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// Copyright 2018 The gVisor Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package tcp
import (
// segmentQueue is a bounded, thread-safe queue of TCP segments.
// +stateify savable
type segmentQueue struct {
mu sync.Mutex `state:"nosave"`
list segmentList `state:"wait"`
ep *endpoint
frozen bool
// emptyLocked determines if the queue is empty.
// Preconditions: must be held.
func (q *segmentQueue) emptyLocked() bool {
return q.list.Empty()
// empty determines if the queue is empty.
func (q *segmentQueue) empty() bool {
r := q.emptyLocked()
return r
// enqueue adds the given segment to the queue.
// Returns true when the segment is successfully added to the queue, in which
// case ownership of the reference is transferred to the queue. And returns
// false if the queue is full, in which case ownership is retained by the
// caller.
func (q *segmentQueue) enqueue(s *segment) bool {
// q.ep.receiveBufferParams() must be called without holding to
// avoid lock order inversion.
bufSz := q.ep.receiveBufferSize()
used := q.ep.receiveMemUsed()
// Allow zero sized segments (ACK/FIN/RSTs etc even if the segment queue
// is currently full).
allow := (used <= bufSz || s.payloadSize() == 0) && !q.frozen
if allow {
// Set the owner now that the endpoint owns the segment.
s.setOwner(q.ep, recvQ)
return allow
// dequeue removes and returns the next segment from queue, if one exists.
// Ownership is transferred to the caller, who is responsible for decrementing
// the ref count when done.
func (q *segmentQueue) dequeue() *segment {
s := q.list.Front()
if s != nil {
return s
// freeze prevents any more segments from being added to the queue. i.e all
// future segmentQueue.enqueue will return false and not add the segment to the
// queue till the queue is unfroze with a corresponding segmentQueue.thaw call.
func (q *segmentQueue) freeze() {
q.frozen = true
// thaw unfreezes a previously frozen queue using segmentQueue.freeze() and
// allows new segments to be queued again.
func (q *segmentQueue) thaw() {
q.frozen = false