cache_handler.c

snmp的源代码,已经在我的ubuntu下编译通过

#include <net-snmp/net-snmp-config.h>

#if HAVE_STRING_H
#include <string.h>
#else
#include <strings.h>
#endif

#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/agent/net-snmp-agent-includes.h>

#include <net-snmp/agent/cache_handler.h>

#if HAVE_DMALLOC_H
#include <dmalloc.h>
#endif

static netsnmp_cache  *cache_head = NULL;
static int             cache_outstanding_valid = 0;
static int             _cache_load( netsnmp_cache *cache );

#define CACHE_RELEASE_FREQUENCY 60      /* Check for expired caches every 60s */

void            release_cached_resources(unsigned int regNo,
                                         void *clientargs);

/** @defgroup cache_handler cache_handler: Maintains a cache of data for use by lower level handlers.
 *  @ingroup utilities
 *  This helper checks to see whether the data has been loaded "recently"
 *  (according to the timeout for that particular cache) and calls the
 *  registered "load_cache" routine if necessary.
 *  The lower handlers can then work with this local cached data.
 *
 *  A timeout value of -1 will cause netsnmp_cache_check_expired() to
 *  always return true, and thus the cache will be reloaded for every
 *  request.
 *
 *  To minimze resource use by the agent, a periodic callback checks for
 *  expired caches, and will call the free_cache function for any expired
 *  cache.
 *
 *  The load_cache route should return a negative number if the cache
 *  was not successfully loaded. 0 or any positive number indicates successs.
 *
 *
 *  Several flags can be set to affect the operations on the cache.
 *
 *  If NETSNMP_CACHE_DONT_INVALIDATE_ON_SET is set, the free_cache method
 *  will not be called after a set request has processed. It is assumed that
 *  the lower mib handler using the cache has maintained cache consistency.
 *
 *  If NETSNMP_CACHE_DONT_FREE_BEFORE_LOAD is set, the free_cache method
 *  will not be called before the load_cache method is called. It is assumed
 *  that the load_cache routine will properly deal with being called with a
 *  valid cache.
 *
 *  If NETSNMP_CACHE_DONT_FREE_EXPIRED is set, the free_cache method will
 *  not be called with the cache expires. The expired flag will be set, but
 *  the valid flag will not be cleared. It is assumed that the load_cache
 *  routine will properly deal with being called with a valid cache.
 *
 *  If NETSNMP_CACHE_PRELOAD is set when a the cache handler is created,
 *  the cache load routine will be called immediately.
 *
 *  If NETSNMP_CACHE_DONT_AUTO_RELEASE is set, the periodic callback that
 *  checks for expired caches will skip the cache. The cache will only be
 *  checked for expiration when a request triggers the cache handler. This
 *  is useful if the cache has it's own periodic callback to keep the cache
 *  fresh.
 *
 *  If NETSNMP_CACHE_AUTO_RELOAD is set, a timer will be set up to reload
 *  the cache when it expires. This is useful for keeping the cache fresh,
 *  even in the absence of incoming snmp requests.
 *
 *
 *  Here are some suggestions for some common situations.
 *
 *  Cached File:
 *      If your table is based on a file that may periodically change,
 *      you can test the modification date to see if the file has
 *      changed since the last cache load. To get the cache helper to call
 *      the load function for every request, set the timeout to -1, which
 *      will cause the cache to always report that it is expired. This means
 *      that you will want to prevent the agent from flushing the cache when
 *      it has expired, and you will have to flush it manually if you
 *      detect that the file has changed. To accomplish this, set the
 *      following flags:
 *
 *          NETSNMP_CACHE_DONT_FREE_EXPIRED
 *          NETSNMP_CACHE_DONT_AUTO_RELEASE
 *
 *
 *  Constant (periodic) reload:
 *      If you want the cache kept up to date regularly, even if no requests
 *      for the table are received, you can have your cache load routine
 *      called periodically. This is very useful if you need to monitor the
 *      data for changes (eg a <i>LastChanged</i> object). You will need to
 *      prevent the agent from flushing the cache when it expires. Set the
 *      cache timeout to the frequency, in seconds, that you wish to
 *      reload your cache, and set the following flags:
 *
 *          NETSNMP_CACHE_DONT_FREE_EXPIRED
 *          NETSNMP_CACHE_DONT_AUTO_RELEASE
 *          NETSNMP_CACHE_AUTO_RELOAD
 *
 *  @{
 */

/** get cache head
 * @internal
 * unadvertised function to get cache head. You really should not
 * do this, since the internal storage mechanism might change.
 */
netsnmp_cache *
netsnmp_cache_get_head(void)
{
    return cache_head;
}

/** find existing cache
 */
netsnmp_cache *
netsnmp_cache_find_by_oid(oid * rootoid, int rootoid_len)
{
    netsnmp_cache  *cache;

    for (cache = cache_head; cache; cache = cache->next) {
        if (0 == netsnmp_oid_equals(cache->rootoid, cache->rootoid_len,
                                    rootoid, rootoid_len))
            return cache;
    }
    
    return NULL;
}

/** returns a cache
 */
netsnmp_cache *
netsnmp_cache_create(int timeout, NetsnmpCacheLoad * load_hook,
                     NetsnmpCacheFree * free_hook,
                     oid * rootoid, int rootoid_len)
{
    netsnmp_cache  *cache = NULL;

    cache = SNMP_MALLOC_TYPEDEF(netsnmp_cache);
    if (NULL == cache) {
        snmp_log(LOG_ERR,"malloc error in netsnmp_cache_create\n");
        return NULL;
    }
    cache->timeout = timeout;
    cache->load_cache = load_hook;
    cache->free_cache = free_hook;
    cache->enabled = 1;

    if(0 == cache->timeout)
        cache->timeout = netsnmp_ds_get_int(NETSNMP_DS_APPLICATION_ID,
                                            NETSNMP_DS_AGENT_CACHE_TIMEOUT);

    
    /*
     * Add the registered OID information, and tack
     * this onto the list for cache SNMP management
     *
     * Note that this list is not ordered.
     *    table_iterator rules again!
     */
    if (rootoid) {
        cache->rootoid = snmp_duplicate_objid(rootoid, rootoid_len);
        cache->rootoid_len = rootoid_len;
        cache->next = cache_head;
        if (cache_head)
            cache_head->prev = cache;
        cache_head = cache;
    }

    return cache;
}

/** callback function to call cache load function */
static void
_timer_reload(unsigned int regNo, void *clientargs)
{
    netsnmp_cache *cache = (netsnmp_cache *)clientargs;

    DEBUGMSGT(("cache_timer:start", "loading cache %p\n", cache));

    cache->expired = 1;

    _cache_load(cache);
}

/** starts the recurring cache_load callback */
unsigned int
netsnmp_cache_timer_start(netsnmp_cache *cache)
{
    if(NULL == cache)
        return 0;

    if(0 != cache->timer_id) {
        snmp_log(LOG_WARNING, "cache has existing timer id.\n");
        return cache->timer_id;
    }
    
    if(! (cache->flags & NETSNMP_CACHE_AUTO_RELOAD)) {
        snmp_log(LOG_ERR,
                 "cache_timer_start called but auto_reload not set.\n");
        return 0;
    }

    cache->timer_id = snmp_alarm_register(cache->timeout, SA_REPEAT,
                                          _timer_reload, cache);
    if(0 == cache->timer_id) {
        snmp_log(LOG_ERR,"could not register alarm\n");
        return 0;
    }

    DEBUGMSGT(("cache_timer:start",
               "starting timer %d for cache %p\n", cache->timer_id, cache));
    return cache->timer_id;
}

/** stops the recurring cache_load callback */
void
netsnmp_cache_timer_stop(netsnmp_cache *cache)
{
    if(NULL == cache)
        return;

    if(0 == cache->timer_id) {
        snmp_log(LOG_WARNING, "cache has no timer id.\n");
        return;
    }

    DEBUGMSGT(("cache_timer:stop",
               "stopping timer %d for cache %p\n", cache->timer_id, cache));

    snmp_alarm_unregister(cache->timer_id);
}


/** returns a cache handler that can be injected into a given handler chain.  
 */
netsnmp_mib_handler *
netsnmp_cache_handler_get(netsnmp_cache* cache)
{
    netsnmp_mib_handler *ret = NULL;
    
    ret = netsnmp_create_handler("cache_handler",
                                 netsnmp_cache_helper_handler);
    if (ret) {
        ret->flags |= MIB_HANDLER_AUTO_NEXT;
        ret->myvoid = (void *) cache;
        
        if(NULL != cache) {
            if ((cache->flags & NETSNMP_CACHE_PRELOAD) && ! cache->valid) {
                /*
                 * load cache, ignore rc
                 * (failed load doesn't affect registration)
                 */
                (void)_cache_load(cache);
            }
            if (cache->flags & NETSNMP_CACHE_AUTO_RELOAD)
                netsnmp_cache_timer_start(cache);
            
        }
    }
    return ret;
}

/** returns a cache handler that can be injected into a given handler chain.  
 */
netsnmp_mib_handler *
netsnmp_get_cache_handler(int timeout, NetsnmpCacheLoad * load_hook,
                          NetsnmpCacheFree * free_hook,
                          oid * rootoid, int rootoid_len)
{
    netsnmp_mib_handler *ret = NULL;
    netsnmp_cache  *cache = NULL;

    ret = netsnmp_cache_handler_get(NULL);
    if (ret) {
        cache = netsnmp_cache_create(timeout, load_hook, free_hook,
                                     rootoid, rootoid_len);
        ret->myvoid = (void *) cache;
    }
    return ret;
}

/** functionally the same as calling netsnmp_register_handler() but also
 * injects a cache handler at the same time for you. */
int
netsnmp_cache_handler_register(netsnmp_handler_registration * reginfo,
                               netsnmp_cache* cache)
{
    netsnmp_mib_handler *handler = NULL;
    handler = netsnmp_cache_handler_get(cache);

    netsnmp_inject_handler(reginfo, handler);
    return netsnmp_register_handler(reginfo);
}

/** functionally the same as calling netsnmp_register_handler() but also
 * injects a cache handler at the same time for you. */
int
netsnmp_register_cache_handler(netsnmp_handler_registration * reginfo,