github.com/attic-labs/noms@v0.0.0-20210827224422-e5fa29d95e8b/go/util/outputpager/page_output.go (about)

     1  // Copyright 2016 Attic Labs, Inc. All rights reserved.
     2  // Licensed under the Apache License, version 2.0:
     3  // http://www.apache.org/licenses/LICENSE-2.0
     4  
     5  package outputpager
     6  
     7  import (
     8  	"io"
     9  	"os"
    10  	"os/exec"
    11  	"sync"
    12  
    13  	"github.com/attic-labs/kingpin"
    14  	goisatty "github.com/mattn/go-isatty"
    15  
    16  	"github.com/attic-labs/noms/go/d"
    17  )
    18  
    19  var (
    20  	noPager bool
    21  )
    22  
    23  type Pager struct {
    24  	Writer        io.Writer
    25  	stdin, stdout *os.File
    26  	mtx           *sync.Mutex
    27  	doneCh        chan struct{}
    28  }
    29  
    30  func Start() *Pager {
    31  	if noPager || !IsStdoutTty() {
    32  		return &Pager{os.Stdout, nil, nil, nil, nil}
    33  	}
    34  
    35  	lessPath, err := exec.LookPath("less")
    36  	d.Chk.NoError(err)
    37  
    38  	// -F ... Quit if entire file fits on first screen.
    39  	// -S ... Chop (truncate) long lines rather than wrapping.
    40  	// -R ... Output "raw" control characters.
    41  	// -X ... Don't use termcap init/deinit strings.
    42  	cmd := exec.Command(lessPath, "-FSRX")
    43  
    44  	stdin, stdout, err := os.Pipe()
    45  	d.Chk.NoError(err)
    46  	cmd.Stdout = os.Stdout
    47  	cmd.Stderr = os.Stderr
    48  	cmd.Stdin = stdin
    49  	cmd.Start()
    50  
    51  	p := &Pager{stdout, stdin, stdout, &sync.Mutex{}, make(chan struct{})}
    52  	go func() {
    53  		err := cmd.Wait()
    54  		d.Chk.NoError(err)
    55  		p.closePipe()
    56  		p.doneCh <- struct{}{}
    57  	}()
    58  	return p
    59  }
    60  
    61  func (p *Pager) Stop() {
    62  	if p.Writer != os.Stdout {
    63  		p.closePipe()
    64  		// Wait until less has fully exited, otherwise it might not have printed the terminal restore characters.
    65  		<-p.doneCh
    66  	}
    67  }
    68  
    69  func (p *Pager) closePipe() {
    70  	p.mtx.Lock()
    71  	defer p.mtx.Unlock()
    72  	if p.stdin != nil {
    73  		// Closing the pipe will cause any outstanding writes to stdout fail, and fail from now on.
    74  		p.stdin.Close()
    75  		p.stdout.Close()
    76  		p.stdin, p.stdout = nil, nil
    77  	}
    78  }
    79  
    80  func RegisterOutputpagerFlags(cmd *kingpin.CmdClause) {
    81  	cmd.Flag("no-pager", "suppress paging functionality").BoolVar(&noPager)
    82  }
    83  
    84  func IsStdoutTty() bool {
    85  	return goisatty.IsTerminal(os.Stdout.Fd())
    86  }