Feature: CancelGuard

[vorner]

Hello

I've noticed that usually when I want to have something cancelable (eg. I have a CancelHandle or anything that implements Cancel), I often want to wrap it into a guard that cancels on drop.

I wonder if it makes sense to:

• Introduce a CancelGuard<C: Cancel> wrapper that calls self.0.cancel in its Drop.
• Add a (provided) method to Cancel to wrap itself into the wrapper.

I can send the pull request if you like the idea.

[fralalonde]

The behavior would certainly be desired. How is the Guard different from having blanket impl Drop for Cancel? Currently, buffered Scopes impl Drop to flush their buffer as a final action. Would the Guard pattern be preferable there too?

[vorner]

I think in three ways:

• I don't think you can do blanked impl on Drop. One could do an implementation for each type in the library, though.
• That would be a breaking change, one that would not be discovered by a compiler screaming at the user, but would change behaviour.
• I believe for things that can be cancelled, it can be desirable to just not cancel. Someone might want to just start the thing, drop the cancel handle and leave it running until the end of the world/application. This will not be easily doable with the drop on the basic handle.

In other word, the guard makes it an explicit request to „activate“ the cancel on drop.

[fralalonde]

Oh, I got my "blanket" wrong. So, would the Handle need to be explicitly wrapped with a Guard by the client?

I had also thought about making Flush-On-Drop behavior optional but would have done so with an Output attribute flag, e.g. output.flush_on_drop(false).new_scope(). The flag value would be copied to the Scope upon creation and looked at in the Drop impl to decide if flushshould be called or not. This way the same type (Scope) is returned not matter the behavior.

[vorner]

Flush on drop probably doesn't hurt if it happens and you didn't want it. Cancel on drop does hurt when you don't want it.

Anyway, this is what we have now:

let observe_cancel = bucket
    .observe(bucket.gauge("answer"), |_| 42))
    .on_flush();
// A bulk of computation or something happens here
observe_cancel.cancel();

What I propose is this:

let _guard = bucket
    .observe(bucket.gauge("answer"), |_| 42))
    .on_flush()
    .into_guard(); // Provided method in Cancel, that just takes self and calls CancelGuard::new(self)
// The computation happens here
// Canceled here by drop

[fralalonde]

Ok, if you have a PR I'll merge it