storage: use object to pin and unpin version

[skyzh]

risinglight/src/storage/secondary/version_manager.rs

Lines 258 to 263 in 7b5bc7a

	pub fn pin(&self) -> (u64, Arc<Snapshot>) {
	let mut inner = self.inner.lock();
	let epoch = inner.epoch;
	*inner.ref_cnt.entry(epoch).or_default() += 1;
	(epoch, inner.status.get(&epoch).unwrap().clone())
	}

Currently, users will need to manually call pin and unpin. If there are futures cancelled or somehow, it is possible that we fail to unpin by ourselves.

We should implement a struct called Version, and implement Drop for it. pin will return a Arc<Version> instead of some strange things. When a Version gets dropped (which means that all reference of Arc<Version> is now dropped), we can call unpin in Drop.

[skyzh]

pub struct Version(u64, Arc<Snapshot>);

impl Drop for Version {
 // ...
}

pub fn pin(&self) -> Arc<Version> { /* .. */ }

[Yisaer]

I'm not familiar with rust, May I try this issue? :)

[skyzh]

Sure, thanks for that!