put_record_into_cache( ulong recno, const char *data )
{
    CACHE_CTRL r, unused;
    int dirty_count = 0;
    int clean_count = 0;

    /* see whether we already cached this one */
    for( unused = NULL, r = cache_list; r; r = r->next ) {
	if( !r->flags.used ) {
	    if( !unused )
		unused = r;
	}
	else if( r->recno == recno ) {
	    if( !r->flags.dirty ) {
		/* Hmmm: should we use a a copy and compare? */
		if( memcmp(r->data, data, TRUST_RECORD_LEN ) ) {
		    r->flags.dirty = 1;
		    cache_is_dirty = 1;
		}
	    }
	    memcpy( r->data, data, TRUST_RECORD_LEN );
	    return 0;
	}
	if( r->flags.used ) {
	    if( r->flags.dirty )
		dirty_count++;
	    else
		clean_count++;
	}
    }
    /* not in the cache: add a new entry */
    if( unused ) { /* reuse this entry */
	r = unused;
	r->flags.used = 1;
	r->recno = recno;
	memcpy( r->data, data, TRUST_RECORD_LEN );
	r->flags.dirty = 1;
	cache_is_dirty = 1;
	cache_entries++;
	return 0;
    }
    /* see whether we reached the limit */
    if( cache_entries < MAX_CACHE_ENTRIES_SOFT ) { /* no */
	r = xmalloc( sizeof *r );
	r->flags.used = 1;
	r->recno = recno;
	memcpy( r->data, data, TRUST_RECORD_LEN );
	r->flags.dirty = 1;
	r->next = cache_list;
	cache_list = r;
	cache_is_dirty = 1;
	cache_entries++;
	return 0;
    }
    /* cache is full: discard some clean entries */
    if( clean_count ) {
	int n = clean_count / 3; /* discard a third of the clean entries */
	if( !n )
	    n = 1;
	for( unused = NULL, r = cache_list; r; r = r->next ) {
	    if( r->flags.used && !r->flags.dirty ) {
		if( !unused )
		    unused = r;
		r->flags.used = 0;
		cache_entries--;
		if( !--n )
		    break;
	    }
	}
	assert( unused );
	r = unused;
	r->flags.used = 1;
	r->recno = recno;
	memcpy( r->data, data, TRUST_RECORD_LEN );
	r->flags.dirty = 1;
	cache_is_dirty = 1;
	cache_entries++;
	return 0;
    }
    /* no clean entries: have to flush some dirty entries */
    if( in_transaction ) {
	/* but we can't do this while in a transaction
	 * we increase the cache size instead */
	if( cache_entries < MAX_CACHE_ENTRIES_HARD ) { /* no */
	    if( opt.debug && !(cache_entries % 100) )
		log_debug("increasing tdbio cache size\n");
	    r = xmalloc( sizeof *r );
	    r->flags.used = 1;
	    r->recno = recno;
	    memcpy( r->data, data, TRUST_RECORD_LEN );
	    r->flags.dirty = 1;
	    r->next = cache_list;
	    cache_list = r;
	    cache_is_dirty = 1;
	    cache_entries++;
	    return 0;
	}
	log_info(_("trustdb transaction too large\n"));
	return GPG_ERR_RESOURCE_LIMIT;
    }
    if( dirty_count ) {
	int n = dirty_count / 5; /* discard some dirty entries */
	if( !n )
	    n = 1;
	if( !is_locked ) {
	    if( dotlock_take( lockhandle, -1 ) )
		log_fatal("can't acquire lock - giving up\n");
	    else
		is_locked = 1;
	}
	for( unused = NULL, r = cache_list; r; r = r->next ) {
	    if( r->flags.used && r->flags.dirty ) {
		int rc = write_cache_item( r );
		if( rc )
		    return rc;
		if( !unused )
		    unused = r;
		r->flags.used = 0;
		cache_entries--;
		if( !--n )
		    break;
	    }
	}
	if( !opt.lock_once ) {
	    if( !dotlock_release( lockhandle ) )
		is_locked = 0;
	}
	assert( unused );
	r = unused;
	r->flags.used = 1;
	r->recno = recno;
	memcpy( r->data, data, TRUST_RECORD_LEN );
	r->flags.dirty = 1;
	cache_is_dirty = 1;
	cache_entries++;
	return 0;
    }
    BUG();
}