http://example.com')
+    conn.get('http://example.com/')
+    conn.get('http://example.com/')
+  end
 end