resolver/dns: error if target ends with a colon instead of assuming the default port (#2150)

Technically this is a behavior change, but any usage of "host:" or ":" should be considered invalid, since dialing a target with a trailing ":" attempts to dial port zero, which is never valid (and we always used to pass the target directly to `net.Dial`).

In https://github.com/grpc/grpc/pull/15618, the target format will be defined as: `host[:port]` -- not: `host[:[port]]`.  (This is potentially subject to change.)

resolver/dns/dns_resolver.go

@@ -51,7 +51,13 @@ const (
 )
 
 var (
-	errMissingAddr = errors.New("missing address")
+	errMissingAddr = errors.New("dns resolver: missing address")
+
+	// Addresses ending with a colon that is supposed to be the separator
+	// between host and port is not allowed.  E.g. "::" is a valid address as
+	// it is an IPv6 address (host only) and "[::]:" is invalid as it ends with
+	// a colon as the host and port separator
+	errEndsWithColon = errors.New("dns resolver: missing port after port-separator colon")
 )
 
 // NewBuilder creates a dnsBuilder which is used to factory DNS resolvers.
@@ -297,7 +303,6 @@ func formatIP(addr string) (addrIP string, ok bool) {
 // target: "ipv4-host:80" returns host: "ipv4-host", port: "80"
 // target: "[ipv6-host]" returns host: "ipv6-host", port: "443"
 // target: ":80" returns host: "localhost", port: "80"
-// target: ":" returns host: "localhost", port: "443"
 func parseTarget(target string) (host, port string, err error) {
 	if target == "" {
 		return "", "", errMissingAddr
@@ -307,15 +312,15 @@ func parseTarget(target string) (host, port string, err error) {
 		return target, defaultPort, nil
 	}
 	if host, port, err = net.SplitHostPort(target); err == nil {
+		if port == "" {
+			// If the port field is empty (target ends with colon), e.g. "[::1]:", this is an error.
+			return "", "", errEndsWithColon
+		}
 		// target has port, i.e ipv4-host:port, [ipv6-host]:port, host-name:port
 		if host == "" {
 			// Keep consistent with net.Dial(): If the host is empty, as in ":80", the local system is assumed.
 			host = "localhost"
 		}
-		if port == "" {
-			// If the port field is empty(target ends with colon), e.g. "[::1]:", defaultPort is used.
-			port = defaultPort
-		}
 		return host, port, nil
 	}
 	if host, port, err = net.SplitHostPort(target + ":" + defaultPort); err == nil {

resolver/dns/dns_resolver_test.go

@@ -815,7 +815,7 @@ func testIPResolver(t *testing.T) {
 		{"127.0.0.1:12345", []resolver.Address{{Addr: "127.0.0.1:12345"}}},
 		{"::1", []resolver.Address{{Addr: "[::1]" + colonDefaultPort}}},
 		{"[::1]:12345", []resolver.Address{{Addr: "[::1]:12345"}}},
-		{"[::1]:", []resolver.Address{{Addr: "[::1]:443"}}},
+		{"[::1]", []resolver.Address{{Addr: "[::1]:443"}}},
 		{"2001:db8:85a3::8a2e:370:7334", []resolver.Address{{Addr: "[2001:db8:85a3::8a2e:370:7334]" + colonDefaultPort}}},
 		{"[2001:db8:85a3::8a2e:370:7334]", []resolver.Address{{Addr: "[2001:db8:85a3::8a2e:370:7334]" + colonDefaultPort}}},
 		{"[2001:db8:85a3::8a2e:370:7334]:12345", []resolver.Address{{Addr: "[2001:db8:85a3::8a2e:370:7334]:12345"}}},
@@ -867,6 +867,7 @@ func TestResolveFunc(t *testing.T) {
 		{"www.google.com", nil},
 		{"foo.bar:12345", nil},
 		{"127.0.0.1", nil},
+		{"::", nil},
 		{"127.0.0.1:12345", nil},
 		{"[::1]:80", nil},
 		{"[2001:db8:a0b:12f0::1]:21", nil},
@@ -875,7 +876,8 @@ func TestResolveFunc(t *testing.T) {
 		{"[fe80::1%lo0]:80", nil},
 		{"golang.org:http", nil},
 		{"[2001:db8::1]:http", nil},
-		{":", nil},
+		{"[2001:db8::1]:", errEndsWithColon},
+		{":", errEndsWithColon},
 		{"", errMissingAddr},
 		{"[2001:db8:a0b:12f0::1", errForInvalidTarget},
 	}