client: Discovery via SRV lookups

Based on code from discovery/srv.go.  The returns the target as DNS
returns it. In the case of SSL, certs are tied to the hostname and not
the IP address generally.

Solves #2547

client/discover.go

@@ -0,0 +1,21 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package client
+
+// Discoverer is an interface that wraps the Discover method.
+type Discoverer interface {
+	// Dicover looks up the etcd servers for the domain.
+	Discover(domain string) ([]string, error)
+}

client/srv.go

@@ -0,0 +1,65 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package client
+
+import (
+	"fmt"
+	"net"
+	"net/url"
+)
+
+var (
+	// indirection for testing
+	lookupSRV = net.LookupSRV
+)
+
+type srvDiscover struct{}
+
+// NewSRVDiscover constructs a new Dicoverer that uses the stdlib to lookup SRV records.
+func NewSRVDiscover() Discoverer {
+	return &srvDiscover{}
+}
+
+// Discover looks up the etcd servers for the domain.
+func (d *srvDiscover) Discover(domain string) ([]string, error) {
+	var urls []*url.URL
+
+	updateURLs := func(service, scheme string) error {
+		_, addrs, err := lookupSRV(service, "tcp", domain)
+		if err != nil {
+			return err
+		}
+		for _, srv := range addrs {
+			urls = append(urls, &url.URL{
+				Scheme: scheme,
+				Host:   net.JoinHostPort(srv.Target, fmt.Sprintf("%d", srv.Port)),
+			})
+		}
+		return nil
+	}
+
+	errHTTPS := updateURLs("etcd-server-ssl", "https")
+	errHTTP := updateURLs("etcd-server", "http")
+
+	if errHTTPS != nil && errHTTP != nil {
+		return nil, fmt.Errorf("dns lookup errors: %s and %s", errHTTPS, errHTTP)
+	}
+
+	endpoints := make([]string, len(urls))
+	for i := range urls {
+		endpoints[i] = urls[i].String()
+	}
+	return endpoints, nil
+}

client/srv_test.go

@@ -0,0 +1,102 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package client
+
+import (
+	"errors"
+	"net"
+	"reflect"
+	"testing"
+)
+
+func TestSRVDiscover(t *testing.T) {
+	defer func() { lookupSRV = net.LookupSRV }()
+
+	tests := []struct {
+		withSSL    []*net.SRV
+		withoutSSL []*net.SRV
+		expected   []string
+	}{
+		{
+			[]*net.SRV{},
+			[]*net.SRV{},
+			[]string{},
+		},
+		{
+			[]*net.SRV{
+				&net.SRV{Target: "10.0.0.1", Port: 2480},
+				&net.SRV{Target: "10.0.0.2", Port: 2480},
+				&net.SRV{Target: "10.0.0.3", Port: 2480},
+			},
+			[]*net.SRV{},
+			[]string{"https://10.0.0.1:2480", "https://10.0.0.2:2480", "https://10.0.0.3:2480"},
+		},
+		{
+			[]*net.SRV{
+				&net.SRV{Target: "10.0.0.1", Port: 2480},
+				&net.SRV{Target: "10.0.0.2", Port: 2480},
+				&net.SRV{Target: "10.0.0.3", Port: 2480},
+			},
+			[]*net.SRV{
+				&net.SRV{Target: "10.0.0.1", Port: 7001},
+			},
+			[]string{"https://10.0.0.1:2480", "https://10.0.0.2:2480", "https://10.0.0.3:2480", "http://10.0.0.1:7001"},
+		},
+		{
+			[]*net.SRV{
+				&net.SRV{Target: "10.0.0.1", Port: 2480},
+				&net.SRV{Target: "10.0.0.2", Port: 2480},
+				&net.SRV{Target: "10.0.0.3", Port: 2480},
+			},
+			[]*net.SRV{
+				&net.SRV{Target: "10.0.0.1", Port: 7001},
+			},
+			[]string{"https://10.0.0.1:2480", "https://10.0.0.2:2480", "https://10.0.0.3:2480", "http://10.0.0.1:7001"},
+		},
+		{
+			[]*net.SRV{
+				&net.SRV{Target: "a.example.com", Port: 2480},
+				&net.SRV{Target: "b.example.com", Port: 2480},
+				&net.SRV{Target: "c.example.com", Port: 2480},
+			},
+			[]*net.SRV{},
+			[]string{"https://a.example.com:2480", "https://b.example.com:2480", "https://c.example.com:2480"},
+		},
+	}
+
+	for i, tt := range tests {
+		lookupSRV = func(service string, proto string, domain string) (string, []*net.SRV, error) {
+			if service == "etcd-server-ssl" {
+				return "", tt.withSSL, nil
+			}
+			if service == "etcd-server" {
+				return "", tt.withoutSSL, nil
+			}
+			return "", nil, errors.New("Unkown service in mock")
+		}
+
+		d := NewSRVDiscover()
+
+		endpoints, err := d.Discover("example.com")
+		if err != nil {
+			t.Fatalf("%d: err: %#v", i, err)
+		}
+
+		if !reflect.DeepEqual(endpoints, tt.expected) {
+			t.Errorf("#%d: endpoints = %v, want %v", i, endpoints, tt.expected)
+		}
+
+	}
+}