downloader.copyLocal should use Clonefile on macOS

[jandubois]

Description

Most of the copies made by downloader will remain identical copies of files in the cache (e.g. basedisk), and the files are often quite large.

On macOS (with apfs) using unix.Clonefile instead of fs.CopyFile will be much faster and save disk space, as the copy will point to the same datablocks as the source until they are modified (via "Copy on Write"). Only file metadata is copied.

Clonefile was added to x/sys/unix in golang/sys@2334cc1

[afbjorklund]

There is reflink on Linux, but only on select filesystems. (i.e. btrfs or xfs)

https://github.com/KarpelesLab/reflink

Seems like reflink.Auto could work ?

"Other OSes have similar features, to be implemented in the future." (2020)

[AkihiroSuda]

This should be implemented in

• fs.CopyFile should use clonefile(2) on macOS when VOL_CAP_INT_CLONE is available containerd/continuity#181

[AkihiroSuda]

Implemented in:

• build(deps): bump github.com/containerd/continuity from 0.3.0 to 0.4.0 #1544