MDEV-6543 Crash if enable 'federatedx' when 'federated' plugin already enabled, and vice-versa

INSTALL SONAME ignores attempts to load the same plugin twice, it's not an error
(because one can load one plugin by name and then install soname for the rest).
But Federated and FederatedX are different plugins, despite having the same name.

Now plugin_add() only considers two plugins identical if their names are the same
string (compared as pointers). Otherwise it reports an error.,

mysql-test/suite/plugins/r/false_dupes-6543.result

+install soname 'ha_federated';
+install soname 'ha_federated';
+install soname 'ha_federatedx';
+ERROR HY000: Function 'FEDERATED' already exists
+uninstall soname 'ha_federated';

mysql-test/suite/plugins/t/false_dupes-6543.test

+#
+#    MDEV-6543 Crash if enable 'federatedx' when 'federated' plugin already enabled, and vice-versa
+#
+if(!$HA_FEDERATED_SO) {
+  skip Needs ha_federated.so;
+}
+if(!$HA_FEDERATEDX_SO) {
+  skip Needs ha_federatedx.so;
+}
+
+install soname 'ha_federated';
+# note: no error below! install soname ignores already loaded plugins
+install soname 'ha_federated';
+# note: an error here, even though plugin name is the same!
+--error ER_UDF_EXISTS
+install soname 'ha_federatedx';
+uninstall soname 'ha_federated';
+

sql/sql_plugin.cc

@@ -1040,7 +1040,7 @@ static st_plugin_int *plugin_insert_or_reuse(struct st_plugin_int *plugin)
 static bool plugin_add(MEM_ROOT *tmp_root,
                        const LEX_STRING *name, LEX_STRING *dl, int report)
 {
-  struct st_plugin_int tmp;
+  struct st_plugin_int tmp, *maybe_dupe;
   struct st_maria_plugin *plugin;
   uint oks= 0, errs= 0, dupes= 0;
   DBUG_ENTER("plugin_add");
@@ -1070,8 +1070,14 @@ static bool plugin_add(MEM_ROOT *tmp_root,
                                   (const uchar *)tmp.name.str, tmp.name.length))
       continue; // plugin name doesn't match
 
-    if (!name->str && plugin_find_internal(&tmp.name, MYSQL_ANY_PLUGIN))
+    if (!name->str &&
+        (maybe_dupe= plugin_find_internal(&tmp.name, MYSQL_ANY_PLUGIN)))
     {
+      if (plugin->name != maybe_dupe->plugin->name)
+      {
+        report_error(report, ER_UDF_EXISTS, plugin->name);
+        DBUG_RETURN(TRUE);
+      }
       dupes++;
       continue; // already installed
     }