TUN-8822: Prevent concurrent usage of ICMPDecoder

## Summary
Some description...

Closes TUN-8822

quic/v3/muxer.go

@@ -65,12 +65,11 @@ type datagramConn struct {
 	icmpRouter     ingress.ICMPRouter
 	metrics        Metrics
 	logger         *zerolog.Logger
-
-	datagrams  chan []byte
-	readErrors chan error
+	datagrams      chan []byte
+	readErrors     chan error
 
 	icmpEncoderPool sync.Pool // a pool of *packet.Encoder
-	icmpDecoder     *packet.ICMPDecoder
+	icmpDecoderPool sync.Pool
 }
 
 func NewDatagramConn(conn QuicConnection, sessionManager SessionManager, icmpRouter ingress.ICMPRouter, index uint8, metrics Metrics, logger *zerolog.Logger) DatagramConn {
@@ -89,7 +88,11 @@ func NewDatagramConn(conn QuicConnection, sessionManager SessionManager, icmpRou
 				return packet.NewEncoder()
 			},
 		},
-		icmpDecoder: packet.NewICMPDecoder(),
+		icmpDecoderPool: sync.Pool{
+			New: func() any {
+				return packet.NewICMPDecoder()
+			},
+		},
 	}
 }
 
@@ -367,7 +370,16 @@ func (c *datagramConn) handleICMPPacket(datagram *ICMPDatagram) {
 
 	// Decode the provided ICMPDatagram as an ICMP packet
 	rawPacket := packet.RawPacket{Data: datagram.Payload}
-	icmp, err := c.icmpDecoder.Decode(rawPacket)
+	cachedDecoder := c.icmpDecoderPool.Get()
+	defer c.icmpDecoderPool.Put(cachedDecoder)
+	decoder, ok := cachedDecoder.(*packet.ICMPDecoder)
+	if !ok {
+		c.logger.Error().Msg("Could not get ICMPDecoder from the pool. Dropping packet")
+		return
+	}
+
+	icmp, err := decoder.Decode(rawPacket)
+
 	if err != nil {
 		c.logger.Err(err).Msgf("unable to marshal icmp packet")
 		return

quic/v3/muxer_test.go

@@ -4,13 +4,17 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"net"
 	"net/netip"
 	"slices"
+	"sort"
 	"sync"
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
+
 	"github.com/google/gopacket/layers"
 	"github.com/rs/zerolog"
 	"golang.org/x/net/icmp"
@@ -304,6 +308,91 @@ func TestDatagramConnServe(t *testing.T) {
 	assertContextClosed(t, ctx, done, cancel)
 }
 
+// This test exists because decoding multiple packets in parallel with the same decoder
+// instances causes inteference resulting in multiple different raw packets being decoded
+// as the same decoded packet.
+func TestDatagramConnServeDecodeMultipleICMPInParallel(t *testing.T) {
+	log := zerolog.Nop()
+	quic := newMockQuicConn()
+	session := newMockSession()
+	sessionManager := mockSessionManager{session: &session}
+	router := newMockICMPRouter()
+	conn := v3.NewDatagramConn(quic, &sessionManager, router, 0, &noopMetrics{}, &log)
+
+	// Setup the muxer
+	ctx, cancel := context.WithCancelCause(context.Background())
+	defer cancel(errors.New("other error"))
+	done := make(chan error, 1)
+	go func() {
+		done <- conn.Serve(ctx)
+	}()
+
+	packetCount := 100
+	packets := make([]*packet.ICMP, 100)
+	ipTemplate := "10.0.0.%d"
+	for i := 1; i <= packetCount; i++ {
+		packets[i-1] = &packet.ICMP{
+			IP: &packet.IP{
+				Src:      netip.MustParseAddr("192.168.1.1"),
+				Dst:      netip.MustParseAddr(fmt.Sprintf(ipTemplate, i)),
+				Protocol: layers.IPProtocolICMPv4,
+				TTL:      20,
+			},
+			Message: &icmp.Message{
+				Type: ipv4.ICMPTypeEcho,
+				Code: 0,
+				Body: &icmp.Echo{
+					ID:   25821,
+					Seq:  58129,
+					Data: []byte("test"),
+				},
+			},
+		}
+	}
+
+	wg := sync.WaitGroup{}
+	var receivedPackets []*packet.ICMP
+	go func() {
+		for ctx.Err() == nil {
+			select {
+			case icmpPacket := <-router.recv:
+				receivedPackets = append(receivedPackets, icmpPacket)
+				wg.Done()
+			}
+		}
+	}()
+
+	for _, p := range packets {
+		// We increment here but only decrement when receiving the packet
+		wg.Add(1)
+		go func() {
+			datagram := newICMPDatagram(p)
+			quic.send <- datagram
+		}()
+	}
+
+	wg.Wait()
+
+	// If there were duplicates then we won't have the same number of IPs
+	packetSet := make(map[netip.Addr]*packet.ICMP, 0)
+	for _, p := range receivedPackets {
+		packetSet[p.Dst] = p
+	}
+	assert.Equal(t, len(packetSet), len(packets))
+
+	// Sort the slice by last byte of IP address (the one we increment for each destination)
+	// and then check that we have one match for each packet sent
+	sort.Slice(receivedPackets, func(i, j int) bool {
+		return receivedPackets[i].Dst.As4()[3] < receivedPackets[j].Dst.As4()[3]
+	})
+	for i, p := range receivedPackets {
+		assert.Equal(t, p.Dst, packets[i].Dst)
+	}
+
+	// Cancel the muxer Serve context and make sure it closes with the expected error
+	assertContextClosed(t, ctx, done, cancel)
+}
+
 func TestDatagramConnServe_RegisterTwice(t *testing.T) {
 	log := zerolog.Nop()
 	quic := newMockQuicConn()