... and that doesn't necessarily mean that malloc() failed.
We do *not* want to evict a heavy-hitter by an unfrequent element.

Note: even the implementation currently in master *did* return NULL,
so some parts of the code were just wrongly returning ENOMEM.

The implementation is now similar to set-associative caches
that x86 CPU use.  Also the API is changed a bit, leading to
slight simplification of our use patterns.

Fixes https://gitlab.labs.nic.cz/knot/resolver/issues/93.
API of a KR_EXPORT function is changed, so ABIVER is bumped.

Karel Slaný authored 8 years ago and Ondřej Surý committed 8 years ago

Introduced kr_rplan_push_empty() to create empty query for DNS cookies.

the e638f9fb introduced deduplication
for queries over UDP, the idea is to track same queries and answer
only once, however that introduces both constant processing and
memory overhead and could break clients who count packet loss
by queries sent/received. disabling for now.

when opening an empty database, an invalidated key
value was written resulting in possible corruption
of the cache

this change introduces new API for cache backends,
that is a subset of knot_db_api_t from libknot
with several cache-specific operations

major changes are:
* merged 'cachectl' module into 'cache' as it is
  99% default-on and it simplifies things
* not transaction oriented, transactions may be
  reused and cached for higher performance
* scatter/gather API, this is important for
  latency and performance of non-local backends
  like Redis
* faster and reliable cache clearing
* cache-specific operations (prefix scan, ...) in
  the API not hacked in
* simpler code for both backends and caller

* daemon now processes messages over TCP stream
out-of-order and concurrently
* support for TCP_DEFER_ACCEPT
* support for TCP Fast-Open
* there are now deadlines for TCP for idle/slow
streams (to prevent slowloris; pruning)
* there is now per-request limit on timeouts
(each request is allowed 4 timeouts before bailing)
* faster request closing, unified retry/timeout timers
* rare race condition in timer closing fixed

in permissive mode, resolver is free to use
(but not cache) non-mandatory glue records even
if they're not resolvable. this is great as a 
workaround for broken child-side zones, but
not great for security of, well, insecure
delegations. it's off by default.

the validator module should ignore any data that
will be scrubbed, that includes non-authoritative
data outside current bailiwick. previously, 
validator attempted to ignore these records only
for answer section and had a special case for NS
records.

cache: non-authoritative NS records are always
unchecked and must be treated as insecure

affected: www.iana.org trying to provide
delegation information for CNAME target, which is
moot with CNAME target explicit-fetch policy unless
the the resolver already knows DNSKEY with which
is could verify the records

updated tests

this is broken by changes in libknot2/db api,
needs to be done differently later

this is not going to be backwards compatible change, but it will be the first tagged libknot release sufficient for resolver