cel_pgsql: deadlock on unload and core_event_dispatcher

A deadlock can happen between a thread unloading or reloading the cel_pgsql
module and the core_event_dispatcher taskprocessor thread. Description of
what is happening:

Thread 1 (for example, a netconsole thread):

    a "module reload cel_pgsql" is launched
    the thread enter the "my_unload_module" function (cel_pgsql.c)
    the thread acquire the write lock on psql_columns
    the thread enter the "ast_event_unsubscribe" function (event.c)
    the thread try to acquire the write lock on ast_event_subs[sub->type]

Thread 2 (core_event_dispatcher taskprocessor thread):

    the taskprocessor pop a CEL event
    the thread enter the "handle_event" function (event.c)
    the thread acquire the read lock on ast_event_subs[sub->type]
    the thread callback the "pgsql_log" function (cel_pgsql.c), since it's a subscriber of CEL events
    the thread try to acquire a read lock on psql_columns

(closes issue ASTERISK-22854)
Reported by: Etienne Lessard
Patches:
     cel_pgsql_fix_deadlock_event.patch uploaded by hexanol (license 6394)
........

Merged revisions 404603 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@404604 65c4cc65-6c06-0410-ace0-fbb531ad65f3

cel/cel_pgsql.c

@@ -345,11 +345,12 @@ ast_log_cleanup:
 static int my_unload_module(void)
 {
 	struct columns *current;
-	AST_RWLIST_WRLOCK(&psql_columns);
+
 	if (event_sub) {
 		event_sub = ast_event_unsubscribe(event_sub);
-		event_sub = NULL;
 	}
+
+	AST_RWLIST_WRLOCK(&psql_columns);
 	if (conn) {
 		PQfinish(conn);
 		conn = NULL;