codel implementation diverges from spec

[smklein]

The implementation of ControlledDelay: https://github.com/TritonDataCenter/node-cueball/blob/master/lib/codel.js

References this specification: https://queue.acm.org/appendices/codel.html

I believe the implementation diverges from this reference specification in cases where a queue has entries that each take CODEL_INTERVAL duration to complete through the queue.

In the reference specification, this would place the queue in the "dropping" state, but the value of drop_next would be re-calculated each time.

In the current implementation, the following would happen:

okToDrop would be set to true:

node-cueball/lib/codel.js

Line 59 in b66d96e

var okToDrop = this.canDrop(now, start);

We'd be in the drop state:

node-cueball/lib/codel.js

Line 62 in b66d96e

if (this.cd_dropping) {

And for requests arriving at CODEL_INTERVAL, we'd drop them:

node-cueball/lib/codel.js

Lines 65 to 68 in b66d96e

	} else if (now >= this.cd_drop_next) {
	dropClaim = true;
	this.cd_count += 1;
	}

This doesn't invoke getDropNext to calculate the next plausible drop time -- it just leaves the old value, which is already in the past. This is where the behavior diverges from the reference specification -- in the reference spec, the value of drop_next is recalculated anytime the queue attempts to drop a value