querier: do A lookups after SRV lookups for store discovery (#865)

* do A lookups after SRV lookups for store discovery

* add missing trailing period

* re-add old behaviour under dnssrvnoa+

Author: mjd95

pkg/discovery/dns/resolver.go

@@ -12,8 +12,9 @@ import (
 type QType string
 
 const (
-	A   = QType("dns")
-	SRV = QType("dnssrv")
+	A      = QType("dns")
+	SRV    = QType("dnssrv")
+	SRVNoA = QType("dnssrvnoa")
 )
 
 type Resolver interface {
@@ -72,6 +73,26 @@ func (s *dnsSD) Resolve(ctx context.Context, name string, qtype QType) ([]string
 			res = append(res, appendScheme(scheme, net.JoinHostPort(ip.String(), port)))
 		}
 	case SRV:
+		_, recs, err := s.resolver.LookupSRV(ctx, "", "", host)
+		if err != nil {
+			return nil, errors.Wrapf(err, "lookup SRV records %q", host)
+		}
+		for _, rec := range recs {
+			// Only use port from SRV record if no explicit port was specified.
+			resPort := port
+			if resPort == "" {
+				resPort = strconv.Itoa(int(rec.Port))
+			}
+			// Do A lookup for the domain in SRV answer.
+			resIPs, err := s.resolver.LookupIPAddr(ctx, rec.Target)
+			if err != nil {
+				return nil, errors.Wrapf(err, "look IP addresses %q", rec.Target)
+			}
+			for _, resIP := range resIPs {
+				res = append(res, appendScheme(scheme, net.JoinHostPort(resIP.String(), resPort)))
+			}
+		}
+	case SRVNoA:
 		_, recs, err := s.resolver.LookupSRV(ctx, "", "", host)
 		if err != nil {
 			return nil, errors.Wrapf(err, "lookup SRV records %q", host)

pkg/discovery/dns/resolver_test.go

@@ -77,11 +77,57 @@ var (
 			resolver:       &mockHostnameResolver{},
 		},
 		{
-			testName:       "multiple srv records from srv lookup",
+			testName:       "multiple SRV records from SRV lookup",
 			addr:           "_test._tcp.mycompany.com",
 			qtype:          SRV,
 			expectedResult: []string{"192.168.0.1:8080", "192.168.0.2:8081"},
 			expectedErr:    nil,
+			resolver: &mockHostnameResolver{
+				resultSRVs: map[string][]*net.SRV{
+					"_test._tcp.mycompany.com": {
+						&net.SRV{Target: "alt1.mycompany.com.", Port: 8080},
+						&net.SRV{Target: "alt2.mycompany.com.", Port: 8081},
+					},
+				},
+				resultIPs: map[string][]net.IPAddr{
+					"alt1.mycompany.com.": {net.IPAddr{IP: net.ParseIP("192.168.0.1")}},
+					"alt2.mycompany.com.": {net.IPAddr{IP: net.ParseIP("192.168.0.2")}},
+				},
+			},
+		},
+		{
+			testName:       "multiple SRV records from SRV lookup with specified port",
+			addr:           "_test._tcp.mycompany.com:8082",
+			qtype:          SRV,
+			expectedResult: []string{"192.168.0.1:8082", "192.168.0.2:8082"},
+			expectedErr:    nil,
+			resolver: &mockHostnameResolver{
+				resultSRVs: map[string][]*net.SRV{
+					"_test._tcp.mycompany.com": {
+						&net.SRV{Target: "alt1.mycompany.com.", Port: 8080},
+						&net.SRV{Target: "alt2.mycompany.com.", Port: 8081},
+					},
+				},
+				resultIPs: map[string][]net.IPAddr{
+					"alt1.mycompany.com.": {net.IPAddr{IP: net.ParseIP("192.168.0.1")}},
+					"alt2.mycompany.com.": {net.IPAddr{IP: net.ParseIP("192.168.0.2")}},
+				},
+			},
+		},
+		{
+			testName:       "error from SRV resolver",
+			addr:           "_test._tcp.mycompany.com",
+			qtype:          SRV,
+			expectedResult: nil,
+			expectedErr:    errors.Wrapf(errorFromResolver, "lookup SRV records \"_test._tcp.mycompany.com\""),
+			resolver:       &mockHostnameResolver{err: errorFromResolver},
+		},
+		{
+			testName:       "multiple SRV records from SRV no A lookup",
+			addr:           "_test._tcp.mycompany.com",
+			qtype:          SRVNoA,
+			expectedResult: []string{"192.168.0.1:8080", "192.168.0.2:8081"},
+			expectedErr:    nil,
 			resolver: &mockHostnameResolver{
 				resultSRVs: map[string][]*net.SRV{
 					"_test._tcp.mycompany.com": {
@@ -92,9 +138,9 @@ var (
 			},
 		},
 		{
-			testName:       "multiple srv records from srv lookup",
+			testName:       "multiple SRV records from SRV no A lookup with specified port",
 			addr:           "_test._tcp.mycompany.com:8082",
-			qtype:          SRV,
+			qtype:          SRVNoA,
 			expectedResult: []string{"192.168.0.1:8082", "192.168.0.2:8082"},
 			expectedErr:    nil,
 			resolver: &mockHostnameResolver{
@@ -107,7 +153,7 @@ var (
 			},
 		},
 		{
-			testName:       "error from SRV resolver",
+			testName:       "error from SRV no A lookup",
 			addr:           "_test._tcp.mycompany.com",
 			qtype:          SRV,
 			expectedResult: nil,