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Fix 'request(0)' issue in Scan #1957

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Dec 13, 2014
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Fix 'request(0)' issue in Scan #1957

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16 changes: 7 additions & 9 deletions src/main/java/rx/internal/operators/OperatorScan.java
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Original file line number Diff line number Diff line change
Expand Up @@ -147,19 +147,17 @@ public void request(long n) {
if (once.compareAndSet(false, true)) {
if (initialValue == NO_INITIAL_VALUE || n == Long.MAX_VALUE) {
producer.request(n);
} else if (n == 1) {
excessive.set(true);
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What does excessive mean? I'm re-reading this code and the intent of that variable doesn't jump out at me.

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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.

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

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Got it, thanks for the explanation.

producer.request(1); // request at least 1
} else {
if (n == Long.MAX_VALUE) {
producer.request(Long.MAX_VALUE);
} else if (n == 1) {
excessive.set(true);
producer.request(1); // request at least 1
} else {
producer.request(n - 1);
}
// n != Long.MAX_VALUE && n != 1
producer.request(n - 1);
}
} else {
// pass-thru after first time
if (excessive.compareAndSet(true, false) && n != Long.MAX_VALUE) {
if (n > 1 // avoid to request 0
&& excessive.compareAndSet(true, false) && n != Long.MAX_VALUE) {
producer.request(n - 1);
} else {
producer.request(n);
Expand Down
56 changes: 52 additions & 4 deletions src/test/java/rx/internal/operators/OperatorScanTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -20,22 +20,22 @@
import static org.mockito.Matchers.any;
import static org.mockito.Matchers.anyInt;
import static org.mockito.Matchers.anyString;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.never;
import static org.mockito.Mockito.times;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.*;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;

import org.junit.Before;
import org.junit.Test;
import org.mockito.MockitoAnnotations;

import rx.Observable;
import rx.Observer;
import rx.Producer;
import rx.Subscriber;
import rx.functions.Action2;
import rx.functions.Func0;
Expand Down Expand Up @@ -312,4 +312,52 @@ public Integer call(Integer t1, Integer t2) {
subscriber.assertTerminalEvent();
subscriber.assertNoErrors();
}

@Test
public void testScanShouldNotRequestZero() {
final AtomicReference<Producer> producer = new AtomicReference<Producer>();
Observable<Integer> o = Observable.create(new Observable.OnSubscribe<Integer>() {
@Override
public void call(final Subscriber subscriber) {
Producer p = spy(new Producer() {

private AtomicBoolean requested = new AtomicBoolean(false);

@Override
public void request(long n) {
if (requested.compareAndSet(false, true)) {
subscriber.onNext(1);
} else {
subscriber.onCompleted();
}
}
});
producer.set(p);
subscriber.setProducer(p);
}
}).scan(100, new Func2<Integer, Integer, Integer>() {

@Override
public Integer call(Integer t1, Integer t2) {
return t1 + t2;
}

});

o.subscribe(new TestSubscriber<Integer>() {

@Override
public void onStart() {
request(1);
}

@Override
public void onNext(Integer integer) {
request(1);
}
});

verify(producer.get(), never()).request(0);
verify(producer.get(), times(2)).request(1);
}
}