Bug#46261 Plugins can be installed with --skip-grant-tables

Previously installed dynamic plugins are explicitly not loaded
on startup with --skip-grant-tables enabled. However, INSTALL
PLUGIN/UNINSTALL PLUGIN commands are allowed, and result in
inconsistent error messages (reporting duplicate plugin or
plugin does not exist).

This patch adds a check for --skip-grant-tables mode, and
returns error ER_OPTION_PREVENTS_STATEMENT to the user when
the above commands are attempted.

mysql-test/r/bug46261.result

+#
+# Bug#46261 Plugins can be installed with --skip-grant-tables
+#
+INSTALL PLUGIN example SONAME 'ha_example.so';
+ERROR HY000: The MySQL server is running with the --skip-grant-tables option so it cannot execute this statement
+UNINSTALL PLUGIN example;
+ERROR HY000: The MySQL server is running with the --skip-grant-tables option so it cannot execute this statement
+End of 5.1 tests

mysql-test/t/bug46261-master.opt

+--skip-grant-tables $EXAMPLE_PLUGIN_OPT

mysql-test/t/bug46261.test

+--source include/not_embedded.inc
+--source include/have_example_plugin.inc
+
+--echo #
+--echo # Bug#46261 Plugins can be installed with --skip-grant-tables
+--echo #
+
+--replace_regex /\.dll/.so/
+--error ER_OPTION_PREVENTS_STATEMENT
+eval INSTALL PLUGIN example SONAME $HA_EXAMPLE_SO;
+
+--replace_regex /\.dll/.so/
+--error ER_OPTION_PREVENTS_STATEMENT
+eval UNINSTALL PLUGIN example;
+
+--echo End of 5.1 tests

sql/sql_plugin.cc

@@ -1665,6 +1665,12 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING *dl
   struct st_plugin_int *tmp;
   DBUG_ENTER("mysql_install_plugin");
 
+  if (opt_noacl)
+  {
+    my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0), "--skip-grant-tables");
+    DBUG_RETURN(TRUE);
+  }
+
   bzero(&tables, sizeof(tables));
   tables.db= (char *)"mysql";
   tables.table_name= tables.alias= (char *)"plugin";
@@ -1741,6 +1747,12 @@ bool mysql_uninstall_plugin(THD *thd, const LEX_STRING *name)
   struct st_plugin_int *plugin;
   DBUG_ENTER("mysql_uninstall_plugin");
 
+  if (opt_noacl)
+  {
+    my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0), "--skip-grant-tables");
+    DBUG_RETURN(TRUE);
+  }
+
   bzero(&tables, sizeof(tables));
   tables.db= (char *)"mysql";
   tables.table_name= tables.alias= (char *)"plugin";