Track number of documents in SessionCollectionView

[koszta]

Fixes issue #9828

[veered]

@koszta You verified that this resolved your performance issues?

[koszta]

@veered Yes, thanks for the hint!

In the reproduction repo there is a fix branch using the patched ddp-server:
https://github.com/fuww/blocked-unsub/tree/fix

Now it is not blocking the event loop for more than ~20-30ms while unsubscribing with 2000 published documents.

[nathan-muir]

Awesome! Removing the call to _.isEmpty reduces the time to unsub from 5s to 500ms (for a production dataset)

Just a thought: instead of the counter, we could also use a Map which has a .size property & better suited to adding/removing items and iteration. (eg. devel...nathan-muir:nm-ddp-perf)

[veered]

This definitely seems worth merging (perhaps after making the change suggested by @nathan-muir )!

[benjamn]

I agree that using a Map would make this easier!

[koszta]

I also agree, but there is DiffSequence.diffObjects called with self.documents, which I assume would not work with a Map

[koszta]

Closing this in favor of #10053