tailscale/ssh/tailssh/ctxreader.go

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// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tailssh
import (
"context"
"io"
"sync"
"tailscale.com/tempfork/gliderlabs/ssh"
)
// readResult is a result from a io.Reader.Read call,
// as used by contextReader.
type readResult struct {
buf []byte // ownership passed on chan send
err error
}
// contextReader wraps an io.Reader, providing a ReadContext method
// that can be aborted before yielding bytes. If it's aborted, subsequent
// reads can get those byte(s) later.
type contextReader struct {
r io.Reader
// buffered is leftover data from a previous read call that wasn't entirely
// consumed.
buffered []byte
// readErr is a previous read error that was seen while filling buffered. It
// should be returned to the caller after bufffered is consumed.
readErr error
mu sync.Mutex // guards ch only
// ch is non-nil if a goroutine had been started and has a result to be
// read. The goroutine may be either still running or done and has
// send to the channel.
ch chan readResult
}
// HasOutstandingRead reports whether there's an oustanding Read call that's
// either currently blocked in a Read or whose result hasn't been consumed.
func (w *contextReader) HasOutstandingRead() bool {
w.mu.Lock()
defer w.mu.Unlock()
return w.ch != nil
}
func (w *contextReader) setChan(c chan readResult) {
w.mu.Lock()
defer w.mu.Unlock()
w.ch = c
}
// ReadContext is like Read, but takes a context permitting the read to be canceled.
//
// If the context becomes done, the underlying Read call continues and its result
// will be given to the next caller to ReadContext.
func (w *contextReader) ReadContext(ctx context.Context, p []byte) (n int, err error) {
if len(p) == 0 {
return 0, nil
}
n = copy(p, w.buffered)
if n > 0 {
w.buffered = w.buffered[n:]
if len(w.buffered) == 0 {
err = w.readErr
}
return n, err
}
if w.ch == nil {
ch := make(chan readResult, 1)
w.setChan(ch)
go func() {
rbuf := make([]byte, len(p))
n, err := w.r.Read(rbuf)
ch <- readResult{rbuf[:n], err}
}()
}
select {
case <-ctx.Done():
return 0, ctx.Err()
case rr := <-w.ch:
w.setChan(nil)
n = copy(p, rr.buf)
w.buffered = rr.buf[n:]
w.readErr = rr.err
if len(w.buffered) == 0 {
err = rr.err
}
return n, err
}
}
// contextReaderSesssion implements ssh.Session, wrapping another
// ssh.Session but changing its Read method to use contextReader.
type contextReaderSesssion struct {
ssh.Session
cr *contextReader
}
func (a contextReaderSesssion) Read(p []byte) (n int, err error) {
if a.cr.HasOutstandingRead() {
return a.cr.ReadContext(context.Background(), p)
}
return a.Session.Read(p)
}