etcdmain: support SRV discovery for gRPC proxy

[fanminshi]

FIX #7562

[heyitsanthony]

try to limit indent depth:

if dns == "" {
    return nil
}

[heyitsanthony]

can exit early here too:

if insecure {
    return endpoints
}

[fanminshi]

All fixed. PTAL

[heyitsanthony]

lgtm

[xiang90]

@fanminshi can you try this out manually? we can merge it when we know it works.

[fanminshi]

@xiang90 will do.

[fanminshi]

test results; succeeded.

# Insecure 

# dns records
example.com.    IN    SOA    bindhostname. admin.example.com. (
1452607488
10800
3600
604800
38400 )
example.com.    IN    NS    bindhostname.
infra0.example.com.    300    IN    A    127.0.0.1
_etcd-client._tcp    300    IN    SRV    0 0 2379 infra0.example.com.
_etcd-server._tcp    300    IN    SRV    0 0 2380 infra0.example.com.


$ bin/etcd \
--name infra0 \
--data-dir /var/lib/etcd \
--discovery-srv example.com \
--listen-client-urls http://127.0.0.1:2379 \
--advertise-client-urls http://127.0.0.1:2379 \
--listen-peer-urls http://127.0.0.1:2380 \
--initial-advertise-peer-urls http://127.0.0.1:2380 \
--initial-cluster-token my-etcd-token \
--initial-cluster-state new \
--initial-cluster infra0=http://127.0.0.1:2380 \
  
  
$ bin/etcd grpc-proxy start --discovery-srv example.com --insecure-discovery
2017-04-06 18:01:45.425201 I | etcdmain: discovered the cluster [http://infra0.example.com.:2379] from example.com
2017-04-06 18:01:45.425289 I | etcdmain: listening for grpc-proxy client requests on 127.0.0.1:23790
2017-04-06 18:01:45.426274 E | etcdmain: forgot to set Type=notify in systemd service file?

# Secure

# dns record
example.com.    IN    SOA    bindhostname. admin.example.com. (
1452607488
10800
3600
604800
38400 )
example.com.    IN    NS    bindhostname.
infra0.example.com.    300    IN    A    127.0.0.1
_etcd-client-ssl._tcp    300    IN    SRV    0 0 2379 infra0.example.com.
_etcd-server-ssl._tcp    300    IN    SRV    0 0 2380 infra0.example.com.

$ bin/etcd --name infra0 \
    --data-dir /var/lib/etcd \
    --discovery-srv example.com \
    --listen-client-urls https://127.0.0.1:2379 \
    --advertise-client-urls https://127.0.0.1:2379 \
    --listen-peer-urls https://127.0.0.1:2380 \
    --initial-advertise-peer-urls https://127.0.0.1:2380 \
    --initial-cluster-token my-etcd-token \
    --initial-cluster-state new \
    --auto-compaction-retention 1 \
    --client-cert-auth \
    --trusted-ca-file /etc/ssl/certs/etcd-root-ca.pem \
    --cert-file /etc/ssl/certs/infra0.pem \
    --key-file /etc/ssl/certs/infra0-key.pem \
    --peer-client-cert-auth \
    --peer-trusted-ca-file /etc/ssl/certs/etcd-root-ca.pem \
    --peer-cert-file /etc/ssl/certs/infra0.pem \
    --peer-key-file /etc/ssl/certs/infra0-key.pem

bin/etcd grpc-proxy start --discovery-srv example.com \ 
--cacert  /etc/ssl/certs/etcd-root-ca.pem  \
--cert /etc/ssl/certs/infra0.pem  \
--key /etc/ssl/certs/infra0-key.pem \
2017-04-06 19:12:26.156269 I | etcdmain: discovered the cluster [https://infra0.example.com.:2379] from example.com
2017-04-06 19:12:26.156317 I | etcdmain: validating discovered endpoints [https://infra0.example.com.:2379]
2017-04-06 19:12:26.156991 I | etcdmain: using discovered endpoints [https://infra0.example.com.:2379]
2017-04-06 19:12:26.157057 I | etcdmain: listening for grpc-proxy client requests on 127.0.0.1:23790
2017-04-06 19:12:26.199309 E | etcdmain: forgot to set Type=notify in systemd service file?

[xiang90]

lgtm

[fanminshi]

Unrelated proxy-ci failure via #7346