#30 Shim: fix the "deadlock avoided" error during the rootfs mount

[akhramov]

Issue number: #30

Description of changes:

Currently, the sporadic "mount_nullfs: /path/to/rootfs: resource
deadlock avoided" error occurs during containerd shim task creation.
The exact reason for this error is unknown, but a mounter that
executes the system call instead of running a shell command seems to
fix the problem.

This change replaces the containerd's mounter with moby/sys/mount

Testing done:

I ran the buildkit's integration test locally with local vagrant: moby/buildkit#2376
Reproduced the error, and ensured that the error is not reproducible with the updated containerd shim.

Terms of contribution:

By submitting this pull request, I agree that this contribution is licensed under the terms found in the LICENSE file.

[samuelkarp]

The exact reason for this error is unknown, but a mounter that executes the system call instead of running a shell command seems to fix the problem.

The existing implementation also executes a system call (calls unix.Unmount, which calls unmount(2)). I think the difference is that the moby/sys/mount implementation has a slow path which sorts and unmounts nested mountpoints.

[samuelkarp]

samuelkarp force-pushed the fix/shim-mount-rootfs branch from 1caf307 to aa66a93

I reworded the commit message but otherwise didn't change any of the code. Thanks for the contribution!