Add request pool to improve delivery performance (#10353)

* Add request pool to improve delivery performance

Fix #7909

* Ensure connection is closed when exception interrupts execution

* Remove Timeout#timeout from socket connection

* Fix infinite retrial loop on HTTP::ConnectionError

* Close sockets on failure, reduce idle time to 90 seconds

* Add MAX_REQUEST_POOL_SIZE option to limit concurrent connections to the same server

* Use a shared pool size, 512 by default, to stay below open file limit

* Add some tests

* Add more tests

* Reduce MAX_IDLE_TIME from 90 to 30 seconds, reap every 30 seconds

* Use a shared pool that returns preferred connection but re-purposes other ones when needed

* Fix wrong connection being returned on subsequent calls within the same thread

* Reduce mutex calls on flushes from 2 to 1 and add test for reaping

spec/lib/connection_pool/shared_timed_stack_spec.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe ConnectionPool::SharedTimedStack do
+  class MiniConnection
+    attr_reader :site
+
+    def initialize(site)
+      @site = site
+    end
+  end
+
+  subject { described_class.new(5) { |site| MiniConnection.new(site) } }
+
+  describe '#push' do
+    it 'keeps the connection in the stack' do
+      subject.push(MiniConnection.new('foo'))
+      expect(subject.size).to eq 1
+    end
+  end
+
+  describe '#pop' do
+    it 'returns a connection' do
+      expect(subject.pop('foo')).to be_a MiniConnection
+    end
+
+    it 'returns the same connection that was pushed in' do
+      connection = MiniConnection.new('foo')
+      subject.push(connection)
+      expect(subject.pop('foo')).to be connection
+    end
+
+    it 'does not create more than maximum amount of connections' do
+      expect { 6.times { subject.pop('foo', 0) } }.to raise_error Timeout::Error
+    end
+
+    it 'repurposes a connection for a different site when maximum amount is reached' do
+      5.times { subject.push(MiniConnection.new('foo')) }
+      expect(subject.pop('bar')).to be_a MiniConnection
+    end
+  end
+
+  describe '#empty?' do
+    it 'returns true when no connections on the stack' do
+      expect(subject.empty?).to be true
+    end
+
+    it 'returns false when there are connections on the stack' do
+      subject.push(MiniConnection.new('foo'))
+      expect(subject.empty?).to be false
+    end
+  end
+
+  describe '#size' do
+    it 'returns the number of connections on the stack' do
+      2.times { subject.push(MiniConnection.new('foo')) }
+      expect(subject.size).to eq 2
+    end
+  end
+end