From 33b72ca8f64755bb7a13bf36190210ad1178b6da Mon Sep 17 00:00:00 2001
From: Mark Reynolds <mreynolds@redhat.com>
Date: May 19 2020 15:25:13 +0000
Subject: Issue 50745: ns-slapd hangs during CleanAllRUV tests


Bug Description:
	The hang condition:
		- is not systematic
		- occurs in rare case, for example here during the deletion of a replica.
		- a thread is waiting for a dblock that an other thread "forgot" to
		  release.
		- have always existed, at least since 1.4.0 but likely since 1.2.x

	When deleting a replica, the replica is retrieved from
	mapping tree structure (mtnode).
	The replica is also retrieved through the mapping tree
	when writing updates to the changelog.

	When deleting the replica, mapping tree structure is cleared
	after the changelog is deleted (that can take some cycles).
	There is a window where an update can retrieve the replica,
	from the not yet cleared MT, while the changelog being removed.

	At the end, the update will update the changelog that is
	currently removed and keeps an unfree lock in the DB.

Fix description:
	Ideally mapping tree should be protected by a lock but it
	is not done systematically (e.g.  slapi_get_mapping_tree_node).
	Using a lock looks an overkill and can probably introduce
	deadlock and performance hit.
	The idea of the fix is to reduce the window, moving the
	mapping tree clear before the changelog removal.

https://pagure.io/389-ds-base/issue/50745

Reviewed by: Mark Reynolds, Ludwig Krispenz

---

diff --git a/ldap/servers/plugins/replication/repl5_replica_config.c b/ldap/servers/plugins/replication/repl5_replica_config.c
index 80a0797..95b7fa5 100644
--- a/ldap/servers/plugins/replication/repl5_replica_config.c
+++ b/ldap/servers/plugins/replication/repl5_replica_config.c
@@ -735,18 +735,18 @@ replica_config_delete(Slapi_PBlock *pb __attribute__((unused)),
     PR_ASSERT(mtnode_ext);
 
     if (mtnode_ext->replica) {
+        Object *repl_obj = mtnode_ext->replica;
         /* remove object from the hash */
         r = (Replica *)object_get_data(mtnode_ext->replica);
+        mtnode_ext->replica = NULL;
         PR_ASSERT(r);
         /* The changelog for this replica is no longer valid, so we should remove it. */
         slapi_log_err(SLAPI_LOG_WARNING, repl_plugin_name, "replica_config_delete - "
                                                            "The changelog for replica %s is no longer valid since "
                                                            "the replica config is being deleted.  Removing the changelog.\n",
                       slapi_sdn_get_dn(replica_get_root(r)));
-        cl5DeleteDBSync(mtnode_ext->replica);
+        cl5DeleteDBSync(repl_obj);
         replica_delete_by_name(replica_get_name(r));
-        object_release(mtnode_ext->replica);
-        mtnode_ext->replica = NULL;
     }
 
     PR_Unlock(s_configLock);