Source file src/cmd/go/internal/lockedfile/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	// +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	}
    94	

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