elvish/pkg/eval/port.go

package eval

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"sync"

	"github.com/elves/elvish/pkg/strutil"
)

// Port conveys data stream. It always consists of a byte band and a channel band.
type Port struct {
	File      *os.File
	Chan      chan interface{}
	CloseFile bool
	CloseChan bool
}

// Fork returns a copy of a Port with the Close* flags unset.
func (p *Port) Fork() *Port {
	return &Port{p.File, p.Chan, false, false}
}

// Close closes a Port.
func (p *Port) Close() {
	if p == nil {
		return
	}
	if p.CloseFile {
		p.File.Close()
	}
	if p.CloseChan {
		close(p.Chan)
	}
}

var (
	// ClosedChan is a closed channel, suitable as a placeholder input channel.
	ClosedChan = getClosedChan()
	// BlackholeChan is a channel that absorbs all values written to it,
	// suitable as a placeholder output channel.
	BlackholeChan = getBlackholeChan()
	// DevNull is /dev/null, suitable as a placeholder file for either input or
	// output.
	DevNull = getDevNull()

	// DevNullClosedChan is a port made up from DevNull and ClosedChan, suitable
	// as a placeholder input port.
	DevNullClosedChan = &Port{File: DevNull, Chan: ClosedChan}
	// DevNullBlackholeChan is a port made up from DevNull and BlackholeChan,
	// suitable as a placeholder output port.
	DevNullBlackholeChan = &Port{File: DevNull, Chan: BlackholeChan}
)

func getClosedChan() chan interface{} {
	ch := make(chan interface{})
	close(ch)
	return ch
}

func getBlackholeChan() chan interface{} {
	ch := make(chan interface{})
	go func() {
		for range ch {
		}
	}()
	return ch
}

func getDevNull() *os.File {
	f, err := os.Open(os.DevNull)
	if err != nil {
		fmt.Fprintf(os.Stderr,
			"cannot open %s, shell might not function normally\n", os.DevNull)
	}
	return f
}

// PipePort returns an output *Port whose value and byte components are both
// piped. The supplied functions are called on a separate goroutine with the
// read ends of the value and byte components of the port. It also returns a
// function to clean up the port and wait for the callbacks to finish.
func PipePort(vCb func(<-chan interface{}), bCb func(*os.File)) (*Port, func(), error) {
	r, w, err := os.Pipe()
	if err != nil {
		return nil, nil, err
	}
	ch := make(chan interface{}, outputCaptureBufferSize)

	var wg sync.WaitGroup
	wg.Add(2)
	go func() {
		defer wg.Done()
		vCb(ch)
	}()
	go func() {
		defer wg.Done()
		defer r.Close()
		bCb(r)
	}()

	port := &Port{Chan: ch, CloseChan: true, File: w, CloseFile: true}
	done := func() {
		port.Close()
		wg.Wait()
	}
	return port, done, nil
}

// CapturePort returns an output *Port whose value and byte components are
// both connected to an internal pipe that saves the output. It also returns a
// function to call to obtain the captured output.
func CapturePort() (*Port, func() []interface{}, error) {
	vs := []interface{}{}
	var m sync.Mutex
	port, done, err := PipePort(
		func(ch <-chan interface{}) {
			for v := range ch {
				m.Lock()
				vs = append(vs, v)
				m.Unlock()
			}
		},
		func(r *os.File) {
			buffered := bufio.NewReader(r)
			for {
				line, err := buffered.ReadString('\n')
				if line != "" {
					v := strutil.ChopLineEnding(line)
					m.Lock()
					vs = append(vs, v)
					m.Unlock()
				}
				if err != nil {
					if err != io.EOF {
						logger.Println("error on reading:", err)
					}
					break
				}
			}
		})
	if err != nil {
		return nil, nil, err
	}
	return port, func() []interface{} {
		done()
		return vs
	}, nil
}