Add new exception when attempting to connect to MySQL running on loca…

…lhost.

Add test to MySQL connection error on localhost via socket.

Address Rubocop warnings.

Update CHANGELOG.

Move exception to MySQL2 adapter.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Raise new exception when connecting to MySQL on `localhost` and it attempts to connect via a socket.
+
+    This issue does not happen with TrilogyAdapter.
+
+    *Ruy Rocha*
+
 *   Fix single quote escapes on default generated MySQL columns
 
     MySQL 5.7.5+ supports generated columns, which can be used to create a column that is computed from an expression.

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -20,6 +20,17 @@ class Mysql2Adapter < AbstractMysqlAdapter
 
       include Mysql2::DatabaseStatements
 
+      # Raised when a connection could not be obtained to the server running
+      # on localhost because it attempts to connect through a socket.
+      CannotConnectThroughSocketOnLocalhost = Class.new(ConnectionNotEstablished) do
+        def initialize(message = nil, connection_pool: nil)
+          message = "The MySQL client library translates localhost address to" \
+                    " a socket, so try to explicitly use the IP address (127.0.0.1) instead."
+
+          super(message, connection_pool: connection_pool)
+        end
+      end
+
       class << self
         def new_client(config)
           ::Mysql2::Client.new(config)
@@ -140,7 +151,11 @@ def text_type?(type)
         def connect
           @raw_connection = self.class.new_client(@connection_parameters)
         rescue ConnectionNotEstablished => ex
-          raise ex.set_pool(@pool)
+          if ex.message.match?(/Can't connect to local server through socket/) && @connection_parameters[:host] == "localhost"
+            raise CannotConnectThroughSocketOnLocalhost
+          else
+            raise ex.set_pool(@pool)
+          end
         end
 
         def reconnect

activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb

@@ -18,6 +18,12 @@ def test_connection_error
     assert_kind_of ActiveRecord::ConnectionAdapters::NullPool, error.connection_pool
   end
 
+  def test_connection_error_via_socket_on_localhost
+    assert_raises ActiveRecord::ConnectionAdapters::Mysql2Adapter::CannotConnectThroughSocketOnLocalhost do
+      ActiveRecord::ConnectionAdapters::Mysql2Adapter.new(host: "localhost", prepared_statements: false).connect!
+    end
+  end
+
   def test_reconnection_error
     fake_connection = Class.new do
       def query_options