fix: adds new connection pool which will discard failed clients

closes #2780

spec/lib/postal/message_db/connection_pool_spec.rb

@@ -0,0 +1,45 @@
+require "rails_helper"
+
+describe Postal::MessageDB::ConnectionPool do
+
+  subject(:pool) { described_class.new }
+
+  describe '#use' do
+    it 'yields a connection' do
+      counter = 0
+      pool.use do |connection|
+        expect(connection).to be_a Mysql2::Client
+        counter += 1
+      end
+      expect(counter).to eq 1
+    end
+
+    it 'checks in a connection after the block has executed' do
+      connection = nil
+      pool.use do |c|
+        expect(pool.connections).to be_empty
+        connection = c
+      end
+      expect(pool.connections).to eq [connection]
+    end
+
+    it 'checks in a connection if theres an error in the block' do
+      expect do
+        pool.use do |c|
+          raise StandardError
+        end
+      end.to raise_error StandardError
+      expect(pool.connections).to match [kind_of(Mysql2::Client)]
+    end
+
+    it 'does not check in connections when there is a connection error' do
+      expect do
+        pool.use do
+          raise Mysql2::Error, "lost connection to server"
+        end
+      end.to raise_error Mysql2::Error
+      expect(pool.connections).to eq []
+    end
+  end
+
+end