Fix 'request(0)' issue in Scan

[zsxwing]

#1904 didn't all cases.

[benjchristensen]

What does excessive mean? I'm re-reading this code and the intent of that variable doesn't jump out at me.

[zsxwing]

It means we request more than what we should.

If a user requests 1 from scan in onStart, scan need to request 1 to the upstream. However, when scan receives an onNext, it will emit 2 onNext to the downstream: the initial value and accumulator.call(this.value, initialvalue). So the downstream only requests 1, but scan emits 2 values. I use excessive to indicate we are in an excessive state.

So the next time when the downstream requests, I will try to request n-1.

[zsxwing]

If a user requests X (X > 1) from scan in onStart, scan can simply request (X-1). In such case, excessive will be false.

[benjchristensen]

Got it, thanks for the explanation.

[benjchristensen]

The logic seems correct to me on a re-read of this code.