api/client.go: in Subscribe() handle nil handler correctly.

api/client.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"fmt"
+	"reflect"
 	"sync"
 
 	"github.com/eosswedenorg/thalos/api/message"
@@ -81,24 +82,30 @@ func (c *Client) hbHandler(payload []byte) {
 }
 
 func (c *Client) Subscribe(channel Channel) error {
-	var handler handler
+	handlers := map[string]struct {
+		handler  handler
+		callback any
+	}{
+		HeartbeatChannel.Type():              {c.hbHandler, c.OnHeartbeat},
+		ActionChannel{}.Channel().Type():     {c.actHandler, c.OnAction},
+		TableDeltaChannel{}.Channel().Type(): {c.tableDeltaHandler, c.OnTableDelta},
+	}
 
-	switch t := channel.Type(); t {
-	case HeartbeatChannel.Type():
-		handler = c.hbHandler
-	case ActionChannel{}.Channel().Type():
-		handler = c.actHandler
-	case TableDeltaChannel{}.Channel().Type():
-		handler = c.tableDeltaHandler
-	default:
-		return fmt.Errorf("invalid channel type. %s", t)
+	h, ok := handlers[channel.Type()]
+
+	if !ok {
+		return fmt.Errorf("invalid channel type. %s", channel.Type())
+	}
+
+	if h.callback == nil || reflect.ValueOf(h.callback).IsNil() {
+		return fmt.Errorf("please set an handler before calling Subscribe")
 	}
 
 	// Start a worker for this channel.
 	c.wg.Add(1)
 	go func() {
 		defer c.wg.Done()
-		c.worker(channel, handler)
+		c.worker(channel, h.handler)
 	}()
 
 	return nil