MDEV-26774 Compression provider unloading at runtime has no effect but doesn't produce a warning

mysql-test/suite/plugins/r/compression_load.result

@@ -17,6 +17,8 @@ a	left(b, 9)	length(b)
 1	defdefdef	3000
 2	ghighighi	30000
 uninstall plugin provider_lz4;
+Warnings:
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
 select plugin_name, plugin_status from information_schema.plugins where plugin_name='provider_lz4';
 plugin_name	plugin_status
 provider_lz4	INACTIVE

sql/sql_plugin.cc

@@ -1267,7 +1267,13 @@ static void plugin_deinitialize(struct st_plugin_int *plugin, bool ref_check)
   if (!deinit)
     deinit= (plugin_type_init)(plugin->plugin->deinit);
 
-  if (!deinit || !deinit(plugin))
+  if (deinit && deinit(plugin))
+  {
+    if (THD *thd= current_thd)
+      push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
+                   WARN_PLUGIN_BUSY, ER_THD(thd, WARN_PLUGIN_BUSY));
+  }
+  else
     plugin->state= PLUGIN_IS_UNINITIALIZED; // free to unload
 
   if (ref_check && plugin->ref_count)

storage/rocksdb/mysql-test/rocksdb/r/mariadb_plugin.result

@@ -10,6 +10,19 @@ connection default;
 UNINSTALL SONAME 'ha_rocksdb';
 Warnings:
 Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
 SELECT ENGINE, SUPPORT FROM INFORMATION_SCHEMA.ENGINES WHERE ENGINE='ROCKSDB';
 ENGINE	SUPPORT
 ROCKSDB	NO