add send logic to handleUser

stack/socket_tcp.go

@@ -1,6 +1,7 @@
 package stack
 
 import (
+	"cmp"
 	"errors"
 	"io"
 	"net/netip"
@@ -19,6 +20,7 @@ type tcp struct {
 	remote    netip.AddrPort
 	remoteMAC [6]byte
 	tx        ring
+	rx        ring
 }
 
 func (t *tcp) State() seqs.State {
@@ -28,7 +30,7 @@ func (t *tcp) State() seqs.State {
 func (t *tcp) Send(b []byte) error {
 	if t.tx.buf == nil {
 		t.tx = ring{
-			buf: make([]byte, 2048),
+			buf: make([]byte, max(2048, len(b))),
 		}
 	}
 	_, err := t.tx.Write(b)
@@ -132,20 +134,42 @@ func (t *tcp) handleRecv(response []byte, pkt *TCPPacket) (n int, err error) {
 }
 
 func (t *tcp) handleUser(response []byte, pkt *TCPPacket) (n int, err error) {
-	return 0, nil
-}
+	available := t.tx.Buffered()
+	if available == 0 {
+		return 0, nil // No data to send.
+	}
+	seg, ok := t.scb.PendingSegment(available)
+	if !ok {
+		return 0, errors.New("possible segment not found") // No pending control segment. Yield to handleUser.
+	}
+	err = t.scb.Send(seg)
+	if err != nil {
+		return 0, err
+	}
+	t.setSrcDest(pkt)
+	pkt.PutHeaders(response)
+	payloadPlace := response[54:]
+	n, err = t.tx.Read(payloadPlace[:seg.DATALEN])
+	if err != nil || n != int(seg.DATALEN) {
+		panic("bug in handleUser") // This is a bug in ring buffer or a race condition.
+	}
 
-func (t *tcp) handleInitSyn(response []byte, pkt *TCPPacket) (n int, err error) {
-	// Uninitialized TCB, we start the handshake.
+	return 54 + n, err
+}
 
+func (t *tcp) setSrcDest(pkt *TCPPacket) {
 	copy(pkt.Eth.Source[:], t.stack.MAC)
 	pkt.IP.Source = t.stack.IP.As4()
 	pkt.TCP.SourcePort = t.localPort
 
 	pkt.IP.Destination = t.remote.Addr().As4()
 	pkt.TCP.DestinationPort = t.remote.Port()
 	pkt.Eth.Destination = t.remoteMAC
+}
 
+func (t *tcp) handleInitSyn(response []byte, pkt *TCPPacket) (n int, err error) {
+	// Uninitialized TCB, we start the handshake.
+	t.setSrcDest(pkt)
 	pkt.CalculateHeaders(t.synsentSegment(), nil)
 	pkt.PutHeaders(response)
 	return 54, nil
@@ -260,3 +284,17 @@ func (r *ring) onReadEnd() {
 		r.end = 0
 	}
 }
+
+func max[T cmp.Ordered](a, b T) T {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func min[T cmp.Ordered](a, b T) T {
+	if a < b {
+		return a
+	}
+	return b
+}