138 lines
3.1 KiB
Go
138 lines
3.1 KiB
Go
// Handle pid file based locking.
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package lockfile
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import (
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"errors"
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"fmt"
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"io/ioutil"
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"os"
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"path/filepath"
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)
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type Lockfile string
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var (
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ErrBusy = errors.New("Locked by other process") // If you get this, retry after a short sleep might help
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ErrNeedAbsPath = errors.New("Lockfiles must be given as absolute path names")
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ErrInvalidPid = errors.New("Lockfile contains invalid pid for system")
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ErrDeadOwner = errors.New("Lockfile contains pid of process not existent on this system anymore")
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)
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// Describe a new filename located at path. It is expected to be an absolute path
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func New(path string) (Lockfile, error) {
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if !filepath.IsAbs(path) {
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return Lockfile(""), ErrNeedAbsPath
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}
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return Lockfile(path), nil
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}
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// Who owns the lockfile?
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func (l Lockfile) GetOwner() (*os.Process, error) {
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name := string(l)
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// Ok, see, if we have a stale lockfile here
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content, err := ioutil.ReadFile(name)
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if err != nil {
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return nil, err
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}
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var pid int
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_, err = fmt.Sscanln(string(content), &pid)
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if err != nil {
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return nil, ErrInvalidPid
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}
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// try hard for pids. If no pid, the lockfile is junk anyway and we delete it.
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if pid > 0 {
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running, err := isRunning(pid)
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if err != nil {
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return nil, err
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}
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if running {
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proc, err := os.FindProcess(pid)
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if err != nil {
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return nil, err
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}
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return proc, nil
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} else {
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return nil, ErrDeadOwner
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}
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} else {
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return nil, ErrInvalidPid
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}
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panic("Not reached")
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}
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// Try to get Lockfile lock. Returns nil, if successful and and error describing the reason, it didn't work out.
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// Please note, that existing lockfiles containing pids of dead processes and lockfiles containing no pid at all
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// are deleted.
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func (l Lockfile) TryLock() error {
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name := string(l)
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// This has been checked by New already. If we trigger here,
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// the caller didn't use New and re-implemented it's functionality badly.
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// So panic, that he might find this easily during testing.
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if !filepath.IsAbs(string(name)) {
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panic(ErrNeedAbsPath)
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}
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tmplock, err := ioutil.TempFile(filepath.Dir(name), "")
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if err != nil {
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return err
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} else {
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defer func(){
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tmplock.Close()
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os.Remove(tmplock.Name())
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}()
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}
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_, err = tmplock.WriteString(fmt.Sprintf("%d\n", os.Getpid()))
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if err != nil {
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return err
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}
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// return value intentionally ignored, as ignoring it is part of the algorithm
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_ = os.Link(tmplock.Name(), name)
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fiTmp, err := os.Lstat(tmplock.Name())
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if err != nil {
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return err
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}
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fiLock, err := os.Lstat(name)
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if err != nil {
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return err
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}
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// Success
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if os.SameFile(fiTmp, fiLock) {
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return nil
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}
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_, err = l.GetOwner()
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switch err {
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default:
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// Other errors -> defensively fail and let caller handle this
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return err
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case nil:
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return ErrBusy
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case ErrDeadOwner, ErrInvalidPid:
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// cases we can fix below
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}
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// clean stale/invalid lockfile
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err = os.Remove(name)
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if err != nil {
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return err
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}
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// now that we cleaned up the stale lockfile, let's recurse
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return l.TryLock()
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}
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// Release a lock again. Returns any error that happend during release of lock.
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func (l Lockfile) Unlock() error {
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return os.Remove(string(l))
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}
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