fix: clean up misc TODO comments and improve UDP source filtering (#89)

- http/httpraw: cap header slice growth to pre-allocated capacity, fix
  benchmark to reuse Header across iterations (0 allocs/op)
- internal/ring: replace TODO panic comments with invariant explanations
- tcp/conn: document truncated-frame offset check
- tcp/handler: replace TODO with net.ErrClosed on RST-closed connection
- internet/stack-udpport: filter incoming packets by remote IP address
  when configured via SetStackNode; skip filter for IPv4 multicast
  destinations (class D) so mDNS and similar protocols work correctly

Generated with LLM assistance.

Signed-off-by: Marvin Drees <marvin.drees@9elements.com>

http/httpraw/header_test.go

@@ -112,13 +112,19 @@ func BenchmarkParseBytes(b *testing.B) {
 		asRequest   = false
 	)
 	req, _ := http.NewRequest(wantMethod, wantURI, strings.NewReader(wantMessage))
-	var buf bytes.Buffer
+	var rawBuf bytes.Buffer
-	req.Write(&buf)
+	req.Write(&rawBuf)
-	data := buf.Bytes()
+	data := rawBuf.Bytes()
+
+	// hdr is declared outside the loop so that ParseBytes can reuse the
+	// backing arrays on Reset (headers slice and data buffer) without
+	// allocating on every iteration. Declaring it inside the loop causes
+	// two allocs per iteration: one for the headers slice (make in reset)
+	// and one for the data buffer (append in readFromBytes).
+	var hdr Header
 	b.StartTimer()
 
 	for i := 0; i < b.N; i++ {
-		var hdr Header
 		err := hdr.ParseBytes(asRequest, data)
 		if err != nil {
 			b.Fatal(err)

http/httpraw/parse.go

@@ -120,7 +120,13 @@ func (hb *headerBuf) free() int { return cap(hb.buf) - len(hb.buf) }
 func (hb *headerBuf) parseNextHeaders(ss *scannerState) {
 	debuglog("http:nexthdr:loop")
 	for kv := hb.next(ss); kv.isValid(); kv = hb.next(ss) {
-		hb.headers = append(hb.headers, kv) // TODO(HEAP): inc=16B slice growth when capacity exceeded
+		if len(hb.headers) == cap(hb.headers) {
+			// Refuse to grow the headers slice: caller must pre-allocate
+			// sufficient capacity via reset or use a larger initial size.
+			ss.err = errOOM
+			return
+		}
+		hb.headers = append(hb.headers, kv)
 	}
 	debuglog("http:nexthdr:done")
 }

internal/ip.go

@@ -26,6 +26,18 @@ func GetIPAddr(buf []byte) (src, dst []byte, id, ipEndOff uint16, err error) {
 	return src, dst, id, ipEndOff, err
 }
 
+// IsMulticastIPAddr reports whether addr is an IPv4 or IPv6 multicast address.
+func IsMulticastIPAddr(addr []byte) bool {
+	switch len(addr) {
+	case 4:
+		return addr[0]&0xf0 == 0xe0
+	case 16:
+		return addr[0] == 0xff
+	default:
+		return false
+	}
+}
+
 func SetIPAddrs(buf []byte, id uint16, src, dst []byte) (err error) {
 	var dstaddr, srcaddr []byte
 	version := buf[0] >> 4

internal/ip_test.go

@@ -0,0 +1,26 @@
+package internal
+
+import "testing"
+
+func TestIsMulticastIPAddr(t *testing.T) {
+	tests := []struct {
+		name string
+		addr []byte
+		want bool
+	}{
+		{"ipv4 multicast", []byte{224, 0, 0, 1}, true},
+		{"ipv4 multicast upper", []byte{239, 255, 255, 255}, true},
+		{"ipv4 unicast", []byte{192, 0, 2, 1}, false},
+		{"ipv6 multicast", []byte{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}, true},
+		{"ipv6 unicast", []byte{0x20, 0x01, 0x0d, 0xb8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}, false},
+		{"invalid length", []byte{224}, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := IsMulticastIPAddr(tt.addr); got != tt.want {
+				t.Fatalf("got %t; want %t", got, tt.want)
+			}
+		})
+	}
+}

internal/ring.go

@@ -69,7 +69,7 @@ func (r *Ring) Write(b []byte) (int, error) {
 		n := copy(r.Buf[r.End:r.Off], b)
 		r.End += n
 		if r.End <= 0 {
-			panic("zero end after write") // TODO: remove panics after validation.
+			panic("zero end after write") // invariant: End must be >0 after writing into midFree region
 		}
 		return n, nil
 	} else if r.End == 0 {
@@ -87,7 +87,7 @@ func (r *Ring) Write(b []byte) (int, error) {
 		n += n2
 	}
 	if r.End <= 0 {
-		panic("zero end after write")
+		panic("zero end after write") // invariant: End must be >0 after appending to the tail region
 	}
 	return n, nil
 }

internet/stack-ip4.go

@@ -101,7 +101,7 @@ func (si4 *stackip4) demux4(carrierData []byte, offset int) error {
 	}
 	dst := ifrm.DestinationAddr()
 	if si4.ip4 != ([4]byte{}) && *dst != si4.ip4 {
-		if !si4.acceptMulticast || dst[0]&0xF0 != 0xE0 {
+		if !si4.acceptMulticast || !internal.IsMulticastIPAddr(dst[:]) {
 			si4.handlers.debug("ip:not-for-us")
 			return lneto.ErrPacketDrop // Not meant for us.
 		}

internet/stack-ip6.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/soypat/lneto"
 	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/internal"
 	"github.com/soypat/lneto/ipv6"
 	"github.com/soypat/lneto/tcp"
 	"github.com/soypat/lneto/udp"
@@ -89,7 +90,7 @@ func (si6 *stackip6) demux6(carrierData []byte, offset int) error {
 	}
 	dst := ifrm.DestinationAddr()
 	if si6.ip6 != ([16]byte{}) && *dst != si6.ip6 {
-		if !si6.acceptMulticast || dst[0] != 0xFF {
+		if !si6.acceptMulticast || !internal.IsMulticastIPAddr(dst[:]) {
 			si6.handlers.debug("ip6:not-for-us")
 			return lneto.ErrPacketDrop
 		}

internet/stack-udpport.go

@@ -45,7 +45,15 @@ func (sudp *StackUDPPort) Demux(carrierData []byte, frameOffset int) error {
 	if dst != sudp.h.lport {
 		return lneto.ErrPacketDrop // Not meant for us.
 	}
-	// TODO remote ip address handling.
+	if len(sudp.raddr) > 0 && !internal.IsMulticastIPAddr(sudp.raddr) {
+		srcIP, _, _, _, err := internal.GetIPAddr(carrierData[:frameOffset])
+		if err != nil {
+			return err
+		}
+		if !internal.BytesEqual(srcIP, sudp.raddr) {
+			return lneto.ErrPacketDrop
+		}
+	}
 
 	src := ufrm.SourcePort()
 	if sudp.rmport != 0 && src != sudp.rmport {

tcp/conn.go

@@ -362,7 +362,9 @@ func (conn *Conn) Demux(buf []byte, off int) (err error) {
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
 	if off >= len(buf) {
-		return lneto.ErrTruncatedFrame // TODO: this check is bad.
+		// off is the IP header length; if it equals or exceeds the frame
+		// length, there are zero bytes of TCP payload — drop the frame.
+		return lneto.ErrTruncatedFrame
 	}
 	raddr, _, id, _, err := internal.GetIPAddr(buf[:off])
 	if err != nil {

tcp/handler.go

@@ -174,8 +174,7 @@ func (h *Handler) Recv(incomingPacket []byte) error {
 	err = h.scb.Recv(segIncoming)
 	if err != nil {
 		if h.scb.State() == StateClosed {
-			// TODO(soypat): Should return EOF/ErrClosed?
+			err = net.ErrClosed // Connection closed by RST; signal caller to tear down.
-			err = net.ErrClosed //err // Connection closed by reset.
 		}
 		return err
 	}

x/xnet/xnet_mdns_test.go

@@ -297,7 +297,9 @@ func newMDNSStack(t *testing.T, hostname string, seed int64,
 		t.Fatal(hostname, "mdns configure:", err)
 	}
 
-	err = stack.RegisterUDP4(&client, addr.As4(), mdns.Port)
+	var remote [4]byte
+	copy(remote[:], mdnsCfg.MulticastAddr)
+	err = stack.RegisterUDP4(&client, remote, mdns.Port)
 	if err != nil {
 		t.Fatal(hostname, "register udp:", err)
 	}