lockedfile_plan9.go

     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  //go:build plan9
     6  
     7  package lockedfile
     8  
     9  import (
    10  	"io/fs"
    11  	"math/rand"
    12  	"os"
    13  	"strings"
    14  	"time"
    15  )
    16  
    17  // Opening an exclusive-use file returns an error.
    18  // The expected error strings are:
    19  //
    20  //   - "open/create -- file is locked" (cwfs, kfs)
    21  //   - "exclusive lock" (fossil)
    22  //   - "exclusive use file already open" (ramfs)
    23  var lockedErrStrings = [...]string{
    24  	"file is locked",
    25  	"exclusive lock",
    26  	"exclusive use file already open",
    27  }
    28  
    29  // Even though plan9 doesn't support the Lock/RLock/Unlock functions to
    30  // manipulate already-open files, IsLocked is still meaningful: os.OpenFile
    31  // itself may return errors that indicate that a file with the ModeExclusive bit
    32  // set is already open.
    33  func isLocked(err error) bool {
    34  	s := err.Error()
    35  
    36  	for _, frag := range lockedErrStrings {
    37  		if strings.Contains(s, frag) {
    38  			return true
    39  		}
    40  	}
    41  
    42  	return false
    43  }
    44  
    45  func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
    46  	// Plan 9 uses a mode bit instead of explicit lock/unlock syscalls.
    47  	//
    48  	// Per http://man.cat-v.org/plan_9/5/stat: “Exclusive use files may be open
    49  	// for I/O by only one fid at a time across all clients of the server. If a
    50  	// second open is attempted, it draws an error.”
    51  	//
    52  	// So we can try to open a locked file, but if it fails we're on our own to
    53  	// figure out when it becomes available. We'll use exponential backoff with
    54  	// some jitter and an arbitrary limit of 500ms.
    55  
    56  	// If the file was unpacked or created by some other program, it might not
    57  	// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
    58  	// can be confident that a successful OpenFile implies exclusive use.
    59  	if fi, err := os.Stat(name); err == nil {
    60  		if fi.Mode()&fs.ModeExclusive == 0 {
    61  			if err := os.Chmod(name, fi.Mode()|fs.ModeExclusive); err != nil {
    62  				return nil, err
    63  			}
    64  		}
    65  	} else if !os.IsNotExist(err) {
    66  		return nil, err
    67  	}
    68  
    69  	nextSleep := 1 * time.Millisecond
    70  	const maxSleep = 500 * time.Millisecond
    71  	for {
    72  		f, err := os.OpenFile(name, flag, perm|fs.ModeExclusive)
    73  		if err == nil {
    74  			return f, nil
    75  		}
    76  
    77  		if !isLocked(err) {
    78  			return nil, err
    79  		}
    80  
    81  		time.Sleep(nextSleep)
    82  
    83  		nextSleep += nextSleep
    84  		if nextSleep > maxSleep {
    85  			nextSleep = maxSleep
    86  		}
    87  		// Apply 10% jitter to avoid synchronizing collisions.
    88  		nextSleep += time.Duration((0.1*rand.Float64() - 0.05) * float64(nextSleep))
    89  	}
    90  }
    91  
    92  func closeFile(f *os.File) error {
    93  	return f.Close()
    94  }
    95
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