request(0) is supposed to work?

[zsxwing]

The docs about request doesn't say anything requirement about n

public interface Producer {

    /**
     * Request a certain maximum number of items from this Producer. This is a way of requesting backpressure.
     * To disable backpressure, pass {@code Long.MAX_VALUE} to this method.
     *
     * @param n the maximum number of items you want this Producer to produce, or {@code Long.MAX_VALUE} if you
     *          want the Producer to produce items at its own pace
     */
    public void request(long n);

}

    /**
     * Request a certain maximum number of emitted items from the Observable this Subscriber is subscribed to.
     * This is a way of requesting backpressure. To disable backpressure, pass {@code Long.MAX_VALUE} to this
     * method.
     *
     * @param n the maximum number of items you want the Observable to emit to the Subscriber at this time, or
     *           {@code Long.MAX_VALUE} if you want the Observable to emit items at its own pace
     */
    protected final void request(long n) {

I assume n must be greater than 0. But I also notice I may write something like this occasionally:

long getTheNextRequestNumber() {
  if (...) {
       ...
       return x; // x>0
  } else {
        ...
       return 0;
   }
}

long n = getTheNextRequestNumber();
request(n);

So request(0) is fine? If not, I think it's better to document it.

[benjchristensen]

request(0) should do nothing, be a no-op.

[akarnokd]

I concur yet some places, a request(0) may trigger a loop and decrement the request counter below zero on the exit edge or do some other sideeffects. (Search for .getAndAdd( in the code)

• OnSubscribeCombineLatest: will subscribe to sources and fill the internal buffers but not emit anything
• OnSubscribeRedo: will trigger a reschedule and clears the boundary indicator
• OperatorConcat: will deliver oncompleted
• OperatorGroupBy: does an atomic increment and atomic decrement
• OperatorObserveOn: does schedule a thread and may decrement request below 0 for a short amount of time
• OnBackpressureBlock: is incorrect anyway, but triggers an empty emission loop
• OnBackpressureBuffer: triggers a drain loop and may decrement request below 0 for a short time

[benjchristensen]

Then it looks like we should better document that request(0) should do nothing, and fix these places in code so they really do nothing.

Do we want to ignore a negative number as well (like 0), or throw an IllegalArgumentException? As a data point the Reactive Streams spec throws an IAE.

[zsxwing]

Do we want to ignore a negative number as well (like 0), or throw an IllegalArgumentException? As a data point the Reactive Streams spec throws an IAE.

Usually a negative number is not intentional, usually means a bug in some place. So I like IAE rather than ignoring them.

[benjchristensen]

I'm okay with IAE.

[akarnokd]

I'm using request(0) some places/tests to suppress the default behavior of requesting unlimited amount.
Negative, I think, is rather a bug than a convenience for no-op.