NetBSD/dist/bind/lib/dns/db.c

/*	$NetBSD: db.c,v 1.1.1.3 2005/12/21 23:16:05 christos Exp $	*/

/*
 * Copyright (C) 2004  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1999-2001, 2003  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

/* Id: db.c,v 1.69.2.1.10.4 2004/03/08 02:07:52 marka Exp */

/***
 *** Imports
 ***/

#include <config.h>

#include <isc/buffer.h>
#include <isc/mem.h>
#include <isc/once.h>
#include <isc/rwlock.h>
#include <isc/string.h>
#include <isc/util.h>

#include <dns/callbacks.h>
#include <dns/db.h>
#include <dns/log.h>
#include <dns/master.h>
#include <dns/rdata.h>
#include <dns/rdataset.h>
#include <dns/result.h>

/***
 *** Private Types
 ***/

struct dns_dbimplementation {
	const char *				name;
	dns_dbcreatefunc_t			create;
	isc_mem_t *				mctx;
	void *					driverarg;
	ISC_LINK(dns_dbimplementation_t)	link;
};

/***
 *** Supported DB Implementations Registry
 ***/

/*
 * Built in database implementations are registered here.
 */

#include "rbtdb.h"
#include "rbtdb64.h"

static ISC_LIST(dns_dbimplementation_t) implementations;
static isc_rwlock_t implock;
static isc_once_t once = ISC_ONCE_INIT;

static dns_dbimplementation_t rbtimp;
static dns_dbimplementation_t rbt64imp;

static void
initialize(void) {
	RUNTIME_CHECK(isc_rwlock_init(&implock, 0, 0) == ISC_R_SUCCESS);

	rbtimp.name = "rbt";
	rbtimp.create = dns_rbtdb_create;
	rbtimp.mctx = NULL;
	rbtimp.driverarg = NULL;
	ISC_LINK_INIT(&rbtimp, link);

	rbt64imp.name = "rbt64";
	rbt64imp.create = dns_rbtdb64_create;
	rbt64imp.mctx = NULL;
	rbt64imp.driverarg = NULL;
	ISC_LINK_INIT(&rbt64imp, link);

	ISC_LIST_INIT(implementations);
	ISC_LIST_APPEND(implementations, &rbtimp, link);
	ISC_LIST_APPEND(implementations, &rbt64imp, link);
}

static inline dns_dbimplementation_t *
impfind(const char *name) {
	dns_dbimplementation_t *imp;

	for (imp = ISC_LIST_HEAD(implementations); 
	     imp != NULL;
	     imp = ISC_LIST_NEXT(imp, link))
		if (strcasecmp(name, imp->name) == 0)
			return (imp);
	return (NULL);
}


/***
 *** Basic DB Methods
 ***/

isc_result_t
dns_db_create(isc_mem_t *mctx, const char *db_type, dns_name_t *origin,
	      dns_dbtype_t type, dns_rdataclass_t rdclass,
	      unsigned int argc, char *argv[], dns_db_t **dbp)
{
	dns_dbimplementation_t *impinfo;

	RUNTIME_CHECK(isc_once_do(&once, initialize) == ISC_R_SUCCESS);

	/*
	 * Create a new database using implementation 'db_type'.
	 */

	REQUIRE(dbp != NULL && *dbp == NULL);
	REQUIRE(dns_name_isabsolute(origin));

	RWLOCK(&implock, isc_rwlocktype_read);
	impinfo = impfind(db_type);
	if (impinfo != NULL) {
		isc_result_t result;
		result = ((impinfo->create)(mctx, origin, type,
					    rdclass, argc, argv,
					    impinfo->driverarg, dbp));
		RWUNLOCK(&implock, isc_rwlocktype_read);
		return (result);
	}

	RWUNLOCK(&implock, isc_rwlocktype_read);

	isc_log_write(dns_lctx, DNS_LOGCATEGORY_DATABASE,
		      DNS_LOGMODULE_DB, ISC_LOG_ERROR,
		      "unsupported database type '%s'", db_type);

	return (ISC_R_NOTFOUND);
}

void
dns_db_attach(dns_db_t *source, dns_db_t **targetp) {

	/*
	 * Attach *targetp to source.
	 */

	REQUIRE(DNS_DB_VALID(source));
	REQUIRE(targetp != NULL && *targetp == NULL);

	(source->methods->attach)(source, targetp);

	ENSURE(*targetp == source);
}

void
dns_db_detach(dns_db_t **dbp) {

	/*
	 * Detach *dbp from its database.
	 */

	REQUIRE(dbp != NULL);
	REQUIRE(DNS_DB_VALID(*dbp));

	((*dbp)->methods->detach)(dbp);

	ENSURE(*dbp == NULL);
}

isc_result_t
dns_db_ondestroy(dns_db_t *db, isc_task_t *task, isc_event_t **eventp)
{
	REQUIRE(DNS_DB_VALID(db));

	return (isc_ondestroy_register(&db->ondest, task, eventp));
}


isc_boolean_t
dns_db_iscache(dns_db_t *db) {

	/*
	 * Does 'db' have cache semantics?
	 */

	REQUIRE(DNS_DB_VALID(db));

	if ((db->attributes & DNS_DBATTR_CACHE) != 0)
		return (ISC_TRUE);

	return (ISC_FALSE);
}

isc_boolean_t
dns_db_iszone(dns_db_t *db) {

	/*
	 * Does 'db' have zone semantics?
	 */

	REQUIRE(DNS_DB_VALID(db));

	if ((db->attributes & (DNS_DBATTR_CACHE|DNS_DBATTR_STUB)) == 0)
		return (ISC_TRUE);

	return (ISC_FALSE);
}

isc_boolean_t
dns_db_isstub(dns_db_t *db) {

	/*
	 * Does 'db' have stub semantics?
	 */

	REQUIRE(DNS_DB_VALID(db));

	if ((db->attributes & DNS_DBATTR_STUB) != 0)
		return (ISC_TRUE);

	return (ISC_FALSE);
}

isc_boolean_t
dns_db_issecure(dns_db_t *db) {

	/*
	 * Is 'db' secure?
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0);

	return ((db->methods->issecure)(db));
}

isc_boolean_t
dns_db_ispersistent(dns_db_t *db) {

	/*
	 * Is 'db' persistent?
	 */

	REQUIRE(DNS_DB_VALID(db));

	return ((db->methods->ispersistent)(db));
}

dns_name_t *
dns_db_origin(dns_db_t *db) {
	/*
	 * The origin of the database.
	 */

	REQUIRE(DNS_DB_VALID(db));

	return (&db->origin);
}

dns_rdataclass_t
dns_db_class(dns_db_t *db) {
	/*
	 * The class of the database.
	 */

	REQUIRE(DNS_DB_VALID(db));

	return (db->rdclass);
}

isc_result_t
dns_db_beginload(dns_db_t *db, dns_addrdatasetfunc_t *addp,
		 dns_dbload_t **dbloadp) {
	/*
	 * Begin loading 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(addp != NULL && *addp == NULL);
	REQUIRE(dbloadp != NULL && *dbloadp == NULL);

	return ((db->methods->beginload)(db, addp, dbloadp));
}

isc_result_t
dns_db_endload(dns_db_t *db, dns_dbload_t **dbloadp) {
	/*
	 * Finish loading 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(dbloadp != NULL && *dbloadp != NULL);

	return ((db->methods->endload)(db, dbloadp));
}

isc_result_t
dns_db_load(dns_db_t *db, const char *filename) {
	isc_result_t result, eresult;
	dns_rdatacallbacks_t callbacks;
	unsigned int options = 0;

	/*
	 * Load master file 'filename' into 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));

	if ((db->attributes & DNS_DBATTR_CACHE) != 0)
		options |= DNS_MASTER_AGETTL;

	dns_rdatacallbacks_init(&callbacks);

	result = dns_db_beginload(db, &callbacks.add, &callbacks.add_private);
	if (result != ISC_R_SUCCESS)
		return (result);
	result = dns_master_loadfile(filename, &db->origin, &db->origin,
				     db->rdclass, options,
				     &callbacks, db->mctx);
	eresult = dns_db_endload(db, &callbacks.add_private);
	/*
	 * We always call dns_db_endload(), but we only want to return its
	 * result if dns_master_loadfile() succeeded.  If dns_master_loadfile()
	 * failed, we want to return the result code it gave us.
	 */
	if (eresult != ISC_R_SUCCESS &&
	    (result == ISC_R_SUCCESS || result == DNS_R_SEENINCLUDE))
		result = eresult;

	return (result);
}

isc_result_t
dns_db_dump(dns_db_t *db, dns_dbversion_t *version, const char *filename) {
	/*
	 * Dump 'db' into master file 'filename'.
	 */

	REQUIRE(DNS_DB_VALID(db));

	return ((db->methods->dump)(db, version, filename));
}

/***
 *** Version Methods
 ***/

void
dns_db_currentversion(dns_db_t *db, dns_dbversion_t **versionp) {

	/*
	 * Open the current version for reading.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0);
	REQUIRE(versionp != NULL && *versionp == NULL);

	(db->methods->currentversion)(db, versionp);
}

isc_result_t
dns_db_newversion(dns_db_t *db, dns_dbversion_t **versionp) {

	/*
	 * Open a new version for reading and writing.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0);
	REQUIRE(versionp != NULL && *versionp == NULL);

	return ((db->methods->newversion)(db, versionp));
}

void
dns_db_attachversion(dns_db_t *db, dns_dbversion_t *source,
		     dns_dbversion_t **targetp)
{
	/*
	 * Attach '*targetp' to 'source'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0);
	REQUIRE(source != NULL);
	REQUIRE(targetp != NULL && *targetp == NULL);

	(db->methods->attachversion)(db, source, targetp);

	ENSURE(*targetp != NULL);
}

void
dns_db_closeversion(dns_db_t *db, dns_dbversion_t **versionp,
		    isc_boolean_t commit)
{

	/*
	 * Close version '*versionp'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0);
	REQUIRE(versionp != NULL && *versionp != NULL);

	(db->methods->closeversion)(db, versionp, commit);

	ENSURE(*versionp == NULL);
}

/***
 *** Node Methods
 ***/

isc_result_t
dns_db_findnode(dns_db_t *db, dns_name_t *name,
		isc_boolean_t create, dns_dbnode_t **nodep)
{

	/*
	 * Find the node with name 'name'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(nodep != NULL && *nodep == NULL);

	return ((db->methods->findnode)(db, name, create, nodep));
}

isc_result_t
dns_db_find(dns_db_t *db, dns_name_t *name, dns_dbversion_t *version,
	    dns_rdatatype_t type, unsigned int options, isc_stdtime_t now,
	    dns_dbnode_t **nodep, dns_name_t *foundname,
	    dns_rdataset_t *rdataset, dns_rdataset_t *sigrdataset)
{

	/*
	 * Find the best match for 'name' and 'type' in version 'version'
	 * of 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(type != dns_rdatatype_rrsig);
	REQUIRE(nodep == NULL || (nodep != NULL && *nodep == NULL));
	REQUIRE(dns_name_hasbuffer(foundname));
	REQUIRE(rdataset == NULL ||
		(DNS_RDATASET_VALID(rdataset) &&
		 ! dns_rdataset_isassociated(rdataset)));
	REQUIRE(sigrdataset == NULL ||
		(DNS_RDATASET_VALID(sigrdataset) &&
		 ! dns_rdataset_isassociated(sigrdataset)));

	return ((db->methods->find)(db, name, version, type, options, now,
				    nodep, foundname, rdataset, sigrdataset));
}

isc_result_t
dns_db_findzonecut(dns_db_t *db, dns_name_t *name,
		   unsigned int options, isc_stdtime_t now,
		   dns_dbnode_t **nodep, dns_name_t *foundname,
		   dns_rdataset_t *rdataset, dns_rdataset_t *sigrdataset)
{
	/*
	 * Find the deepest known zonecut which encloses 'name' in 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) != 0);
	REQUIRE(nodep == NULL || (nodep != NULL && *nodep == NULL));
	REQUIRE(dns_name_hasbuffer(foundname));
	REQUIRE(sigrdataset == NULL ||
		(DNS_RDATASET_VALID(sigrdataset) &&
		 ! dns_rdataset_isassociated(sigrdataset)));

	return ((db->methods->findzonecut)(db, name, options, now, nodep,
					   foundname, rdataset, sigrdataset));
}

void
dns_db_attachnode(dns_db_t *db, dns_dbnode_t *source, dns_dbnode_t **targetp) {

	/*
	 * Attach *targetp to source.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(source != NULL);
	REQUIRE(targetp != NULL && *targetp == NULL);

	(db->methods->attachnode)(db, source, targetp);
}

void
dns_db_detachnode(dns_db_t *db, dns_dbnode_t **nodep) {

	/*
	 * Detach *nodep from its node.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(nodep != NULL && *nodep != NULL);

	(db->methods->detachnode)(db, nodep);

	ENSURE(*nodep == NULL);
}

isc_result_t
dns_db_expirenode(dns_db_t *db, dns_dbnode_t *node, isc_stdtime_t now) {

	/*
	 * Mark as stale all records at 'node' which expire at or before 'now'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) != 0);
	REQUIRE(node != NULL);

	return ((db->methods->expirenode)(db, node, now));
}

void
dns_db_printnode(dns_db_t *db, dns_dbnode_t *node, FILE *out) {
	/*
	 * Print a textual representation of the contents of the node to
	 * 'out'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(node != NULL);

	(db->methods->printnode)(db, node, out);
}

/***
 *** DB Iterator Creation
 ***/

isc_result_t
dns_db_createiterator(dns_db_t *db, isc_boolean_t relative_names,
		      dns_dbiterator_t **iteratorp)
{
	/*
	 * Create an iterator for version 'version' of 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(iteratorp != NULL && *iteratorp == NULL);

	return (db->methods->createiterator(db, relative_names, iteratorp));
}

/***
 *** Rdataset Methods
 ***/

isc_result_t
dns_db_findrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
		    dns_rdatatype_t type, dns_rdatatype_t covers,
		    isc_stdtime_t now, dns_rdataset_t *rdataset,
		    dns_rdataset_t *sigrdataset)
{
	/*
	 * Search for an rdataset of type 'type' at 'node' that are in version
	 * 'version' of 'db'.  If found, make 'rdataset' refer to it.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(node != NULL);
	REQUIRE(DNS_RDATASET_VALID(rdataset));
	REQUIRE(! dns_rdataset_isassociated(rdataset));
	REQUIRE(covers == 0 || type == dns_rdatatype_rrsig);
	REQUIRE(type != dns_rdatatype_any);
	REQUIRE(sigrdataset == NULL ||
		(DNS_RDATASET_VALID(sigrdataset) &&
		 ! dns_rdataset_isassociated(sigrdataset)));

	return ((db->methods->findrdataset)(db, node, version, type, covers,
					    now, rdataset, sigrdataset));
}

isc_result_t
dns_db_allrdatasets(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
		    isc_stdtime_t now, dns_rdatasetiter_t **iteratorp)
{
	/*
	 * Make '*iteratorp' an rdataset iteratator for all rdatasets at
	 * 'node' in version 'version' of 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(iteratorp != NULL && *iteratorp == NULL);

	return ((db->methods->allrdatasets)(db, node, version, now,
					    iteratorp));
}

isc_result_t
dns_db_addrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
		   isc_stdtime_t now, dns_rdataset_t *rdataset,
		   unsigned int options, dns_rdataset_t *addedrdataset)
{
	/*
	 * Add 'rdataset' to 'node' in version 'version' of 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(node != NULL);
	REQUIRE(((db->attributes & DNS_DBATTR_CACHE) == 0 && version != NULL)||
		((db->attributes & DNS_DBATTR_CACHE) != 0 &&
		 version == NULL && (options & DNS_DBADD_MERGE) == 0));
	REQUIRE((options & DNS_DBADD_EXACT) == 0 ||
		(options & DNS_DBADD_MERGE) != 0);
	REQUIRE(DNS_RDATASET_VALID(rdataset));
	REQUIRE(dns_rdataset_isassociated(rdataset));
	REQUIRE(rdataset->rdclass == db->rdclass);
	REQUIRE(addedrdataset == NULL ||
		(DNS_RDATASET_VALID(addedrdataset) &&
		 ! dns_rdataset_isassociated(addedrdataset)));

	return ((db->methods->addrdataset)(db, node, version, now, rdataset,
					   options, addedrdataset));
}

isc_result_t
dns_db_subtractrdataset(dns_db_t *db, dns_dbnode_t *node,
			dns_dbversion_t *version, dns_rdataset_t *rdataset,
			unsigned int options, dns_rdataset_t *newrdataset)
{
	/*
	 * Remove any rdata in 'rdataset' from 'node' in version 'version' of
	 * 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(node != NULL);
	REQUIRE((db->attributes & DNS_DBATTR_CACHE) == 0 && version != NULL);
	REQUIRE(DNS_RDATASET_VALID(rdataset));
	REQUIRE(dns_rdataset_isassociated(rdataset));
	REQUIRE(rdataset->rdclass == db->rdclass);
	REQUIRE(newrdataset == NULL ||
		(DNS_RDATASET_VALID(newrdataset) &&
		 ! dns_rdataset_isassociated(newrdataset)));

	return ((db->methods->subtractrdataset)(db, node, version, rdataset,
						options, newrdataset));
}

isc_result_t
dns_db_deleterdataset(dns_db_t *db, dns_dbnode_t *node,
		      dns_dbversion_t *version, dns_rdatatype_t type,
		      dns_rdatatype_t covers)
{
	/*
	 * Make it so that no rdataset of type 'type' exists at 'node' in
	 * version version 'version' of 'db'.
	 */

	REQUIRE(DNS_DB_VALID(db));
	REQUIRE(node != NULL);
	REQUIRE(((db->attributes & DNS_DBATTR_CACHE) == 0 && version != NULL)||
		((db->attributes & DNS_DBATTR_CACHE) != 0 && version == NULL));

	return ((db->methods->deleterdataset)(db, node, version,
					      type, covers));
}

void 
dns_db_overmem(dns_db_t *db, isc_boolean_t overmem) {

	REQUIRE(DNS_DB_VALID(db));

	(db->methods->overmem)(db, overmem);
}

isc_result_t
dns_db_getsoaserial(dns_db_t *db, dns_dbversion_t *ver, isc_uint32_t *serialp)
{
	isc_result_t result;
	dns_dbnode_t *node = NULL;
	dns_rdataset_t rdataset;
	dns_rdata_t rdata = DNS_RDATA_INIT;
	isc_buffer_t buffer;

	REQUIRE(dns_db_iszone(db) || dns_db_isstub(db));

	result = dns_db_findnode(db, dns_db_origin(db), ISC_FALSE, &node);
	if (result != ISC_R_SUCCESS)
		return (result);

	dns_rdataset_init(&rdataset);
	result = dns_db_findrdataset(db, node, ver, dns_rdatatype_soa, 0,
				     (isc_stdtime_t)0, &rdataset, NULL);
 	if (result != ISC_R_SUCCESS)
		goto freenode;

	result = dns_rdataset_first(&rdataset);
 	if (result != ISC_R_SUCCESS)
		goto freerdataset;
	dns_rdataset_current(&rdataset, &rdata);
	result = dns_rdataset_next(&rdataset);
	INSIST(result == ISC_R_NOMORE);

	INSIST(rdata.length > 20);
	isc_buffer_init(&buffer, rdata.data, rdata.length);
	isc_buffer_add(&buffer, rdata.length);
	isc_buffer_forward(&buffer, rdata.length - 20);
	*serialp = isc_buffer_getuint32(&buffer);

	result = ISC_R_SUCCESS;

 freerdataset:
	dns_rdataset_disassociate(&rdataset);

 freenode:
	dns_db_detachnode(db, &node);
	return (result);
}

unsigned int
dns_db_nodecount(dns_db_t *db) {
	REQUIRE(DNS_DB_VALID(db));

	return ((db->methods->nodecount)(db));
}

void
dns_db_settask(dns_db_t *db, isc_task_t *task) {
	REQUIRE(DNS_DB_VALID(db));

	(db->methods->settask)(db, task);
}

isc_result_t
dns_db_register(const char *name, dns_dbcreatefunc_t create, void *driverarg,
		isc_mem_t *mctx, dns_dbimplementation_t **dbimp)
{
	dns_dbimplementation_t *imp;

	REQUIRE(name != NULL);
	REQUIRE(dbimp != NULL && *dbimp == NULL);

	RUNTIME_CHECK(isc_once_do(&once, initialize) == ISC_R_SUCCESS);

	RWLOCK(&implock, isc_rwlocktype_write);
	imp = impfind(name);
	if (imp != NULL) {
		RWUNLOCK(&implock, isc_rwlocktype_write);
		return (ISC_R_EXISTS);
	}
	
	imp = isc_mem_get(mctx, sizeof(dns_dbimplementation_t));
	if (imp == NULL) {
		RWUNLOCK(&implock, isc_rwlocktype_write);
		return (ISC_R_NOMEMORY);
	}
	imp->name = name;
	imp->create = create;
	imp->mctx = NULL;
	imp->driverarg = driverarg;
	isc_mem_attach(mctx, &imp->mctx);
	ISC_LINK_INIT(imp, link);
	ISC_LIST_APPEND(implementations, imp, link);
	RWUNLOCK(&implock, isc_rwlocktype_write);

	*dbimp = imp;

	return (ISC_R_SUCCESS);
}

void
dns_db_unregister(dns_dbimplementation_t **dbimp) {
	dns_dbimplementation_t *imp;
	isc_mem_t *mctx;

	REQUIRE(dbimp != NULL && *dbimp != NULL);

	RUNTIME_CHECK(isc_once_do(&once, initialize) == ISC_R_SUCCESS);

	imp = *dbimp;
	RWLOCK(&implock, isc_rwlocktype_write);
	ISC_LIST_UNLINK(implementations, imp, link);
	mctx = imp->mctx;
	isc_mem_put(mctx, imp, sizeof(dns_dbimplementation_t));
	isc_mem_detach(&mctx);
	RWUNLOCK(&implock, isc_rwlocktype_write);
}