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Refactor stale lookups, ignore active RRsets

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When doing a staleonly lookup, ignore active RRsets from cache. If we
don't, we may add a duplicate RRset to the message, and hit an
assertion failure in query.c because adding the duplicate RRset to the
ANSWER section failed.

This can happen on a race condition. When a client query is received,
the recursion is started. When 'stale-answer-client-timeout' triggers
around the same time the recursion completes, the following sequence
of events may happen:
1. Queue the "try stale" fetch_callback() event to the client task.
2. Add the RRsets from the authoritative response to the cache.
3. Queue the "fetch complete" fetch_callback() event to the client task.
4. Execute the "try stale" fetch_callback(), which retrieves the
   just-inserted RRset from the database.
5. In "ns_query_done()" we are still recursing, but the "staleonly"
   query attribute has already been cleared. In other words, the
   query will resume when recursion ends (it already has ended but is
   still on the task queue).
6. Execute the "fetch complete" fetch_callback(). It finds the answer
   from recursion in the cache again and tries to add the duplicate to
   the answer section.

This commit changes the logic for finding stale answers in the cache,
such that on "stale_only" lookups actually only stale RRsets are
considered. It refactors the code so that code paths for "dbfind_stale",
"stale_refresh_window", and "stale_only" are more clear.

First we call some generic code that applies in all three cases,
formatting the domain name for logging purposes, increment the
trystale stats, and check if we actually found stale data that we can
use.

The "dbfind_stale" lookup will return SERVFAIL if we didn't found a
usable answer, otherwise we will continue with the lookup
(query_gotanswer()). This is no different as before the introduction of
"stale-answer-client-timeout" and "stale-refresh-time".

The "stale_refresh_window" lookup is similar to the "dbfind_stale"
lookup: return SERVFAIL if we didn't found a usable answer, otherwise
continue with the lookup (query_gotanswer()).

Finally the "stale_only" lookup.

If the "stale_only" lookup was triggered because of an actual client
timeout (stale-answer-client-timeout > 0), and if database lookup
returned a stale usable RRset, trigger a response to the client.
Otherwise return and wait until the recursion completes (or the
resolver query times out).

If the "stale_only" lookup is a "stale-anwer-client-timeout 0" lookup,
preferring stale data over a lookup. In this case if there was no stale
data, or the data was not a positive answer, retry the lookup with the
stale options cleared, a.k.a. a normal lookup. Otherwise, continue
with the lookup (query_gotanswer()) and refresh the stale RRset. This
will trigger a response to the client, but will not detach the handle
because a fetch will be created to refresh the RRset.
This commit is contained in:
Matthijs Mekking
2021-03-25 17:25:32 +01:00
parent fee164243f
commit 92f7a67892
1 changed files with 80 additions and 107 deletions
Download Patch File Download Diff File Expand all files Collapse all files

187
lib/ns/query.c
View File

@@ -5744,6 +5744,7 @@ query_lookup(query_ctx_t *qctx) {
dns_clientinfomethods_t cm;
dns_clientinfo_t ci;
dns_name_t *rpzqname = NULL;
char namebuf[DNS_NAME_FORMATSIZE];
unsigned int dboptions;
dns_ttl_t stale_refresh = 0;
bool dbfind_stale = false;
@@ -5861,7 +5862,8 @@ query_lookup(query_ctx_t *qctx) {
if (dbfind_stale ||
(STALE(qctx->rdataset) && (stale_only || stale_refresh_window)))
{
char namebuf[DNS_NAME_FORMATSIZE];
dns_name_format(qctx->client->query.qname, namebuf,
sizeof(namebuf));
inc_stats(qctx->client, ns_statscounter_trystale);
@@ -5874,127 +5876,98 @@ query_lookup(query_ctx_t *qctx) {
} else {
stale_found = false;
}
}
dns_name_format(qctx->client->query.qname, namebuf,
sizeof(namebuf));
if (dbfind_stale) {
isc_log_write(ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s resolver failure, stale answer %s",
namebuf,
stale_found ? "used" : "unavailable");
if (!stale_found) {
/*
* Resolver failure, no stale data, nothing
* more we can do, return SERVFAIL.
*/
QUERY_ERROR(qctx, DNS_R_SERVFAIL);
return (ns_query_done(qctx));
}
stale_only = false;
} else if (stale_refresh_window) {
if (dbfind_stale) {
isc_log_write(ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s resolver failure, stale answer %s", namebuf,
stale_found ? "used" : "unavailable");
if (!stale_found) {
/*
* A recent lookup failed, so during this time window
* we are allowed to return stale data immediately.
* Resolver failure, no stale data, nothing more we
* can do, return SERVFAIL.
*/
isc_log_write(ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s query within stale refresh time, "
"stale answer %s",
namebuf,
stale_found ? "used" : "unavailable");
QUERY_ERROR(qctx, DNS_R_SERVFAIL);
return (ns_query_done(qctx));
}
qctx->client->nodetach = false;
} else if (stale_refresh_window) {
/*
* A recent lookup failed, so during this time window we are
* allowed to return stale data immediately.
*/
isc_log_write(ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s query within stale refresh time, stale "
"answer %s",
namebuf, stale_found ? "used" : "unavailable");
if (!stale_found) {
if (!stale_found) {
/*
* During the stale refresh window explicitly do not try
* to refresh the data, because a recent lookup failed.
*/
QUERY_ERROR(qctx, DNS_R_SERVFAIL);
return (ns_query_done(qctx));
}
qctx->client->nodetach = false;
} else if (stale_only) {
/*
* This is a staleonly lookup: if there wass no stale answer
* in cache, treat as we don't have an answer and wait for the
* resolver fetch to finish.
*/
qctx->client->nodetach = !stale_found;
if ((qctx->options & DNS_GETDB_STALEFIRST) != 0) {
if (!stale_found || result != ISC_R_SUCCESS) {
/*
* During the stale refresh window explicitly
* do not try to refresh the data, because a
* recent lookup failed.
* We have nothing useful in cache to return
* immediately.
*/
QUERY_ERROR(qctx, DNS_R_SERVFAIL);
return (ns_query_done(qctx));
}
stale_only = false;
} else if ((dboptions & DNS_DBFIND_STALEONLY) != 0) {
if ((qctx->options & DNS_GETDB_STALEFIRST) != 0) {
if (stale_found && result == ISC_R_SUCCESS) {
/*
* Immediately return the stale answer,
* start a resolver fetch to refresh
* the data in cache.
*/
isc_log_write(
ns_lctx,
NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY,
ISC_LOG_INFO,
"%s stale answer used, an "
"attempt to refresh the RRset "
"will still be made",
namebuf);
refresh_rrset = true;
} else {
/*
* We have nothing useful in cache to
* return immediately.
*/
qctx_clean(qctx);
qctx_freedata(qctx);
dns_db_attach(
qctx->client->view->cachedb,
&qctx->db);
qctx->client->query.dboptions &=
~DNS_DBFIND_STALEONLY;
qctx->client->nodetach = false;
qctx->options &= ~DNS_GETDB_STALEFIRST;
if (qctx->client->query.fetch != NULL) {
dns_resolver_destroyfetch(
&qctx->client->query
.fetch);
}
return query_lookup(qctx);
qctx_clean(qctx);
qctx_freedata(qctx);
dns_db_attach(qctx->client->view->cachedb,
&qctx->db);
qctx->client->query.dboptions &=
~DNS_DBFIND_STALEONLY;
qctx->options &= ~DNS_GETDB_STALEFIRST;
if (qctx->client->query.fetch != NULL) {
dns_resolver_destroyfetch(
&qctx->client->query.fetch);
}
return query_lookup(qctx);
} else {
INSIST(stale_found);
INSIST(result == ISC_R_SUCCESS);
/*
* The 'stale-answer-client-timeout' triggered,
* return the stale answer if available,
* otherwise wait until the resolver finishes.
* Immediately return the stale answer, start a
* resolver fetch to refresh the data in cache.
*/
isc_log_write(
ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s client timeout, stale answer %s",
namebuf,
stale_found ? "used" : "unavailable");
if (!stale_found || result != ISC_R_SUCCESS) {
return (result);
}
"%s stale answer used, an attempt to "
"refresh the RRset will still be made",
namebuf);
refresh_rrset = STALE(qctx->rdataset);
}
} else {
/*
* The 'stale-answer-client-timeout' triggered, return
* the stale answer if available, otherwise wait until
* the resolver finishes.
*/
isc_log_write(ns_lctx, NS_LOGCATEGORY_SERVE_STALE,
NS_LOGMODULE_QUERY, ISC_LOG_INFO,
"%s client timeout, stale answer %s",
namebuf,
stale_found ? "used" : "unavailable");
if (!stale_found || result != ISC_R_SUCCESS) {
return (result);
}
}
} else if (stale_only && result != ISC_R_SUCCESS) {
/*
* This is a staleonly lookup and no stale answer was found
* in cache. Treat as we don't have an answer and wait for
* the resolver fetch to finish.
*/
if ((qctx->options & DNS_GETDB_STALEFIRST) == 0) {
return (result);
}
} else {
stale_only = false;
}
if (!stale_only) {
/*
* The DNS_DBFIND_STALEONLY option may be set, but if we
* didn't have stale data in cache, or we responded with
* a stale answer because of 'stale-refresh-time', this is
* actually not a 'stale-only' lookup. Clear the flag to
* allow the client to be detach the handle.
*/
qctx->client->nodetach = false;
}
result = query_gotanswer(qctx, result);