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Ack dropping - fix for #50 (#127)

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* fix for #50

* fix remaining test

* revert changes and instead approach problem on full-duplex case

* fix merge messup

* fix @MDr164 note and upgrade documentation while at it

* gate full buffer exit on rx shutdown since no risk of full buffer error

* add log line to note issue is hit

* document CloseRead relationship with Close
This commit is contained in:
Pat Whittingslow
2026-06-19 18:43:13 -03:00
committed by GitHub
parent 6f5acd1948
commit 8f9d765cb0
4 changed files with 192 additions and 7 deletions
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tcp/handler.go
+23 -4
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@@ -14,7 +14,7 @@ import (
// related to data buffering, frame sequencing and connection state handling.
// Does NOT implement IP related logic, so no CRC calculation/validation or pseudo header logic.
//
// See [Conn] for a higher level abstraction of a TCP connection, and see [ControlBlock] for the lower level bits of a TCP connection.
// See [Conn] for a higher level abstraction of a TCP connection, and see [ControlBlock] for the low level state machine of a TCP connection.
type Handler struct {
connid uint64
scb ControlBlock
@@ -35,6 +35,7 @@ type Handler struct {
nRetransmit uint8
}
// SetLoggers sets the [slog.Logger] for the Handler and internal [ControlBlock].
func (h *Handler) SetLoggers(handler, scb *slog.Logger) {
h.logger.log = handler
h.scb.logger.log = scb
@@ -118,6 +119,7 @@ func (h *Handler) Abort() {
h.reset(0, 0, 0)
}
// reset clears all state except [ControlBlock] state. So [Handler.State] will remain unchanged.
func (h *Handler) reset(localPort, remotePort uint16, iss Value) {
*h = Handler{
connid: h.connid + 1,
@@ -160,7 +162,7 @@ func (h *Handler) Recv(incomingPacket []byte) error {
return lneto.ErrMismatch
}
payload := tfrm.Payload()
if len(payload) > h.bufRx.Free() {
if !h.shutdownRx && len(payload) > h.bufRx.Free() {
return lneto.ErrBufferFull
}
segIncoming := tfrm.Segment(len(payload))
@@ -186,6 +188,15 @@ func (h *Handler) Recv(incomingPacket []byte) error {
if prevState != h.scb.State() {
h.info("tcp.Handler:rx-statechange", slog.Uint64("port", uint64(h.localPort)), slog.String("old", prevState.String()), slog.String("new", h.scb.State().String()), slog.String("rxflags", segIncoming.Flags.String()))
}
if segIncoming.DATALEN != 0 && h.shutdownRx && (h.scb.State() == StateFinWait1 || h.scb.State() == StateFinWait2) {
// soypat/lneto#50: the application is done in both directions — read side
// shut down (CloseRead) and our FIN sent (Close) — so inbound data has no
// consumer. Reply RST instead of the silent ACK-and-drop that leaves the
// peer waiting; the connection is torn down once the RST is sent.
h.info("tcp.Handler:rst-data-after-fullclose", slog.Uint64("lport", uint64(h.localPort)), slog.Uint64("rport", uint64(h.remotePort)), slog.Uint64("datalen", uint64(segIncoming.DATALEN)))
h.scb.QueueRST(segIncoming.ACK)
return nil
}
if segIncoming.DATALEN != 0 && !h.shutdownRx {
_, err = h.bufRx.Write(payload)
if err != nil {
@@ -231,12 +242,16 @@ func (h *Handler) Recv(incomingPacket []byte) error {
}
// ShutdownRead activates local discard mode: incoming payload bytes are dropped
// (ACK/SEQ still advance normally) and Read returns io.EOF immediately.
// Not reversible within the lifetime of a connection; reset clears it.
// (ACK/SEQ still advance normally) and Read returns [io.EOF] immediately.
// Not reversible within the lifetime of a connection.
// If [Handler.Close] and this method are both called then connection will be terminated.
func (h *Handler) ShutdownRead() {
h.shutdownRx = true
}
// Close will initiate the TCP close sequence.
// After Close is called [Handler.Write] will fail with [net.ErrClosed].
// The connection may still receive data to read after Close called.
func (h *Handler) Close() error {
h.trace("tcp.Handler.Close")
if h.closing {
@@ -328,6 +343,10 @@ func (h *Handler) Send(b []byte) (int, error) {
closedSuccess := prevState == StateTimeWait && segment.Flags.HasAny(FlagACK)
if closedSuccess {
h.reset(0, 0, 0)
} else if segment.Flags.HasAny(FlagRST) {
// A sent RST aborts the connection: tear down local state now that the
// reset has been written to the wire (frame already in b).
h.Abort()
}
return datalen, nil
}