fix: increase UDP read buffer length to max datagram size

Co-authored-by: Tom Moulard <tom.moulard@traefik.io>

pkg/udp/conn_test.go

@@ -1,9 +1,11 @@
 package udp
 
 import (
+	"crypto/rand"
 	"errors"
 	"io"
 	"net"
+	"runtime"
 	"testing"
 	"time"
 
@@ -317,6 +319,61 @@ func TestShutdown(t *testing.T) {
 	}
 }
 
+func TestReadLoopMaxDataSize(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		// sudo sysctl -w net.inet.udp.maxdgram=65507
+		t.Skip("Skip test on darwin as the maximum dgram size is set to 9216 bytes by default")
+	}
+
+	// Theoretical maximum size of data in a UDP datagram.
+	// 65535 − 8 (UDP header) − 20 (IP header).
+	dataSize := 65507
+
+	doneCh := make(chan struct{})
+
+	addr, err := net.ResolveUDPAddr("udp", ":0")
+	require.NoError(t, err)
+
+	l, err := Listen("udp", addr, 3*time.Second)
+	require.NoError(t, err)
+
+	defer func() {
+		err := l.Close()
+		require.NoError(t, err)
+	}()
+
+	go func() {
+		defer close(doneCh)
+
+		conn, err := l.Accept()
+		require.NoError(t, err)
+
+		buffer := make([]byte, dataSize)
+
+		n, err := conn.Read(buffer)
+		require.NoError(t, err)
+
+		assert.Equal(t, dataSize, n)
+	}()
+
+	c, err := net.Dial("udp", l.Addr().String())
+	require.NoError(t, err)
+
+	data := make([]byte, dataSize)
+
+	_, err = rand.Read(data)
+	require.NoError(t, err)
+
+	_, err = c.Write(data)
+	require.NoError(t, err)
+
+	select {
+	case <-doneCh:
+	case <-time.Tick(5 * time.Second):
+		t.Fatal("Timeout waiting for datagram read")
+	}
+}
+
 // requireEcho tests that the conn session is live and functional,
 // by writing data through it, and expecting the same data as a response when reading on it.
 // It fatals if the read blocks longer than timeout,