fix(promtail): Fix UDP receiver on syslog transport (#10708)

Co-authored-by: Callum Styan <callumstyan@gmail.com>

CHANGELOG.md

@@ -135,6 +135,7 @@ Starting with the 3.0 release we began using [conventional commits](https://www.
 
 ##### Fixes
 
+* [10708](https://github.com/grafana/loki/pull/10708) **joshuapare**: Fix UDP receiver on syslog transport
 * [10631](https://github.com/grafana/loki/pull/10631) **thampiotr**: Fix race condition in cleaning up metrics when stopping to tail files.
 * [10798](https://github.com/grafana/loki/pull/10798) **hainenber**: Fix agent panicking after reloaded due to duplicate metric collector registration.
 * [10848](https://github.com/grafana/loki/pull/10848) **rgroothuijsen**: Correctly parse list of drop stage sources from YAML.

clients/pkg/promtail/targets/syslog/transport.go

@@ -1,6 +1,7 @@
 package syslog
 
 import (
+	"bytes"
 	"context"
 	"crypto/tls"
 	"crypto/x509"
@@ -380,16 +381,32 @@ func (t *UDPTransport) handleRcv(c *ConnPipe) {
 	defer t.openConnections.Done()
 
 	lbs := t.connectionLabels(c.addr.String())
-	err := syslogparser.ParseStream(c, func(result *syslog.Result) {
-		if err := result.Error; err != nil {
-			t.handleMessageError(err)
-		} else {
-			t.handleMessage(lbs.Copy(), result.Message)
+
+	for {
+		datagram := make([]byte, t.maxMessageLength())
+		n, err := c.Read(datagram)
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+
+			level.Warn(t.logger).Log("msg", "error reading from pipe", "err", err)
+			continue
 		}
-	}, t.maxMessageLength())
 
-	if err != nil {
-		level.Warn(t.logger).Log("msg", "error parsing syslog stream", "err", err)
+		r := bytes.NewReader(datagram[:n])
+
+		err = syslogparser.ParseStream(r, func(result *syslog.Result) {
+			if err := result.Error; err != nil {
+				t.handleMessageError(err)
+			} else {
+				t.handleMessage(lbs.Copy(), result.Message)
+			}
+		}, t.maxMessageLength())
+
+		if err != nil {
+			level.Warn(t.logger).Log("msg", "error parsing syslog stream", "err", err)
+		}
 	}
 }