Return an error rather than do nothing when having no valid result

[comzyh]

Currently, if overture gets nothing from all of its backends, overture will not reply to the DNS query, cause the client to wait for a response.

Actually, overture should return something rather than keep the client waiting.

I'm facing this problem because many domains don't have an AAAA record. When using curl, curl wants to try ipv4 and ipv6, so curl make an AAAA query to the overture, but overture doesn't reply until timeout, cause curl to start ipv4 query very slow.

However, dig has the same problem.

time dig @127.0.0.1 -p 8053 qq.com AAAA

; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @127.0.0.1 -p 8053 qq.com AAAA
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached
dig @127.0.0.1 -p 8053 qq.com AAAA  0.01s user 0.01s system 0% cpu 15.012 total

Dig return after 15 seconds.

[comzyh]

Actually, response without answer indeed is an answer, and it can be cached. If backend returns a message that doesn't have Answer Section, Overture should treat it as an error.

overture/core/outbound/clients/client_remote_bundle.go

Lines 47 to 54 in 21fe533

	for _, o := range cb.clients {
	go func(c *RemoteClient, ch chan *RemoteClient) {
	c.Exchange(isLog)
	if common.HasAnswer(c.responseMessage) {
	ch <- c
	}
	ch <- nil
	}(o, ch)

I think this behavior(ignoring HasAnswer = false) is not correct. But I don't have a good solution.

[shawn1m]

Merged. The HasAnswer problem I will fix it sometime later.
There is no Answer Section when dig AAAA qq.com @114.114.114.114 (qq.com does not have an AAAA record right now). I think this is the common strategy to return a message that doesn't have Answer Section if there is actually no answer.

$ dig AAAA qq.com @114.114.114.114                                                                                                                               

; <<>> DiG 9.10.6 <<>> AAAA qq.com @114.114.114.114
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 30855
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;qq.com.				IN	AAAA

;; AUTHORITY SECTION:
qq.com.			582	IN	SOA	ns1.qq.com. webmaster.qq.com. 1330914143 3600 300 86400 300

;; Query time: 18 msec
;; SERVER: 114.114.114.114#53(114.114.114.114)
;; WHEN: Sun Mar 10 15:29:23 CST 2019
;; MSG SIZE  rcvd: 85