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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.
package gofer
import (
"fmt"
"sync"
"time"
"golang.org/x/sys/unix"
"gvisor.dev/gvisor/pkg/context"
"gvisor.dev/gvisor/pkg/syserror"
)
// Global pipe used by blockUntilNonblockingPipeHasWriter since we can't create
// pipes after sentry initialization due to syscall filters.
var (
tempPipeMu sync.Mutex
tempPipeReadFD int
tempPipeWriteFD int
tempPipeBuf [1]byte
)
func init() {
var pipeFDs [2]int
if err := unix.Pipe(pipeFDs[:]); err != nil {
panic(fmt.Sprintf("failed to create pipe for gofer.blockUntilNonblockingPipeHasWriter: %v", err))
}
tempPipeReadFD = pipeFDs[0]
tempPipeWriteFD = pipeFDs[1]
}
func blockUntilNonblockingPipeHasWriter(ctx context.Context, fd int32) error {
for {
ok, err := nonblockingPipeHasWriter(fd)
if err != nil {
return err
}
if ok {
return nil
}
if sleepErr := sleepBetweenNamedPipeOpenChecks(ctx); sleepErr != nil {
// Another application thread may have opened this pipe for
// writing, succeeded because we previously opened the pipe for
// reading, and subsequently interrupted us for checkpointing (e.g.
// this occurs in mknod tests under cooperative save/restore). In
// this case, our open has to succeed for the checkpoint to include
// a readable FD for the pipe, which is in turn necessary to
// restore the other thread's writable FD for the same pipe
// (otherwise it will get ENXIO). So we have to check
// nonblockingPipeHasWriter() once last time.
ok, err := nonblockingPipeHasWriter(fd)
if err != nil {
return err
}
if ok {
return nil
}
return sleepErr
}
}
}
func nonblockingPipeHasWriter(fd int32) (bool, error) {
tempPipeMu.Lock()
defer tempPipeMu.Unlock()
// Copy 1 byte from fd into the temporary pipe.
n, err := unix.Tee(int(fd), tempPipeWriteFD, 1, unix.SPLICE_F_NONBLOCK)
if err == syserror.EAGAIN {
// The pipe represented by fd is empty, but has a writer.
return true, nil
}
if err != nil {
return false, err
}
if n == 0 {
// The pipe represented by fd is empty and has no writer.
return false, nil
}
// The pipe represented by fd is non-empty, so it either has, or has
// previously had, a writer. Remove the byte copied to the temporary pipe
// before returning.
if n, err := unix.Read(tempPipeReadFD, tempPipeBuf[:]); err != nil || n != 1 {
panic(fmt.Sprintf("failed to drain pipe for gofer.blockUntilNonblockingPipeHasWriter: got (%d, %v), wanted (1, nil)", n, err))
}
return true, nil
}
func sleepBetweenNamedPipeOpenChecks(ctx context.Context) error {
t := time.NewTimer(100 * time.Millisecond)
defer t.Stop()
cancel := ctx.SleepStart()
select {
case <-t.C:
ctx.SleepFinish(true)
return nil
case <-cancel:
ctx.SleepFinish(false)
return syserror.ErrInterrupted
}
}
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