ITS#9015 syncprov: fix for zero-length suffix

If the "" glue entry exists and lacks a contextCSN, must perform
an additional search to be sure the DB is otherwise empty. If so,
skip creating the contextCSN.

servers/slapd/overlays/syncprov.c

@@ -3213,6 +3213,49 @@ syncprov_db_otask(
 	return NULL;
 }
 
+static int
+syncprov_db_ocallback(
+	Operation *op,
+	SlapReply *rs
+)
+{
+	if ( rs->sr_type == REP_SEARCH && rs->sr_err == LDAP_SUCCESS ) {
+		if ( rs->sr_entry->e_name.bv_len )
+			op->o_callback->sc_private = (void *)1;
+	}
+	return LDAP_SUCCESS;
+}
+
+/* ITS#9015 see if the DB is really empty */
+static void *
+syncprov_db_otask2(
+	void *ptr
+)
+{
+	Operation *op = ptr;
+	SlapReply rs = {REP_RESULT};
+	slap_callback cb = {0};
+	int rc;
+
+	cb.sc_response = syncprov_db_ocallback;
+
+	op->o_managedsait = SLAP_CONTROL_CRITICAL;
+	op->o_callback = &cb;
+	op->o_tag = LDAP_REQ_SEARCH;
+	op->ors_scope = LDAP_SCOPE_SUBTREE;
+	op->ors_limit = NULL;
+	op->ors_slimit = 1;
+	op->ors_tlimit = SLAP_NO_LIMIT;
+	op->ors_attrs = slap_anlist_no_attrs;
+	op->ors_attrsonly = 1;
+	op->ors_deref = LDAP_DEREF_NEVER;
+	op->ors_filter = &generic_filter;
+	op->ors_filterstr = generic_filterstr;
+	rc = op->o_bd->be_search( op, &rs );
+	if ( rc == LDAP_SIZELIMIT_EXCEEDED || cb.sc_private )
+		op->ors_slimit = 2;
+	return NULL;
+}
 
 /* Read any existing contextCSN from the underlying db.
  * Then search for any entries newer than that. If no value exists,
@@ -3305,6 +3348,20 @@ syncprov_db_open(
 			/* If the DB is genuinely empty, don't generate one either. */
 			goto out;
 		}
+		if ( !si->si_contextdn.bv_len ) {
+			ldap_pvt_thread_t tid;
+			/* a glue entry here with no contextCSN might mean an empty DB.
+			 * we need to search for children, to be sure.
+			 */
+			op->o_req_dn = be->be_suffix[0];
+			op->o_req_ndn = be->be_nsuffix[0];
+			op->o_bd->bd_info = (BackendInfo *)on->on_info;
+			ldap_pvt_thread_create( &tid, 0, syncprov_db_otask2, op );
+			ldap_pvt_thread_join( tid, NULL );
+			if ( op->ors_slimit == 1 )
+				goto out;
+		}
+
 		csn.bv_val = csnbuf;
 		csn.bv_len = sizeof( csnbuf );
 		slap_get_csn( op, &csn, 0 );