fix(balancer) fix ttl-0 records issues

* the previous record was not properly detected as a ttl=0 record
  by checking on the __ttl0flag we now do

* since the "fake" SRV record wasn't updated with a new expiry
  time the expiry-check-timer would keep updating that record
  every second

Fixes Kong issue Kong/kong#5477

README.md

@@ -43,6 +43,16 @@ History
 
 Versioning is strictly based on [Semantic Versioning](https://semver.org/)
 
+### 4.1.x (xx-xxx-2020)
+
+- Fix: fix ttl-0 records issues with the balancer, see Kong issue
+  https://github.com/Kong/kong/issues/5477
+  * the previous record was not properly detected as a ttl=0 record
+    by checking on the `__ttl0flag` we now do
+  * since the "fake" SRV record wasn't updated with a new expiry
+    time the expiry-check-timer would keep updating that record
+    every second
+
 ### 4.1.2 (10-Dec-2019)
 
 - Fix: handle cases when `lastQuery` is `nil`, see [PR 81](https://github.com/Kong/lua-resty-dns-client/pull/81)

src/resty/dns/balancer/base.lua

@@ -413,7 +413,8 @@ local function update_dns_result(self, newQuery, dns)
   -- 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
+  if (newQuery[1] or empty).ttl == 0 and
+     (((oldQuery[1] or empty).ttl or 0) == 0 or oldQuery.__ttl0Flag) 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
@@ -424,6 +425,8 @@ local function update_dns_result(self, newQuery, dns)
     -- so we fix them at the `nodeWeight` property, as with A and AAAA records.
     if oldQuery.__ttl0Flag then
       -- still ttl 0 so nothing changed
+      oldQuery.touched = time()
+      oldQuery.expire = oldQuery.touched + self.balancer.ttl0Interval
       ngx_log(ngx_DEBUG, self.log_prefix, "no dns changes detected for ",
               self.hostname, ", still using ttl=0")
       return true