github.com/gagliardetto/golang-go@v0.0.0-20201020153340-53909ea70814/cmd/go/not-internal/lockedfile/lockedfile_plan9.go (about)

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // +build plan9
     6  
     7  package lockedfile
     8  
     9  import (
    10  	"math/rand"
    11  	"os"
    12  	"strings"
    13  	"time"
    14  )
    15  
    16  // Opening an exclusive-use file returns an error.
    17  // The expected error strings are:
    18  //
    19  //  - "open/create -- file is locked" (cwfs, kfs)
    20  //  - "exclusive lock" (fossil)
    21  //  - "exclusive use file already open" (ramfs)
    22  var lockedErrStrings = [...]string{
    23  	"file is locked",
    24  	"exclusive lock",
    25  	"exclusive use file already open",
    26  }
    27  
    28  // Even though plan9 doesn't support the Lock/RLock/Unlock functions to
    29  // manipulate already-open files, IsLocked is still meaningful: os.OpenFile
    30  // itself may return errors that indicate that a file with the ModeExclusive bit
    31  // set is already open.
    32  func isLocked(err error) bool {
    33  	s := err.Error()
    34  
    35  	for _, frag := range lockedErrStrings {
    36  		if strings.Contains(s, frag) {
    37  			return true
    38  		}
    39  	}
    40  
    41  	return false
    42  }
    43  
    44  func openFile(name string, flag int, perm os.FileMode) (*os.File, error) {
    45  	// Plan 9 uses a mode bit instead of explicit lock/unlock syscalls.
    46  	//
    47  	// Per http://man.cat-v.org/plan_9/5/stat: “Exclusive use files may be open
    48  	// for I/O by only one fid at a time across all clients of the server. If a
    49  	// second open is attempted, it draws an error.”
    50  	//
    51  	// So we can try to open a locked file, but if it fails we're on our own to
    52  	// figure out when it becomes available. We'll use exponential backoff with
    53  	// some jitter and an arbitrary limit of 500ms.
    54  
    55  	// If the file was unpacked or created by some other program, it might not
    56  	// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
    57  	// can be confident that a successful OpenFile implies exclusive use.
    58  	if fi, err := os.Stat(name); err == nil {
    59  		if fi.Mode()&os.ModeExclusive == 0 {
    60  			if err := os.Chmod(name, fi.Mode()|os.ModeExclusive); err != nil {
    61  				return nil, err
    62  			}
    63  		}
    64  	} else if !os.IsNotExist(err) {
    65  		return nil, err
    66  	}
    67  
    68  	nextSleep := 1 * time.Millisecond
    69  	const maxSleep = 500 * time.Millisecond
    70  	for {
    71  		f, err := os.OpenFile(name, flag, perm|os.ModeExclusive)
    72  		if err == nil {
    73  			return f, nil
    74  		}
    75  
    76  		if !isLocked(err) {
    77  			return nil, err
    78  		}
    79  
    80  		time.Sleep(nextSleep)
    81  
    82  		nextSleep += nextSleep
    83  		if nextSleep > maxSleep {
    84  			nextSleep = maxSleep
    85  		}
    86  		// Apply 10% jitter to avoid synchronizing collisions.
    87  		nextSleep += time.Duration((0.1*rand.Float64() - 0.05) * float64(nextSleep))
    88  	}
    89  }
    90  
    91  func closeFile(f *os.File) error {
    92  	return f.Close()
    93  }