Allow shutdown to unload modules that register bucket scheme's or codec's.

* Change __ast_module_shutdown_ref to be NULL safe (11+).
* Allow modules that call ast_bucket_scheme_register or ast_codec_register
  to be unloaded during graceful shutdown only (13+ only).

ASTERISK-24796 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4428/
........

Merged revisions 432058 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432059 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/bucket.c

@@ -282,7 +282,7 @@ int __ast_bucket_scheme_register(const char *name, struct ast_sorcery_wizard *bu
 
 	ast_verb(2, "Registered bucket scheme '%s'\n", name);
 
-	ast_module_ref(module);
+	ast_module_shutdown_ref(module);
 
 	return 0;
 }

main/codec.c

@@ -296,8 +296,8 @@ int __ast_codec_register(struct ast_codec *codec, struct ast_module *mod)
 
 	ao2_link_flags(codecs, codec_new, OBJ_NOLOCK);
 
-	/* Once registered a codec can not be unregistered, and the module must persist */
-	ast_module_ref(mod);
+	/* Once registered a codec can not be unregistered, and the module must persist until shutdown */
+	ast_module_shutdown_ref(mod);
 
 	ast_verb(2, "Registered '%s' codec '%s' at sample rate '%u' with id '%u'\n",
 		ast_codec_media_type2str(codec->type), codec->name, codec->sample_rate, codec_new->id);

main/loader.c

@@ -1484,10 +1484,12 @@ struct ast_module *__ast_module_ref(struct ast_module *mod, const char *file, in
 
 void __ast_module_shutdown_ref(struct ast_module *mod, const char *file, int line, const char *func)
 {
-	if (!mod->flags.keepuntilshutdown) {
-		__ast_module_ref(mod, file, line, func);
-		mod->flags.keepuntilshutdown = 1;
+	if (!mod || mod->flags.keepuntilshutdown) {
+		return;
 	}
+
+	__ast_module_ref(mod, file, line, func);
+	mod->flags.keepuntilshutdown = 1;
 }
 
 void __ast_module_unref(struct ast_module *mod, const char *file, int line, const char *func)