resolver: make decoders return string instead of byte slice

ip/resolver/http/service.go

@@ -1,6 +1,7 @@
 package http
 
 import (
+	"bufio"
 	"context"
 	"io"
 	"net"
@@ -11,7 +12,7 @@ import (
 	"dnsupdater/ip/internal"
 )
 
-type Decoder func(io.Reader) ([]byte, error)
+type Decoder func(io.Reader) (string, error)
 
 type Service struct {
 	ServiceName string
@@ -31,16 +32,19 @@ func (s Service) Lookup(ctx context.Context) (net.IP, error) {
 	}
 
 	if s.Decoder == nil {
-		s.Decoder = io.ReadAll
+		s.Decoder = func(r io.Reader) (string, error) {
+			line, err := bufio.NewReader(r).ReadString('\n')
+			if err == nil || err == io.EOF {
+				line = strings.TrimRight(line, "\r\n")
+				line = strings.TrimSpace(line)
+			}
+			return line, err
+		}
 	}
 
 	body, err := s.Decoder(resp.Body)
-	if err != nil {
+	if err != nil && err != io.EOF {
 		return nil, err
 	}
-
-	// Trim spaces and stuff.
-	ip_str := strings.TrimSpace(string(body))
-
-	return internal.ParseIP(ip_str)
+	return internal.ParseIP(string(body))
 }