fix(balancer) fix accidental ttl=0 switches

See #51

Some servers will report ttl=0 when they are on the very edge
of their own cached ttl. This should never happen for a record
that has a non-0 ttl.

This fix makes sure we require ttl=0 reported twice in a row before
we switch the loadbalancer.

Fixes #51

README.md

@@ -43,6 +43,8 @@ Versioning is strictly based on [Semantic Versioning](https://semver.org/)
  `ttl` value of any valid answer received. [Issue 48](https://github.com/Kong/lua-resty-dns-client/issues/48).
 - Fix: remove multiline log entries, now encoded as single-line json. [Issue 52](https://github.com/Kong/lua-resty-dns-client/issues/52).
 - Fix: always inject a `localhost` value, even if not in `/etc/hosts`. [Issue 54](https://github.com/Kong/lua-resty-dns-client/issues/54).
+- Fix: added a workaround for Amazon Route 53 nameservers replying with a
+ `ttl=0` whilst the record has a non-0 ttl. [Issue 56](https://github.com/Kong/lua-resty-dns-client/issues/56).
 
 ### 2.1.0 (21-May-2018) Fixes
 

src/resty/dns/balancer.lua

@@ -358,7 +358,12 @@ function objHost:queryDns(cacheOnly)
  return true -- exit, nothing changed
  end
 
- if (newQuery[1] or empty).ttl == 0 then
+ -- To detect ttl = 0 we validate both the old and new record. This is done to ensure
+ -- we do not hit the edgecase of https://github.com/Kong/lua-resty-dns-client/issues/51
+ -- So if we get a ttl=0 twice in a row (the old one, and the new one), we update it. And
+ -- if the very first request ever reports ttl=0 (we assume we're not hitting the edgecase
+ -- in that case)
+ if (newQuery[1] or empty).ttl == 0 and ((oldQuery[1] or empty).ttl or 0) == 0 then
  -- ttl = 0 means we need to lookup on every request.
  -- To enable lookup on each request we 'abuse' a virtual SRV record. We set the ttl
  -- to `ttl0Interval` seconds, and set the `target` field to the hostname that needs