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// Copyright 2019 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
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//go:build go1.1
// +build go1.1
package flipcall
import (
"encoding/json"
"fmt"
"math"
"sync/atomic"
"gvisor.dev/gvisor/pkg/log"
)
type endpointControlImpl struct {
state int32
}
// Bits in endpointControlImpl.state.
const (
epsBlocked = 1 << iota
epsShutdown
)
func (ep *Endpoint) ctrlInit(opts ...EndpointOption) error {
if len(opts) != 0 {
return fmt.Errorf("unknown EndpointOption: %T", opts[0])
}
return nil
}
func (ep *Endpoint) ctrlConnect() error {
if err := ep.enterFutexWait(); err != nil {
return err
}
defer ep.exitFutexWait()
// Write the connection request.
w := ep.NewWriter()
if err := json.NewEncoder(w).Encode(struct{}{}); err != nil {
return fmt.Errorf("error writing connection request: %v", err)
}
*ep.dataLen() = w.Len()
// Exchange control with the server.
if err := ep.futexSetPeerActive(); err != nil {
return err
}
if err := ep.futexWakePeer(); err != nil {
return err
}
if err := ep.futexWaitUntilActive(); err != nil {
return err
}
// Read the connection response.
var resp struct{}
respLen := atomic.LoadUint32(ep.dataLen())
if respLen > ep.dataCap {
return fmt.Errorf("invalid connection response length %d (maximum %d)", respLen, ep.dataCap)
}
if err := json.NewDecoder(ep.NewReader(respLen)).Decode(&resp); err != nil {
return fmt.Errorf("error reading connection response: %v", err)
}
return nil
}
func (ep *Endpoint) ctrlWaitFirst() error {
if err := ep.enterFutexWait(); err != nil {
return err
}
defer ep.exitFutexWait()
// Wait for the connection request.
if err := ep.futexWaitUntilActive(); err != nil {
return err
}
// Read the connection request.
reqLen := atomic.LoadUint32(ep.dataLen())
if reqLen > ep.dataCap {
return fmt.Errorf("invalid connection request length %d (maximum %d)", reqLen, ep.dataCap)
}
var req struct{}
if err := json.NewDecoder(ep.NewReader(reqLen)).Decode(&req); err != nil {
return fmt.Errorf("error reading connection request: %v", err)
}
// Write the connection response.
w := ep.NewWriter()
if err := json.NewEncoder(w).Encode(struct{}{}); err != nil {
return fmt.Errorf("error writing connection response: %v", err)
}
*ep.dataLen() = w.Len()
// Return control to the client.
raceBecomeInactive()
if err := ep.futexSetPeerActive(); err != nil {
return err
}
if err := ep.futexWakePeer(); err != nil {
return err
}
// Wait for the first non-connection message.
return ep.futexWaitUntilActive()
}
func (ep *Endpoint) ctrlRoundTrip() error {
if err := ep.enterFutexWait(); err != nil {
return err
}
defer ep.exitFutexWait()
if err := ep.futexSetPeerActive(); err != nil {
return err
}
if err := ep.futexWakePeer(); err != nil {
return err
}
return ep.futexWaitUntilActive()
}
func (ep *Endpoint) ctrlWakeLast() error {
if err := ep.futexSetPeerActive(); err != nil {
return err
}
return ep.futexWakePeer()
}
func (ep *Endpoint) enterFutexWait() error {
switch eps := atomic.AddInt32(&ep.ctrl.state, epsBlocked); eps {
case epsBlocked:
return nil
case epsBlocked | epsShutdown:
atomic.AddInt32(&ep.ctrl.state, -epsBlocked)
return ShutdownError{}
default:
// Most likely due to ep.enterFutexWait() being called concurrently
// from multiple goroutines.
panic(fmt.Sprintf("invalid flipcall.Endpoint.ctrl.state before flipcall.Endpoint.enterFutexWait(): %v", eps-epsBlocked))
}
}
func (ep *Endpoint) exitFutexWait() {
switch eps := atomic.AddInt32(&ep.ctrl.state, -epsBlocked); eps {
case 0:
return
case epsShutdown:
// ep.ctrlShutdown() was called while we were blocked, so we are
// repsonsible for indicating connection shutdown.
ep.shutdownConn()
default:
panic(fmt.Sprintf("invalid flipcall.Endpoint.ctrl.state after flipcall.Endpoint.exitFutexWait(): %v", eps+epsBlocked))
}
}
func (ep *Endpoint) ctrlShutdown() {
// Set epsShutdown to ensure that future calls to ep.enterFutexWait() fail.
if atomic.AddInt32(&ep.ctrl.state, epsShutdown)&epsBlocked != 0 {
// Wake the blocked thread. This must loop because it's possible that
// FUTEX_WAKE occurs after the waiter sets epsBlocked, but before it
// blocks in FUTEX_WAIT.
for {
// Wake MaxInt32 threads to prevent a broken or malicious peer from
// swallowing our wakeup by FUTEX_WAITing from multiple threads.
if err := ep.futexWakeConnState(math.MaxInt32); err != nil {
log.Warningf("failed to FUTEX_WAKE Endpoints: %v", err)
break
}
yieldThread()
if atomic.LoadInt32(&ep.ctrl.state)&epsBlocked == 0 {
break
}
}
} else {
// There is no blocked thread, so we are responsible for indicating
// connection shutdown.
ep.shutdownConn()
}
}
func (ep *Endpoint) shutdownConn() {
switch cs := atomic.SwapUint32(ep.connState(), csShutdown); cs {
case ep.activeState:
if err := ep.futexWakeConnState(1); err != nil {
log.Warningf("failed to FUTEX_WAKE peer Endpoint for shutdown: %v", err)
}
case ep.inactiveState:
// The peer is currently active and will detect shutdown when it tries
// to update the connection state.
case csShutdown:
// The peer also called Endpoint.Shutdown().
default:
log.Warningf("unexpected connection state before Endpoint.shutdownConn(): %v", cs)
}
}
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