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diff --git a/servers/slapd/overlays/seqmod.c b/servers/slapd/overlays/seqmod.c
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+/* seqmod.c - sequenced modifies */
+/* This work is part of OpenLDAP Software <http://www.openldap.org/>.
+ *
+ * Copyright 2004-2021 The OpenLDAP Foundation.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted only as authorized by the OpenLDAP
+ * Public License.
+ *
+ * A copy of this license is available in the file LICENSE in the
+ * top-level directory of the distribution or, alternatively, at
+ * <http://www.OpenLDAP.org/license.html>.
+ */
+/* ACKNOWLEDGEMENTS:
+ * This work was initially developed by Howard Chu for inclusion in
+ * OpenLDAP Software.
+ */
+
+#include "portable.h"
+
+#ifdef SLAPD_OVER_SEQMOD
+
+#include "slap.h"
+#include "config.h"
+
+/* This overlay serializes concurrent attempts to modify a single entry */
+
+typedef struct modtarget {
+ struct modtarget *mt_next;
+ struct modtarget *mt_tail;
+ Operation *mt_op;
+} modtarget;
+
+typedef struct seqmod_info {
+ Avlnode *sm_mods; /* entries being modified */
+ ldap_pvt_thread_mutex_t sm_mutex;
+} seqmod_info;
+
+static int
+sm_avl_cmp( const void *c1, const void *c2 )
+{
+ const modtarget *m1, *m2;
+ int rc;
+
+ m1 = c1; m2 = c2;
+ rc = m1->mt_op->o_req_ndn.bv_len - m2->mt_op->o_req_ndn.bv_len;
+
+ if ( rc ) return rc;
+ return ber_bvcmp( &m1->mt_op->o_req_ndn, &m2->mt_op->o_req_ndn );
+}
+
+static int
+seqmod_op_cleanup( Operation *op, SlapReply *rs )
+{
+ slap_callback *sc = op->o_callback;
+ seqmod_info *sm = sc->sc_private;
+ modtarget *mt, mtdummy;
+ Avlnode *av;
+
+ mtdummy.mt_op = op;
+ /* This op is done, remove it */
+ ldap_pvt_thread_mutex_lock( &sm->sm_mutex );
+ av = avl_find2( sm->sm_mods, &mtdummy, sm_avl_cmp );
+ assert(av != NULL);
+
+ mt = av->avl_data;
+
+ /* If there are more, promote the next one */
+ if ( mt->mt_next ) {
+ av->avl_data = mt->mt_next;
+ mt->mt_next->mt_tail = mt->mt_tail;
+ } else {
+ avl_delete( &sm->sm_mods, mt, sm_avl_cmp );
+ }
+ ldap_pvt_thread_mutex_unlock( &sm->sm_mutex );
+ op->o_callback = sc->sc_next;
+ op->o_tmpfree( sc, op->o_tmpmemctx );
+
+ return 0;
+}
+
+static int
+seqmod_op_mod( Operation *op, SlapReply *rs )
+{
+ slap_overinst *on = (slap_overinst *)op->o_bd->bd_info;
+ seqmod_info *sm = on->on_bi.bi_private;
+ modtarget *mt;
+ Avlnode *av;
+ slap_callback *cb;
+
+ cb = op->o_tmpcalloc( 1, sizeof(slap_callback) + sizeof(modtarget),
+ op->o_tmpmemctx );
+ mt = (modtarget *)(cb+1);
+ mt->mt_next = NULL;
+ mt->mt_tail = mt;
+ mt->mt_op = op;
+
+ /* See if we're already modifying this entry - don't allow
+ * near-simultaneous mods of the same entry
+ */
+ ldap_pvt_thread_mutex_lock( &sm->sm_mutex );
+ av = avl_find2( sm->sm_mods, mt, sm_avl_cmp );
+ if ( av ) {
+ modtarget *mtp = av->avl_data;
+ mtp->mt_tail->mt_next = mt;
+ mtp->mt_tail = mt;
+ /* Wait for this op to get to head of list */
+ while ( mtp != mt ) {
+ ldap_pvt_thread_mutex_unlock( &sm->sm_mutex );
+ ldap_pvt_thread_yield();
+ /* Let it finish - should use a condition
+ * variable here... */
+ ldap_pvt_thread_mutex_lock( &sm->sm_mutex );
+ mtp = av->avl_data;
+ }
+ } else {
+ /* Record that we're modifying this now */
+ avl_insert( &sm->sm_mods, mt, sm_avl_cmp, avl_dup_error );
+ }
+ ldap_pvt_thread_mutex_unlock( &sm->sm_mutex );
+
+ cb->sc_cleanup = seqmod_op_cleanup;
+ cb->sc_private = sm;
+ cb->sc_next = op->o_callback;
+ op->o_callback = cb;
+
+ return SLAP_CB_CONTINUE;
+}
+
+static int
+seqmod_op_extended(
+ Operation *op,
+ SlapReply *rs
+)
+{
+ if ( exop_is_write( op )) return seqmod_op_mod( op, rs );
+ else return SLAP_CB_CONTINUE;
+}
+
+static int
+seqmod_db_open(
+ BackendDB *be,
+ ConfigReply *cr
+)
+{
+ slap_overinst *on = (slap_overinst *)be->bd_info;
+ seqmod_info *sm;
+
+ sm = ch_calloc(1, sizeof(seqmod_info));
+ on->on_bi.bi_private = sm;
+
+ ldap_pvt_thread_mutex_init( &sm->sm_mutex );
+
+ return 0;
+}
+
+static int
+seqmod_db_close(
+ BackendDB *be,
+ ConfigReply *cr
+)
+{
+ slap_overinst *on = (slap_overinst *)be->bd_info;
+ seqmod_info *sm = (seqmod_info *)on->on_bi.bi_private;
+
+ if ( sm ) {
+ ldap_pvt_thread_mutex_destroy( &sm->sm_mutex );
+
+ ch_free( sm );
+ on->on_bi.bi_private = NULL;
+ }
+
+ return 0;
+}
+
+/* This overlay is set up for dynamic loading via moduleload. For static
+ * configuration, you'll need to arrange for the slap_overinst to be
+ * initialized and registered by some other function inside slapd.
+ */
+
+static slap_overinst seqmod;
+
+int
+seqmod_initialize()
+{
+ seqmod.on_bi.bi_type = "seqmod";
+ seqmod.on_bi.bi_db_open = seqmod_db_open;
+ seqmod.on_bi.bi_db_close = seqmod_db_close;
+
+ seqmod.on_bi.bi_op_modify = seqmod_op_mod;
+ seqmod.on_bi.bi_op_modrdn = seqmod_op_mod;
+ seqmod.on_bi.bi_extended = seqmod_op_extended;
+
+ return overlay_register( &seqmod );
+}
+
+#if SLAPD_OVER_SEQMOD == SLAPD_MOD_DYNAMIC
+int
+init_module( int argc, char *argv[] )
+{
+ return seqmod_initialize();
+}
+#endif /* SLAPD_OVER_SEQMOD == SLAPD_MOD_DYNAMIC */
+
+#endif /* defined(SLAPD_OVER_SEQMOD) */