activerecord/mysql2: Avoid setting @connection to nil, just close it

By doing `@connection = nil` that means that we need nil checks before it
is used anywhere, but we weren't doing those checks.  Instead, we get a
NoMethodError after using a connection after it fails to reconnect.

Neither of the other adapters set @connection to nil, just the mysql2
adapter. By just closing it, we avoid the need to check if we have a
connection object and it will produce an appropriate exception when used.

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -24,11 +24,9 @@ def select_rows(sql, name = nil, binds = [])
 
         # Executes the SQL statement in the context of this connection.
         def execute(sql, name = nil)
-          if @connection
-            # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
-            # made since we established the connection
-            @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone
-          end
+          # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
+          # made since we established the connection
+          @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone
 
           super
         end
@@ -71,11 +69,9 @@ def select_result(sql, name = nil, binds = [])
           end
 
           def exec_stmt_and_free(sql, name, binds, cache_stmt: false)
-            if @connection
-              # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
-              # made since we established the connection
-              @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone
-            end
+            # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
+            # made since we established the connection
+            @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone
 
             type_casted_binds = type_casted_binds(binds)
 

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -90,7 +90,6 @@ def quote_string(string)
       #++
 
       def active?
-        return false unless @connection
         @connection.ping
       end
 
@@ -105,10 +104,7 @@ def reconnect!
       # Otherwise, this method does nothing.
       def disconnect!
         super
-        unless @connection.nil?
-          @connection.close
-          @connection = nil
-        end
+        @connection.close
       end
 
       private

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

@@ -63,6 +63,27 @@ def test_successful_reconnection_after_timeout_with_verify
     assert @connection.active?
   end
 
+  def test_execute_after_disconnect
+    @connection.disconnect!
+    error = assert_raise(ActiveRecord::StatementInvalid) do
+      @connection.execute("SELECT 1")
+    end
+    assert_match /closed MySQL connection/, error.message
+  end
+
+  def test_quote_after_disconnect
+    @connection.disconnect!
+    error = assert_raise(Mysql2::Error) do
+      @connection.quote("string")
+    end
+    assert_match /closed MySQL connection/, error.message
+  end
+
+  def test_active_after_disconnect
+    @connection.disconnect!
+    assert_equal false, @connection.active?
+  end
+
   def test_mysql_connection_collation_is_configured
     assert_equal "utf8_unicode_ci", @connection.show_variable("collation_connection")
     assert_equal "utf8_general_ci", ARUnit2Model.connection.show_variable("collation_connection")