MDEV-33441 Do not deinit plugin variables when retry requested

After MDEV-31400, plugins are allowed to ask for retries when failing
initialisation. However, such failures also cause plugin system
variables to be deleted (plugin_variables_deinit()) before retrying
and are not re-added during retry.

We fix this by checking that if the plugin has requested a retry the
variables are not deleted. Because plugin_deinitialize() also calls
plugin_variables_deinit(), if the retry fails, the variables will
still be deleted.

Alternatives considered:

- remove the plugin_variables_deinit() from plugin_initialize() error
handling altogether. We decide to take a more conservative approach
here.

- re-add the system variables during retry. It is more complicated
than simply iterating over plugin->system_vars and call
my_hash_insert(). For example we will need to assign values to
the test_load field and extract more code from test_plugin_options(),
if that is possible.

sql/sql_plugin.cc

@@ -1505,7 +1505,7 @@ static int plugin_initialize(MEM_ROOT *tmp_root, struct st_plugin_int *plugin,
   else
     ret= plugin_do_initialize(plugin, state);
 
-  if (ret)
+  if (ret && ret != HA_ERR_RETRY_INIT)
     plugin_variables_deinit(plugin);
 
   mysql_mutex_lock(&LOCK_plugin);
@@ -1786,6 +1786,7 @@ int plugin_init(int *argc, char **argv, int flags)
         uint state= plugin_ptr->state;
         mysql_mutex_unlock(&LOCK_plugin);
         error= plugin_do_initialize(plugin_ptr, state);
+        DBUG_EXECUTE_IF("fail_spider_init_retry", error= 1;);
         mysql_mutex_lock(&LOCK_plugin);
         plugin_ptr->state= state;
         if (error == HA_ERR_RETRY_INIT)

storage/spider/mysql-test/spider/bugfix/r/mdev_33441.result

+#
+# MDEV-33441 No spider variables available is Spider is loaded upon server startup
+#
+set spider_same_server_link=0;
+#
+# end of test mdev_33441
+#

storage/spider/mysql-test/spider/bugfix/r/mdev_33441_fail.result

+#
+# MDEV-33441 No spider variables available is Spider is loaded upon server startup
+#
+select * from mysql.plugin;
+name	dl
+show variables like 'spider%';
+Variable_name	Value
+#
+# end of test mdev_33441_fail
+#

storage/spider/mysql-test/spider/bugfix/t/mdev_33441.opt

+--plugin-load-add=ha_spider

storage/spider/mysql-test/spider/bugfix/t/mdev_33441.test

+--echo #
+--echo # MDEV-33441 No spider variables available is Spider is loaded upon server startup
+--echo #
+
+# We test that at least one spider variable exists.
+set spider_same_server_link=0;
+
+--echo #
+--echo # end of test mdev_33441
+--echo #

storage/spider/mysql-test/spider/bugfix/t/mdev_33441_fail.opt

+--plugin-load-add=ha_spider
+--debug-dbug=d,fail_spider_init_retry

storage/spider/mysql-test/spider/bugfix/t/mdev_33441_fail.test

+--source include/have_debug.inc
+--echo #
+--echo # MDEV-33441 No spider variables available is Spider is loaded upon server startup
+--echo #
+# We test that when retry fails, spider variables are deleted.
+select * from mysql.plugin;
+show variables like 'spider%';
+--echo #
+--echo # end of test mdev_33441_fail
+--echo #