Store extended attributes in files in stead of in extended attributes

[aduffeck]

We added a new metadata backend for the decomposed storage driver that uses an additional .ini file to store file metadata. This allows scaling beyond some filesystem specific xattr limitations.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[butonic]

Yuck, the missing abstraction of a node causes a lot of noise and making the linter happy causes even more noise. The prefix usage is sprinkled over all decomposedfs files ... we really need to move that to the node as in 87a0d23 to truly decouple the implementation. The current xattr backend interface only ties the code further together by introducing methods like IsMetaFile(path string) bool, UsesExternalMetadataFile() bool and especially MetadataPath(path string) string.

I'm ok with merging it as is, as it introduces a new experimental backend for metadata which frees us from xattr limitations in some corner cases. But before touching this again we should refactor this mess ASAP.

[C0rby]

LGTM. Just one small nitpick

[kobergj]

LGTM 👍 But can we run a [full-ci] pipeline with this changes on ocis before merging? If you did already please ignore the comment.

[butonic]

LGTM 👍 But can we run a [full-ci] pipeline with this changes on ocis before merging? If you did already please ignore the comment.

running in https://drone.owncloud.com/owncloud/ocis/19330

[butonic]

ocis full-ci green, too https://drone.owncloud.com/owncloud/ocis/19330

[labkode]

Hi, I just want to give my 2 cents on this.

Keeping a metadata file in sync with the file is referring to is very hard task. Atomicity is paramount to avoid race conditions.

• How are you preventing two concurrent updates to the metadata of the file?
• The use of flock is process-dependant. The storage layer dealing with files is usually one single process . How do you flock.
• The use of flock it's has always a pain in the ass on top of NFS (what enterprises usually want). fnctl should be used in these cases.