/*
 * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, you can obtain one at https://mozilla.org/MPL/2.0/.
 *
 * See the COPYRIGHT file distributed with this work for additional
 * information regarding copyright ownership.
 */

#ifndef DNS_ACACHE_H
#define DNS_ACACHE_H 1

/*****
 ***** Module Info
 *****/

/*
 * Acache
 *
 * The Additional Cache Object
 *
 *	This module manages internal caching entries that correspond to
 *	the additional section data of a DNS DB node (an RRset header, more
 *	accurately).  An additional cache entry is expected to be (somehow)
 *	attached to a particular RR in a particular DB node, and contains a set
 *	of information of an additional data for the DB node.
 *
 *	An additional cache object is intended to be created as a per-view
 *	object, and manages all cache entries within the view.
 *
 *	The intended usage of the additional caching is to provide a short cut
 *	to additional glue RRs of an NS RR.  For each NS RR, it is often
 *	necessary to look for glue RRs to make a proper response.  Once the
 *	glue RRs are known, the additional caching allows the client to
 *	associate the information to the original NS RR so that further
 *	expensive lookups can be avoided for the NS RR.
 *
 *	Each additional cache entry contains information to identify a
 *	particular DB node and (optionally) an associated RRset.  The
 *	information consists of its zone, database, the version of the
 *	database, database node, and RRset.
 *
 *	A "negative" information can also be cached.  For example, if a glue
 *	RR does not exist as an authoritative data in the same zone as that
 *	of the NS RR, this fact can be cached by specifying a NULL pointer
 *	for the database, version, and node.  (See the description for
 *	dns_acache_getentry() below for more details.)
 *
 *	Since each member stored in an additional cache entry holds a reference
 *	to a corresponding object, a stale cache entry may cause unnecessary
 *	memory consumption.  For instance, when a zone is reloaded, additional
 *	cache entries that have a reference to the zone (and its DB and/or
 *	DB nodes) can delay the cleanup of the referred objects.  In order to
 *	minimize such a bad effect, this module provides several cleanup
 *	mechanisms.
 *
 *	The first one is a shutdown procedure called when the associated view
 *	is shut down.  In this case, dns_acache_shutdown() will be called and
 *	all cache entries will be purged.  This mechanism will help the
 *	situation when the configuration is reloaded or the main server is
 *	stopped.
 *
 *	Per-DB cleanup mechanism is also provided.  Each additional cache entry
 *	is associated with related DB, which is expected to have been
 *	registered when the DB was created by dns_acache_setdb().  If a
 *	particular DB is going to be destroyed, the primary holder of the DB,
 *	a typical example of which is a zone, will call dns_acache_putdb().
 *	Then this module will clean-up all cache entries associated with the
 *	DB.  This mechanism is effective when a secondary zone DB is going to
 *	be stale after a zone transfer.
 *
 *	Finally, this module supports for periodic clean-up of stale entries.
 *	Each cache entry has a timestamp field, which is updated every time
 *	the entry is referred.  A periodically invoked cleaner checks the
 *	timestamp of each entry, and purge entries that have not been referred
 *	for a certain period.  The cleaner interval can be specified by
 *	dns_acache_setcleaninginterval().  If the periodic clean-up is not
 *	enough, it is also possible to specify the upper limit of entries
 *	in terms of the memory consumption.  If the maximum value is
 *	specified, the cleaner is invoked when the memory consumption reaches
 *	the high watermark inferred from the maximum value.  In this case,
 *	the cleaner will use more aggressive algorithm to decide the "victim"
 *	entries.  The maximum value can be specified by
 *	dns_acache_setcachesize().
 *
 *	When a cache entry is going to be purged within this module, the
 *	callback function specified at the creation time will be called.
 *	The callback function is expected to release all internal resources
 *	related to the entry, which will typically be specific to DB
 *	implementation, and to call dns_acache_detachentry().  The callback
 *	mechanism is very important, since the holder of an additional cache
 *	entry may not be able to initiate the clean-up of the entry, due to
 *	the reference ordering.  For example, as long as an additional cache
 *	entry has a reference to a DB object, the DB cannot be freed, in which
 *	a DB node may have a reference to the cache entry.
 *
 *	Credits:
 *	The basic idea of this kind of short-cut for frequently used
 *	information is similar to the "pre-compiled answer" approach adopted
 *	in nsd by NLnet LABS with RIPE NCC.  Our work here is an independent
 *	effort, but the success of nsd encouraged us to pursue this path.
 *
 *	The design and implementation of the periodic memory management and
 *	the upper limitation of memory consumption was derived from the cache
 *	DB implementation of BIND9.
 *
 * MP:
 *	There are two main locks in this module.  One is for each entry, and
 *	the other is for the additional cache object.
 *
 * Reliability:
 *	The callback function for a cache entry is called with holding the
 *	entry lock.  Thus, it implicitly assumes the callback function does not
 *	call a function that can require the lock.  Typically, the only
 *	function that can be called from the callback function safely is
 *	dns_acache_detachentry().  The breakage of this implicit assumption
 *	may cause a deadlock.
 *
 * Resources:
 *	In a 32-bit architecture (such as i386), the following additional
 *	memory is required comparing to the case that disables this module.
 *	- 76 bytes for each additional cache entry
 *	- if the entry has a DNS name and associated RRset,
 *	  * 44 bytes + size of the name (1-255 bytes)
 *	  * 52 bytes x number_of_RRs
 *	- 28 bytes for each DB related to this module
 *
 *	Using the additional cache also requires extra memory consumption in
 *	the DB implementation.  In the current implementation for rbtdb, we
 *	need:
 *	- two additional pointers for each DB node (8 bytes for a 32-bit
 *	  architecture
 *	- for each RR associated to an RR in a DB node, we also need
 *	  a pointer and management objects to support the additional cache
 *	  function.  These are allocated on-demand.  The total size is
 *	  32 bytes for a 32-bit architecture.
 *
 * Security:
 *	Since this module does not handle any low-level data directly,
 *	no security issue specific to this module is anticipated.
 *
 * Standards:
 *	None.
 */

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

#include <isc/mutex.h>
#include <isc/lang.h>
#include <isc/stdtime.h>

#include <dns/types.h>

/***
 *** Functions
 ***/
ISC_LANG_BEGINDECLS

isc_result_t
dns_acache_create(dns_acache_t **acachep, isc_mem_t *mctx,
		  isc_taskmgr_t *taskmgr, isc_timermgr_t *timermgr);
/*
 * Create a new DNS additional cache object.
 *
 * Requires:
 *
 *	'mctx' is a valid memory context
 *
 *	'taskmgr' is a valid task manager
 *
 *	'timermgr' is a valid timer or NULL.  If NULL, no periodic cleaning of
 *	the cache will take place.
 *
 *	'acachep' is a valid pointer, and *acachep == NULL
 *
 * Ensures:
 *
 *	'*acachep' is attached to the newly created cache
 *
 * Returns:
 *
 *	ISC_R_SUCCESS
 *	ISC_R_NOMEMORY
 *	ISC_R_UNEXPECTED
 */

void
dns_acache_attach(dns_acache_t *source, dns_acache_t **targetp);
/*
 * Attach *targetp to cache.
 *
 * Requires:
 *
 *	'acache' is a valid additional cache.
 *
 *	'targetp' points to a NULL dns_acache_t *.
 *
 * Ensures:
 *
 *	*targetp is attached to the 'source' additional cache.
 */

void
dns_acache_detach(dns_acache_t **acachep);
/*
 * Detach *acachep from its cache.
 *
 * Requires:
 *
 *	'*acachep' points to a valid additional cache.
 *
 * Ensures:
 *
 *	*acachep is NULL.
 *
 *	If '*acachep' is the last reference to the cache and the additional
 *	cache does not have an outstanding task, all resources used by the
 *	cache will be freed.
 */

void
dns_acache_setcleaninginterval(dns_acache_t *acache, unsigned int t);
/*
 * Set the periodic cleaning interval of an additional cache to 'interval'
 * seconds.
 */

void
dns_acache_setcachesize(dns_acache_t *acache, size_t size);
/*
 * Set the maximum additional cache size.  0 means unlimited.
 */

isc_result_t
dns_acache_setdb(dns_acache_t *acache, dns_db_t *db);
/*
 * Set 'db' in 'acache' when the db can be referred from acache, in order
 * to provide a hint for resolving the back reference.
 *
 * Requires:
 *	'acache' is a valid acache pointer.
 *	'db' is a valid DNS DB pointer.
 *
 * Ensures:
 *	'acache' will have a reference to 'db'.
 *
 * Returns:
 *	ISC_R_SUCCESS
 *	ISC_R_EXISTS	(which means the specified 'db' is already set)
 *	ISC_R_NOMEMORY
 */

isc_result_t
dns_acache_putdb(dns_acache_t *acache, dns_db_t *db);
/*
 * Release 'db' from 'acache' if it has been set by dns_acache_setdb().
 *
 * Requires:
 *	'acache' is a valid acache pointer.
 *	'db' is a valid DNS DB pointer.
 *
 * Ensures:
 *	'acache' will release the reference to 'db'.  Additionally, the content
 *	of each cache entry that is related to the 'db' will be released via
 *	the callback function.
 *
 * Returns:
 *	ISC_R_SUCCESS
 *	ISC_R_NOTFOUND	(which means the specified 'db' is not set in 'acache')
 *	ISC_R_NOMEMORY
 */

void
dns_acache_shutdown(dns_acache_t *acache);
/*
 * Shutdown 'acache'.
 *
 * Requires:
 *
 * 	'*acache' is a valid additional cache.
 */

isc_result_t
dns_acache_createentry(dns_acache_t *acache, dns_db_t *origdb,
		       void (*callback)(dns_acacheentry_t *, void **),
		       void *cbarg, dns_acacheentry_t **entryp);
/*
 * Create an additional cache entry.  A new entry is created and attached to
 * the given additional cache object.  A callback function is also associated
 * with the created entry, which will be called when the cache entry is purged
 * for some reason.
 *
 * Requires:
 *
 * 	'acache' is a valid additional cache.
 *	'entryp' is a valid pointer, and *entryp == NULL
 *	'origdb' is a valid DNS DB pointer.
 *	'callback' and 'cbarg' can be NULL.  In this case, however, the entry
 *	is meaningless (and will be cleaned-up in the next periodical
 *	cleaning).
 *
 * Ensures:
 *	'*entryp' will point to a new additional cache entry.
 *
 * Returns:
 *	ISC_R_SUCCESS
 *	ISC_R_NOMEMORY
 */

isc_result_t
dns_acache_getentry(dns_acacheentry_t *entry, dns_zone_t **zonep,
		    dns_db_t **dbp, dns_dbversion_t **versionp,
		    dns_dbnode_t **nodep, dns_name_t *fname,
		    dns_message_t *msg, isc_stdtime_t now);
/*
 * Get content from a particular additional cache entry.
 *
 * Requires:
 *
 * 	'entry' is a valid additional cache entry.
 *	'zonep' is a NULL pointer or '*zonep' == NULL (this is the only
 *	optional parameter.)
 *	'dbp' is a valid pointer, and '*dbp' == NULL
 *	'versionp' is a valid pointer, and '*versionp' == NULL
 *	'nodep' is a valid pointer, and '*nodep' == NULL
 *	'fname' is a valid DNS name.
 *	'msg' is a valid DNS message.
 *
 * Ensures:
 *	Several possible cases can happen according to the content.
 *	1. For a positive cache entry,
 *	'*zonep' will point to the corresponding zone (if zonep is a valid
 *	pointer),
 *	'*dbp' will point to a DB for the zone,
 *	'*versionp' will point to its version, and
 *	'*nodep' will point to the corresponding DB node.
 *	'fname' will have the DNS name of the DB node and contain a list of
 *	rdataset for the node (which can be an empty list).
 *
 * 	2. For a negative cache entry that means no corresponding zone exists,
 *	'*zonep' == NULL (if zonep is a valid pointer)
 *	'*dbp', '*versionp', and '*nodep' will be NULL.
 *
 *	3. For a negative cache entry that means no corresponding DB node
 *	exists, '*zonep' will point to the corresponding zone (if zonep is a
 *	valid pointer),
 *	'*dbp' will point to a corresponding DB for zone,
 *	'*versionp' will point to its version.
 *	'*nodep' will be kept as NULL.
 *	'fname' will not change.
 *
 *	On failure, no new references will be created.
 *
 * Returns:
 *	ISC_R_SUCCESS
 *	ISC_R_NOMEMORY
 */

isc_result_t
dns_acache_setentry(dns_acache_t *acache, dns_acacheentry_t *entry,
		    dns_zone_t *zone, dns_db_t *db, dns_dbversion_t *version,
		    dns_dbnode_t *node, dns_name_t *fname);
/*
 * Set content to a particular additional cache entry.
 *
 * Requires:
 *	'acache' is a valid additional cache.
 *	'entry' is a valid additional cache entry.
 *	All the others pointers are NULL or a valid pointer of the
 *	corresponding type.
 *
 * Returns:
 *	ISC_R_SUCCESS
 *	ISC_R_NOMEMORY
 *	ISC_R_NOTFOUND
 */

bool
dns_acache_cancelentry(dns_acacheentry_t *entry);
/*
 * Cancel the use of the cache entry 'entry'.  This function is supposed to
 * be called when the node that holds the entry finds the content is not
 * correct any more.  This function will try to release as much dependency as
 * possible, and will be ready to be cleaned-up.  The registered callback
 * function will be canceled and will never called.
 *
 * Requires:
 *	'entry' is a valid additional cache entry.
 *
 * Returns:
 * 	true if the entry was active when canceled
 */

void
dns_acache_attachentry(dns_acacheentry_t *source, dns_acacheentry_t **targetp);
/*
 * Attach *targetp to the cache entry 'source'.
 *
 * Requires:
 *
 *	'source' is a valid additional cache entry.
 *
 *	'targetp' points to a NULL dns_acacheentry_t *.
 *
 * Ensures:
 *
 *	*targetp is attached to 'source'.
 */

void
dns_acache_detachentry(dns_acacheentry_t **entryp);
/*
 * Detach *entryp from its cache.
 *
 * Requires:
 *
 *	'*entryp' points to a valid additional cache entry.
 *
 * Ensures:
 *
 *	*entryp is NULL.
 *
 *	If '*entryp' is the last reference to the entry,
 *	cache does not have an outstanding task, all resources used by the
 *	entry (including the entry object itself) will be freed.
 */

void
dns_acache_countquerymiss(dns_acache_t *acache);
/*
 * Count up a missed acache query.  XXXMLG need more docs.
 */

ISC_LANG_ENDDECLS

#endif /* DNS_ACACHE_H */