From 025d2c32b46cf77e6443c3a588df9878bfce8e4f Mon Sep 17 00:00:00 2001
From: Joshua Pare <joshuabpare@gmail.com>
Date: Mon, 25 Sep 2023 21:05:43 -0500
Subject: [PATCH] ensure syslog parser gets an EOF-terminated reader on udp
 receive

---
 CHANGELOG.md                                  |  1 +
 .../pkg/promtail/targets/syslog/transport.go  | 33 ++++++++++++++-----
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b9a7c8af187d..f4309464a1f6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -42,6 +42,7 @@
 ##### Fixes
 
 * [10631](https://github.com/grafana/loki/pull/10631) **thampiotr**: Fix race condition in cleaning up metrics when stopping to tail files.
+* [10708](https://github.com/grafana/loki/pull/10708) **joshuapare**: Fix UDP receiver on syslog transport
 
 #### LogCLI
 
diff --git a/clients/pkg/promtail/targets/syslog/transport.go b/clients/pkg/promtail/targets/syslog/transport.go
index 67a78136e311..aafa86ed26d5 100644
--- a/clients/pkg/promtail/targets/syslog/transport.go
+++ b/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)
+		}
 	}
 }