WIP: Add Multithreading Support

[volfco]

Add Multithreading Support to fuser.

This implementation modifies BackgroundSession to accept a thread quantity. If this number is two or more, then n - 1 background workers will be created. There will always be a primary worker created, hence n - 1.

Session::worker is called to clone the Session, which in turn calls Channel::worker. This method accepts the Mount object, and using .session_fd to return the Session FD, fuse_fd_clone is invoked to clone the file descriptor and run the needed ioctl call to setup the session.

I can't take all the credit, as the main piece of code in channel.rs is from the Datenlord project where they are doing a native async_fuse library.

The code in question is from here: https://github.com/datenlord/datenlord/blob/d90fd43732373451207e56e9b9cd7eef9e7b53e1/src/async_fuse/fuse/session.rs#L73

Additional References:

• https://john-millikin.com/the-fuse-protocol#multi-threading
• https://fuse-devel.narkive.com/CodQceXu/how-could-fuse-support-parallel-opreation

TODO

• Clean up feature gate
• Add more function documentation
• Add changelog entry
• Bump version

[colinmarc]

Sorry for the random drive-by comment. In my opinion, it would be better to require Send + Sync instead of Clone, and change the trait methods to take non-mutable self. Then it's up to the user how they handle mutability, if they need it. Cloning + mutability is a massive footgun.

[volfco]

Thanks for the feedback!

I need to figure out exactly what you're referring to and understand it, but that does make sense and does seem better. Something fun to do this weekend!

[theunkn0wn1]

if a type is Send, it can be safely moved between threads.

if a type is Sync, it can be safely shared between threads.
see https://doc.rust-lang.org/nomicon/send-and-sync.html?highlight=Send#send-and-sync

[colinmarc]

For example, Arc<AtomicU64> is a totally reasonable way to handle mutability, here - but this example could have easily used a normal u64 before these changes and still derived Clone, leading to coinciding file handles.