src/main/java/rx/internal/operators/BackpressureUtils.java
/**
* Copyright 2015 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
package rx.internal.operators;
import java.util.Queue;
import java.util.concurrent.atomic.*;
import rx.Subscriber;
import rx.functions.Func1;
import rx.internal.util.UtilityFunctions;
/**
* Utility functions for use with backpressure.
*
*/
public final class BackpressureUtils {
/**
* Masks the most significant bit, i.e., 0x8000_0000_0000_0000L.
*/
static final long COMPLETED_MASK = Long.MIN_VALUE;
/**
* Masks the request amount bits, i.e., 0x7FFF_FFFF_FFFF_FFFF.
*/
static final long REQUESTED_MASK = Long.MAX_VALUE;
/** Utility class, no instances. */
private BackpressureUtils() {
throw new IllegalStateException("No instances!");
}
/**
* Adds {@code n} (not validated) to {@code requested} and returns the value prior to addition once the
* addition is successful (uses CAS semantics). If overflows then sets
* {@code requested} field to {@code Long.MAX_VALUE}.
*
* @param requested
* atomic long that should be updated
* @param n
* the number of requests to add to the requested count, positive (not validated)
* @return requested value just prior to successful addition
*/
public static long getAndAddRequest(AtomicLong requested, long n) {
// add n to field but check for overflow
while (true) {
long current = requested.get();
long next = addCap(current, n);
if (requested.compareAndSet(current, next)) {
return current;
}
}
}
/**
* Multiplies two positive longs and caps the result at Long.MAX_VALUE.
* @param a the first value
* @param b the second value
* @return the capped product of a and b
*/
public static long multiplyCap(long a, long b) {
long u = a * b;
if (((a | b) >>> 31) != 0) {
if (b != 0L && (u / b != a)) {
u = Long.MAX_VALUE;
}
}
return u;
}
/**
* Adds two positive longs and caps the result at Long.MAX_VALUE.
* @param a the first value
* @param b the second value
* @return the capped sum of a and b
*/
public static long addCap(long a, long b) {
long u = a + b;
if (u < 0L) {
u = Long.MAX_VALUE;
}
return u;
}
/**
* Signals the completion of the main sequence and switches to post-completion replay mode.
*
* <p>
* Don't modify the queue after calling this method!
*
* <p>
* Post-completion backpressure handles the case when a source produces values based on
* requests when it is active but more values are available even after its completion.
* In this case, the onCompleted() can't just emit the contents of the queue but has to
* coordinate with the requested amounts. This requires two distinct modes: active and
* completed. In active mode, requests flow through and the queue is not accessed but
* in completed mode, requests no-longer reach the upstream but help in draining the queue.
* <p>
* The algorithm utilizes the most significant bit (bit 63) of a long value (AtomicLong) since
* request amount only goes up to Long.MAX_VALUE (bits 0-62) and negative values aren't
* allowed.
*
* @param <T> the value type to emit
* @param requested the holder of current requested amount
* @param queue the queue holding values to be emitted after completion
* @param actual the subscriber to receive the values
*/
public static <T> void postCompleteDone(AtomicLong requested, Queue<T> queue, Subscriber<? super T> actual) {
postCompleteDone(requested, queue, actual, UtilityFunctions.<T>identity());
}
/**
* Accumulates requests (validated) and handles the completed mode draining of the queue based on the requests.
*
* <p>
* Post-completion backpressure handles the case when a source produces values based on
* requests when it is active but more values are available even after its completion.
* In this case, the onCompleted() can't just emit the contents of the queue but has to
* coordinate with the requested amounts. This requires two distinct modes: active and
* completed. In active mode, requests flow through and the queue is not accessed but
* in completed mode, requests no-longer reach the upstream but help in draining the queue.
*
* @param <T> the value type to emit
* @param requested the holder of current requested amount
* @param n the value requested;
* @param queue the queue holding values to be emitted after completion
* @param actual the subscriber to receive the values
* @return true if in the active mode and the request amount of n can be relayed to upstream, false if
* in the post-completed mode and the queue is draining.
*/
public static <T> boolean postCompleteRequest(AtomicLong requested, long n, Queue<T> queue, Subscriber<? super T> actual) {
return postCompleteRequest(requested, n, queue, actual, UtilityFunctions.<T>identity());
}
/**
* Signals the completion of the main sequence and switches to post-completion replay mode
* and allows exit transformation on the queued values.
*
* <p>
* Don't modify the queue after calling this method!
*
* <p>
* Post-completion backpressure handles the case when a source produces values based on
* requests when it is active but more values are available even after its completion.
* In this case, the onCompleted() can't just emit the contents of the queue but has to
* coordinate with the requested amounts. This requires two distinct modes: active and
* completed. In active mode, requests flow through and the queue is not accessed but
* in completed mode, requests no-longer reach the upstream but help in draining the queue.
* <p>
* The algorithm utilizes the most significant bit (bit 63) of a long value (AtomicLong) since
* request amount only goes up to Long.MAX_VALUE (bits 0-62) and negative values aren't
* allowed.
*
* @param <T> the value type in the queue
* @param <R> the value type to emit
* @param requested the holder of current requested amount
* @param queue the queue holding values to be emitted after completion
* @param actual the subscriber to receive the values
* @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted
*/
public static <T, R> void postCompleteDone(AtomicLong requested, Queue<T> queue, Subscriber<? super R> actual, Func1<? super T, ? extends R> exitTransform) {
for (;;) {
long r = requested.get();
// switch to completed mode only once
if ((r & COMPLETED_MASK) != 0L) {
return;
}
//
long u = r | COMPLETED_MASK;
if (requested.compareAndSet(r, u)) {
// if we successfully switched to post-complete mode and there
// are requests available start draining the queue
if (r != 0L) {
// if the switch happened when there was outstanding requests, start draining
postCompleteDrain(requested, queue, actual, exitTransform);
}
return;
}
}
}
/**
* Accumulates requests (validated) and handles the completed mode draining of the queue based on the requests
* and allows exit transformation on the queued values.
*
* <p>
* Post-completion backpressure handles the case when a source produces values based on
* requests when it is active but more values are available even after its completion.
* In this case, the onCompleted() can't just emit the contents of the queue but has to
* coordinate with the requested amounts. This requires two distinct modes: active and
* completed. In active mode, requests flow through and the queue is not accessed but
* in completed mode, requests no-longer reach the upstream but help in draining the queue.
*
* @param <T> the value type in the queue
* @param <R> the value type to emit
* @param requested the holder of current requested amount
* @param n the value requested;
* @param queue the queue holding values to be emitted after completion
* @param actual the subscriber to receive the values
* @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted
* @return true if in the active mode and the request amount of n can be relayed to upstream, false if
* in the post-completed mode and the queue is draining.
*/
public static <T, R> boolean postCompleteRequest(AtomicLong requested, long n, Queue<T> queue, Subscriber<? super R> actual, Func1<? super T, ? extends R> exitTransform) {
if (n < 0L) {
throw new IllegalArgumentException("n >= 0 required but it was " + n);
}
if (n == 0) {
return (requested.get() & COMPLETED_MASK) == 0;
}
for (;;) {
long r = requested.get();
// mask of the completed flag
long c = r & COMPLETED_MASK;
// mask of the requested amount
long u = r & REQUESTED_MASK;
// add the current requested amount and the new requested amount
// cap at Long.MAX_VALUE;
long v = addCap(u, n);
// restore the completed flag
v |= c;
if (requested.compareAndSet(r, v)) {
// if there was no outstanding request before and in
// the post-completed state, start draining
if (r == COMPLETED_MASK) {
postCompleteDrain(requested, queue, actual, exitTransform);
return false;
}
// returns true for active mode and false if the completed flag was set
return c == 0L;
}
}
}
/**
* Drains the queue based on the outstanding requests in post-completed mode (only!)
* and allows exit transformation on the queued values.
*
* @param <T> the value type in the queue
* @param <R> the value type to emit
* @param requested the holder of current requested amount
* @param queue the queue holding values to be emitted after completion
* @param subscriber the subscriber to receive the values
* @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted
*/
static <T, R> void postCompleteDrain(AtomicLong requested, Queue<T> queue, Subscriber<? super R> subscriber, Func1<? super T, ? extends R> exitTransform) {
long r = requested.get();
// Run on a fast-path if the downstream is unbounded
if (r == Long.MAX_VALUE) {
for (;;) {
if (subscriber.isUnsubscribed()) {
return;
}
T v = queue.poll();
if (v == null) {
subscriber.onCompleted();
return;
}
subscriber.onNext(exitTransform.call(v));
}
}
/*
* Since we are supposed to be in the post-complete state,
* requested will have its top bit set.
* To allow direct comparison, we start with an emission value which has also
* this flag set, then increment it as usual.
* Since COMPLETED_MASK is essentially Long.MIN_VALUE,
* there won't be any overflow or sign flip.
*/
long e = COMPLETED_MASK;
for (;;) {
/*
* This is an improved queue-drain algorithm with a specialization
* in which we know the queue won't change anymore (i.e., done is always true
* when looking at the classical algorithm and there is no error).
*
* Note that we don't check for cancellation or emptiness upfront for two reasons:
* 1) if e != r, the loop will do this and we quit appropriately
* 2) if e == r, then either there was no outstanding requests or we emitted the requested amount
* and the execution simply falls to the e == r check below which checks for emptiness anyway.
*/
while (e != r) {
if (subscriber.isUnsubscribed()) {
return;
}
T v = queue.poll();
if (v == null) {
subscriber.onCompleted();
return;
}
subscriber.onNext(exitTransform.call(v));
e++;
}
/*
* If the emission count reaches the requested amount the same time the queue becomes empty
* this will make sure the subscriber is completed immediately instead of on the next request.
* This is also true if there are no outstanding requests (this the while loop doesn't run)
* and the queue is empty from the start.
*/
if (e == r) {
if (subscriber.isUnsubscribed()) {
return;
}
if (queue.isEmpty()) {
subscriber.onCompleted();
return;
}
}
/*
* Fast flow: see if more requests have arrived in the meantime.
* This avoids an atomic add (~40 cycles) and resumes the emission immediately.
*/
r = requested.get();
if (r == e) {
/*
* Atomically decrement the requested amount by the emission amount.
* We can't use the full emission value because of the completed flag,
* however, due to two's complement representation, the flag on requested
* is preserved.
*/
r = requested.addAndGet(-(e & REQUESTED_MASK));
// The requested amount actually reached zero, quit
if (r == COMPLETED_MASK) {
return;
}
// reset the emission count
e = COMPLETED_MASK;
}
}
}
/**
* Atomically subtracts a value from the requested amount unless it's at Long.MAX_VALUE.
* @param requested the requested amount holder
* @param n the value to subtract from the requested amount, has to be positive (not verified)
* @return the new requested amount
* @throws IllegalStateException if n is greater than the current requested amount, which
* indicates a bug in the request accounting logic
*/
public static long produced(AtomicLong requested, long n) {
for (;;) {
long current = requested.get();
if (current == Long.MAX_VALUE) {
return Long.MAX_VALUE;
}
long next = current - n;
if (next < 0L) {
throw new IllegalStateException("More produced than requested: " + next);
}
if (requested.compareAndSet(current, next)) {
return next;
}
}
}
/**
* Validates the requested amount and returns true if it is positive.
* @param n the requested amount
* @return true if n is positive
* @throws IllegalArgumentException if n is negative
*/
public static boolean validate(long n) {
if (n < 0) {
throw new IllegalArgumentException("n >= 0 required but it was " + n);
}
return n != 0L;
}
}