messagelog: Use custom errors

messagelog/errors.go

@@ -1,7 +1,19 @@
 package messagelog
 
+import "fmt"
+
 type ErrInvalidMessage struct {
+	message string
+}
+
+func (err *ErrInvalidMessage) Error() string {
+	return fmt.Sprintf("invalid message format: %s", err.message)
 }
 
 type ErrTopicNotFound struct {
+	topic string
+}
+
+func (err *ErrTopicNotFound) Error() string {
+	return fmt.Sprintf("topic `%s` not found", err.topic)
 }

messagelog/log.go

@@ -1,7 +1,6 @@
 package messagelog
 
 import (
-	"errors"
 	"sync"
 )
 
@@ -24,7 +23,7 @@ func (ml *Log) Push(topic string, message Message) error {
 	defer ml.logLock.Unlock()
 	log, ok := ml.log[topic]
 	if !ok {
-		return errors.New("topic doesn't exist")
+		return &ErrTopicNotFound{topic: topic}
 	}
 	message.Index = len(log)
 	ml.log[topic] = append(log, message)
@@ -36,7 +35,7 @@ func (ml *Log) List(topic string, offset int) ([]Message, error) {
 	defer ml.logLock.RUnlock()
 	log, ok := ml.log[topic]
 	if !ok {
-		return nil, errors.New("topic doesn't exist")
+		return nil, &ErrTopicNotFound{topic: topic}
 	}
 	if offset >= len(log) {
 		return []Message{}, nil

messagelog/message.go

@@ -10,6 +10,8 @@ type Message struct {
 }
 
 func (m Message) validate() error {
-	// TODO: Use ErrInvalidMessage!
+	if err := validator.Validate(m); err != nil {
-	return validator.Validate(m)
+		return &ErrInvalidMessage{message: err.Error()}
+	}
+	return nil
 }