Mqtt: limit number of inflight token waits (#10131)

provider/mqtt/client.go

@@ -6,6 +6,7 @@ import (
 	"math/rand"
 	"strings"
 	"sync"
+	"sync/atomic"
 
 	paho "github.com/eclipse/paho.mqtt.golang"
 	"github.com/evcc-io/evcc/api"
@@ -38,6 +39,7 @@ type Client struct {
 	Client   paho.Client
 	broker   string
 	Qos      byte
+	inflight uint32
 	listener map[string][]func(string)
 }
 
@@ -168,6 +170,14 @@ func (m *Client) listen(topic string) {
 
 // WaitForToken synchronously waits until token operation completed
 func (m *Client) WaitForToken(action, topic string, token paho.Token) {
+	if inflight := atomic.LoadUint32(&m.inflight); inflight > 64 {
+		return
+	}
+
+	// track inflight token waits
+	atomic.AddUint32(&m.inflight, 1)
+	defer atomic.AddUint32(&m.inflight, ^uint32(0))
+
 	err := api.ErrTimeout
 	if token.WaitTimeout(request.Timeout) {
 		err = token.Error()