content/local/locks.go

    基于ref的锁，ref可以是image的一个layer或image manifest文件

    这是tryLock的唯一入口，获取writer的函数

    1. func (s *store) Writer(ctx context.Context, opts ...content.WriterOpt) (content.Writer, error) {
    2.     var wOpts content.WriterOpts
    3.     for _, opt := range opts {
    4.         if err := opt(&wOpts); err != nil {
    5.             return nil, err
    6.         }
    7.     }
    8.     // TODO(AkihiroSuda): we could create a random string or one calculated based on the context
    9.     // https://github.com/containerd/containerd/issues/2129#issuecomment-380255019
    10.     if wOpts.Ref == "" {
    11.         return nil, errors.Wrap(errdefs.ErrInvalidArgument, "ref must not be empty")
    12.     }
    13.     var lockErr error
    14.     for count := uint64(0); count < 10; count++ {
    15.         if err := tryLock(wOpts.Ref); err != nil {
    16.             if !errdefs.IsUnavailable(err) {
    17.                 return nil, err
    18.             }
    20.             lockErr = err
    21.         } else {
    22.             lockErr = nil
    23.             break
    24.         }
    25.         time.Sleep(time.Millisecond * time.Duration(rand.Intn(1<<count)))
    26.     }
    28.     if lockErr != nil {
    29.         return nil, lockErr
    30.     }
    32.     w, err := s.writer(ctx, wOpts.Ref, wOpts.Desc.Size, wOpts.Desc.Digest)
    33.     if err != nil {
    34.         unlock(wOpts.Ref)
    35.         return nil, err
    36.     }
    38.     return w, nil // lock is now held by w.
    39. }

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