TUN-6696: Refactor flow into funnel and close idle funnels

A funnel is an abstraction for 1 source to many destinations.
As part of this refactoring, shared logic between Darwin and Linux are moved into icmp_posix

ingress/icmp_posix_test.go

@@ -0,0 +1,76 @@
+//go:build darwin || linux
+
+package ingress
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/google/gopacket/layers"
+	"github.com/rs/zerolog"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+
+	"github.com/cloudflare/cloudflared/packet"
+)
+
+func TestFunnelIdleTimeout(t *testing.T) {
+	const (
+		idleTimeout = time.Second
+		echoID      = 42573
+		startSeq    = 8129
+	)
+	logger := zerolog.New(os.Stderr)
+	proxy, err := newICMPProxy(localhostIP, &logger, idleTimeout)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	proxyDone := make(chan struct{})
+	go func() {
+		proxy.Serve(ctx)
+		close(proxyDone)
+	}()
+
+	// Send a packet to register the flow
+	pk := packet.ICMP{
+		IP: &packet.IP{
+			Src:      localhostIP,
+			Dst:      localhostIP,
+			Protocol: layers.IPProtocolICMPv4,
+		},
+		Message: &icmp.Message{
+			Type: ipv4.ICMPTypeEcho,
+			Code: 0,
+			Body: &icmp.Echo{
+				ID:   echoID,
+				Seq:  startSeq,
+				Data: []byte(t.Name()),
+			},
+		},
+	}
+	responder := echoFlowResponder{
+		decoder:  packet.NewICMPDecoder(),
+		respChan: make(chan []byte),
+	}
+	require.NoError(t, proxy.Request(&pk, &responder))
+	responder.validate(t, &pk)
+
+	// Send second request, should reuse the funnel
+	require.NoError(t, proxy.Request(&pk, nil))
+	responder.validate(t, &pk)
+
+	time.Sleep(idleTimeout * 2)
+	newResponder := echoFlowResponder{
+		decoder:  packet.NewICMPDecoder(),
+		respChan: make(chan []byte),
+	}
+	require.NoError(t, proxy.Request(&pk, &newResponder))
+	newResponder.validate(t, &pk)
+
+	cancel()
+	<-proxyDone
+}