From 66c9a7d763f219591430182b6ff307a3fa70728d Mon Sep 17 00:00:00 2001
From: Patricio Whittingslow <graded.sp@gmail.com>
Date: Thu, 27 Jun 2024 21:09:30 -0300
Subject: [PATCH] start adding debug logic, slightly better Listener recv

---
 stacks/portstack.go   | 16 ++++++++++------
 stacks/tcpconn.go     |  5 ++---
 stacks/tcplistener.go | 38 ++++++++++++++++++++++----------------
 3 files changed, 34 insertions(+), 25 deletions(-)

diff --git a/stacks/portstack.go b/stacks/portstack.go
index c5533f4..89d959e 100644
--- a/stacks/portstack.go
+++ b/stacks/portstack.go
@@ -315,12 +315,6 @@ func (ps *PortStack) RecvEth(ethernetFrame []byte) (err error) {
 		}
 
 		pkt := &ps.auxTCP
-		if pkt == nil {
-			ps.error("TCP packet dropped")
-			ps.droppedPackets++
-			err = ErrDroppedPacket // Our socket needs handling before admitting more packets.
-			break
-		}
 		if isDebug {
 			ps.debug("TCP:recv",
 				slog.Int("opt", len(tcpOptions)),
@@ -347,6 +341,16 @@ func (ps *PortStack) RecvEth(ethernetFrame []byte) (err error) {
 		} else if err == ErrFlagPending {
 			err = nil // TODO(soypat).
 		}
+		if err != nil && isDebug && ps.logger.Enabled(context.Background(), internal.LevelTrace) {
+			seg := pkt.TCP.Segment(len(payload))
+			ps.trace("stack:error",
+				slog.Uint64("seg.seq", uint64(seg.SEQ)),
+				slog.Uint64("seg.ack", uint64(seg.ACK)),
+				slog.Uint64("seg.wnd", uint64(seg.WND)),
+				slog.String("seg.flags", seg.Flags.String()),
+				slog.Uint64("seg.data", uint64(seg.DATALEN)),
+			)
+		}
 	}
 	if err != nil {
 		ps.error("Stack.RecvEth", slog.String("err", err.Error()))
diff --git a/stacks/tcpconn.go b/stacks/tcpconn.go
index 544e885..9e5a02e 100644
--- a/stacks/tcpconn.go
+++ b/stacks/tcpconn.go
@@ -332,13 +332,12 @@ func (sock *TCPConn) checkPipeOpen() error {
 }
 
 func (sock *TCPConn) recv(pkt *TCPPacket) (err error) {
-	sock.trace("TCPConn.recv:start")
+	remotePort := sock.remote.Port()
+	sock.trace("TCPConn.recv:start", slog.Uint64("lport", uint64(sock.localPort)), slog.Uint64("rport", uint64(remotePort)))
 	prevState := sock.scb.State()
 	if prevState.IsClosed() {
 		return io.EOF
 	}
-
-	remotePort := sock.remote.Port()
 	if remotePort != 0 && pkt.TCP.SourcePort != remotePort {
 		return nil // This packet came from a different client to the one we are interacting with.
 	}
diff --git a/stacks/tcplistener.go b/stacks/tcplistener.go
index a9bea17..5a33926 100644
--- a/stacks/tcplistener.go
+++ b/stacks/tcplistener.go
@@ -129,26 +129,32 @@ func (l *TCPListener) recv(pkt *TCPPacket) error {
 	if !l.isOpen() {
 		return io.EOF
 	}
+	l.stack.trace("lst:recv", slog.Uint64("lport", uint64(l.port)), slog.Uint64("rport", uint64(pkt.TCP.SourcePort)))
 	var freeconn *TCPConn
-	isSYN := pkt.TCP.Flags() == seqs.FlagSYN
+	isSYN := pkt.TCP.Ack == 0 && pkt.TCP.Flags() == seqs.FlagSYN
 	var connidx int
-	for connidx = range l.conns {
-		conn := &l.conns[connidx]
-		if pkt.TCP.Ack == 0 && isSYN && !l.used[connidx] && conn.State() == seqs.StateListen {
-			// Get first available connection in list for first SYN packet, initiating connection.
-			freeconn = conn
-			break
-		}
-		if pkt.TCP.SourcePort != conn.remote.Port() ||
-			pkt.IP.Source != conn.remote.Addr().As4() {
-			continue // Not for this connection.
+	if isSYN {
+		for connidx = range l.conns {
+			conn := &l.conns[connidx]
+			if conn.scb.State() == seqs.StateListen && !conn.scb.HasPending() {
+				// Get first available connection in list for first SYN packet, initiating connection.
+				if l.used[connidx] {
+					// See https://github.com/soypat/seqs/issues/25.
+					l.stack.trace("lst:reuse-conn", slog.Uint64("rport", uint64(pkt.TCP.SourcePort)), slog.Uint64("rport-old", uint64(conn.remote.Port())))
+				}
+				freeconn = conn
+				break
+			}
 		}
-		err := conn.recv(pkt)
-		if err == io.EOF {
-			l.freeConnForReuse(connidx)
-			err = nil
+	} else {
+		for connidx = range l.conns {
+			conn := &l.conns[connidx]
+			isMatch := pkt.TCP.SourcePort == conn.remote.Port() && pkt.IP.Source == conn.remote.Addr().As4()
+			if isMatch {
+				freeconn = conn
+				break
+			}
 		}
-		return err
 	}
 	if freeconn == nil {
 		l.trace("lst:noconn2recv")