Description

The push handler does not work properly if there are multiple pushes of the same blob running concurrently. It seems that both pushes write to the same place and one of them will fail with an error saying that twice as many bytes were written as are in the descriptor.

This is not only the case when two competing pushes are pushing same data to the same location but it is quite easy to hit it when just pushing a single manifest list as the subimages can easily contain duplicates. Also possible to hit it by just pushing multiple images that share some layers to the same repo.

Steps to reproduce the issue:

1. Push an image(s)/manifest-list with same layers

Output of containerd --version:

v1.1.3