Overlay for serializing modifications of a specific entry without

using backend-specific locks

servers/slapd/overlays/seqmod.c

+/* seqmod.c - sequenced modifies */
+/* This work is part of OpenLDAP Software <http://www.openldap.org/>.
+ *
+ * Copyright 2004 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"
+
+#define	SLAPD_OVER_SEQMOD	SLAPD_MOD_STATIC
+
+#ifdef SLAPD_OVER_SEQMOD
+
+#include "slap.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 )
+{
+	seqmod_info *sm = op->o_callback->sc_private;
+	modtarget *mt;
+	Avlnode	 *av;
+
+	/* This op is done, remove it */
+	ldap_pvt_thread_mutex_lock( &sm->sm_mutex );
+	av = avl_find( sm->sm_mods, mt, sm_avl_cmp );
+	assert(av);
+
+	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 = op->o_callback->sc_next;
+	op->o_tmpfree( op->o_callback, 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_find( 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
+)
+{
+	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
+)
+{
+	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 );
+	}
+
+	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_init()
+{
+	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_init();
+}
+#endif /* SLAPD_OVER_SEQMOD == SLAPD_MOD_DYNAMIC */
+
+#endif /* defined(SLAPD_OVER_SEQMOD) */