Detect when a fargo error indicates not found

The fargo library already provides the HTTPResponseStatusCode function
to extract an HTTP status code from a returned error, but here we want
to be able to inject similar errors for tests, so look for both a real
fargo error indicating that a heartbeat attempt failed to find its
target instance and a synthetic error returned by a fake connection
during a test.

sd/eureka/registrar.go

@@ -30,6 +30,10 @@ type fargoUnsuccessfulHTTPResponse struct {
 	messagePrefix string
 }
 
+func (u *fargoUnsuccessfulHTTPResponse) Error() string {
+	return fmt.Sprintf("err=%s code=%d", u.messagePrefix, u.statusCode)
+}
+
 // Registrar maintains service instance liveness information in Eureka.
 type Registrar struct {
 	conn     fargoConnection
@@ -110,18 +114,30 @@ func (r *Registrar) loop() {
 	}
 }
 
+func httpResponseStatusCode(err error) (code int, present bool) {
+	if code, ok := fargo.HTTPResponseStatusCode(err); ok {
+		return code, true
+	}
+	// Allow injection of errors for testing.
+	if u, ok := err.(*fargoUnsuccessfulHTTPResponse); ok {
+		return u.statusCode, true
+	}
+	return 0, false
+}
+
+func isNotFound(err error) bool {
+	code, ok := httpResponseStatusCode(err)
+	return ok && code == http.StatusNotFound
+}
+
 func (r *Registrar) heartbeat() error {
 	err := r.conn.HeartBeatInstance(r.instance)
-	if err != nil {
-		if u, ok := err.(*fargoUnsuccessfulHTTPResponse); ok && u.statusCode == http.StatusNotFound {
-			// Instance expired (e.g. network partition). Re-register.
-			r.logger.Log("during", "heartbeat", err.Error())
-			return r.conn.ReregisterInstance(r.instance)
-		}
+	if err == nil {
+		return nil
+	}
+	if isNotFound(err) {
+		// Instance expired (e.g. network partition). Re-register.
+		return r.conn.ReregisterInstance(r.instance)
 	}
 	return err
 }
-
-func (u *fargoUnsuccessfulHTTPResponse) Error() string {
-	return fmt.Sprintf("err=%s code=%d", u.messagePrefix, u.statusCode)
-}