Make transaction log commit with filesystem storage backend to be atomic

[mosyp]

Description

This PR makes transaction log commit by writing file in put_obj first in temporary file and once that successful then uses atomic rename (with checking on destination existence) to move temporary file into final commit file.

Related Issue(s)

Fixes #94

Documentation

• Linux renameat2 https://man7.org/linux/man-pages/man2/rename.2.html
• Mac renamex_np https://www.unix.com/man-page/mojave/2/renamex_np/

[mosyp]

I've got several concern here

• If rename fails due to concurrent commits, it'll start over again with incremented version which means that we'll have it writes tmp file again and again. However the impact will be on higher level within try_commit_loop which is out of backend storage lvl.
• Current impl does not remove tmp files once it fails, but this should be added/revised once previous concern is resolved

[houqp]

The rest looks good to me 👍 just need to fix that build issue :D

[xianwill]

I've got several concern here

• If rename fails due to concurrent commits, it'll start over again with incremented version which means that we'll have it writes tmp file again and again. However the impact will be on higher level within try_commit_loop which is out of backend storage lvl.
• Current impl does not remove tmp files once it fails, but this should be added/revised once previous concern is resolved

@mosyp - Regarding 1 you mentioned above, I'll create a separate issue for that problem. Ultimately, we need to expose a rename_obj on the StorageBackend so DeltaTransaction can handle it the same way across storage backends. I.e.

* write temp file
* optimistic commit loop:
  * atomic rename
  * if version already exists
    * do conflict resolution if update, update temp file
  * increase version

[mosyp]

@xianwill yes, that's a good idea to move it out of this scope. I feel like once we're done with s3 atomic rename, then we're safe to create some abstraction which will work for both fs and s3

[xianwill]

Posted #135

[houqp]

great work @mosyp !

[houqp]

@xianwill want to take a final look before we merge?

[xianwill]

LGTM! Great work @mosyp!