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.

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

@@ -140,7 +140,11 @@ 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.include?("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/lib/active_record/errors.rb

@@ -88,6 +88,16 @@ def set_pool(connection_pool)
   class ConnectionTimeoutError < ConnectionNotEstablished
   end
 
+  # Raised when a connection could not be obtained to the server running
+  # on localhost because it attempts to connect through a socket.
+  class CannotConnectThroughSocketOnLocalhost < ConnectionNotEstablished
+    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
+
   # Raised when connection to the database could not been established because it was not
   # able to connect to the host or when the authorization failed.
   class DatabaseConnectionError < ConnectionNotEstablished

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::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