retry: add optional semaphore for retry receipts

Closes #1119

client.go

@@ -26,6 +26,7 @@ import (
 	"go.mau.fi/util/ptr"
 	"go.mau.fi/util/random"
 	"golang.org/x/net/proxy"
+	"golang.org/x/sync/semaphore"
 
 	"go.mau.fi/whatsmeow/appstate"
 	waBinary "go.mau.fi/whatsmeow/binary"
@@ -119,6 +120,7 @@ type Client struct {
 
 	messageRetries     map[string]int
 	messageRetriesLock sync.Mutex
+	retrySema          *semaphore.Weighted
 
 	incomingRetryRequestCounter     map[incomingRetryKey]int
 	incomingRetryRequestCounterLock sync.Mutex
@@ -403,6 +405,16 @@ func (cli *Client) SetPreLoginHTTPClient(h *http.Client) {
 	cli.preLoginHTTP = h
 }
 
+// SetMaxParallelRetryReceiptHandling sets how many retry receipts can be handled in parallel.
+// Defaults to unlimited. This should only be set before connecting, changing it afterwards can cause data races.
+func (cli *Client) SetMaxParallelRetryReceiptHandling(n int64) {
+	if n <= 0 {
+		cli.retrySema = nil
+	} else {
+		cli.retrySema = semaphore.NewWeighted(n)
+	}
+}
+
 func (cli *Client) getSocketWaitChan() <-chan struct{} {
 	cli.socketLock.RLock()
 	ch := cli.socketWait

go.mod

@@ -13,6 +13,7 @@ require (
 	go.mau.fi/util v0.9.8
 	golang.org/x/crypto v0.50.0
 	golang.org/x/net v0.53.0
+	golang.org/x/sync v0.20.0
 	google.golang.org/protobuf v1.36.11
 )
 

go.sum

@@ -46,6 +46,8 @@ golang.org/x/exp v0.0.0-20260410095643-746e56fc9e2f h1:W3F4c+6OLc6H2lb//N1q4WpJk
 golang.org/x/exp v0.0.0-20260410095643-746e56fc9e2f/go.mod h1:J1xhfL/vlindoeF/aINzNzt2Bket5bjo9sdOYzOsU80=
 golang.org/x/net v0.53.0 h1:d+qAbo5L0orcWAr0a9JweQpjXF19LMXJE8Ey7hwOdUA=
 golang.org/x/net v0.53.0/go.mod h1:JvMuJH7rrdiCfbeHoo3fCQU24Lf5JJwT9W3sJFulfgs=
+golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
+golang.org/x/sync v0.20.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.43.0 h1:Rlag2XtaFTxp19wS8MXlJwTvoh8ArU6ezoyFsMyCTNI=
 golang.org/x/sys v0.43.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=

retry.go

@@ -187,6 +187,13 @@ func (cli *Client) tryHandleRetryReceipt(ctx context.Context, receipt *events.Re
 			cli.Log.Errorf("Retry receipt handler panicked: %v\n%s", err, debug.Stack())
 		}
 	}()
+	if cli.retrySema != nil {
+		err := cli.retrySema.Acquire(ctx, 1)
+		if err != nil {
+			return
+		}
+		defer cli.retrySema.Release(1)
+	}
 	err := cli.handleRetryReceipt(ctx, receipt, node)
 	if err != nil {
 		cli.Log.Errorf("Failed to handle retry receipt for %s/%s from %s: %v", receipt.Chat, receipt.MessageIDs[0], receipt.Sender, err)