io.reactivex.flowable.internal.utils.QueueDrainHelper Maven / Gradle / Ivy
Show all versions of rxjava3-flowable Show documentation
/**
* Copyright (c) 2016-present, RxJava Contributors.
*
* 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 io.reactivex.flowable.internal.utils;
import java.util.Queue;
import java.util.concurrent.atomic.AtomicLong;
import org.reactivestreams.*;
import hu.akarnokd.reactivestreams.extensions.FusedQueue;
import io.reactivex.common.Disposable;
import io.reactivex.common.exceptions.*;
import io.reactivex.common.functions.BooleanSupplier;
import io.reactivex.flowable.internal.queues.*;
import io.reactivex.flowable.internal.queues.SimplePlainQueue;
/**
* Utility class to help with the queue-drain serialization idiom.
*/
public final class QueueDrainHelper {
/** Utility class. */
private QueueDrainHelper() {
throw new IllegalStateException("No instances!");
}
/**
* Drain the queue but give up with an error if there aren't enough requests.
* @param the queue value type
* @param the emission value type
* @param q the queue
* @param a the subscriber
* @param delayError true if errors should be delayed after all normal items
* @param dispose the disposable to call when termination happens and cleanup is necessary
* @param qd the QueueDrain instance that gives status information to the drain logic
*/
public static void drainMaxLoop(SimplePlainQueue q, Subscriber a, boolean delayError,
Disposable dispose, QueueDrain qd) {
int missed = 1;
for (;;) {
for (;;) {
boolean d = qd.done();
T v = q.poll();
boolean empty = v == null;
if (checkTerminated(d, empty, a, delayError, q, qd)) {
if (dispose != null) {
dispose.dispose();
}
return;
}
if (empty) {
break;
}
long r = qd.requested();
if (r != 0L) {
if (qd.accept(a, v)) {
if (r != Long.MAX_VALUE) {
qd.produced(1);
}
}
} else {
q.clear();
if (dispose != null) {
dispose.dispose();
}
a.onError(new MissingBackpressureException("Could not emit value due to lack of requests."));
return;
}
}
missed = qd.leave(-missed);
if (missed == 0) {
break;
}
}
}
public static boolean checkTerminated(boolean d, boolean empty,
Subscriber s, boolean delayError, FusedQueue q, QueueDrain qd) {
if (qd.cancelled()) {
q.clear();
return true;
}
if (d) {
if (delayError) {
if (empty) {
Throwable err = qd.error();
if (err != null) {
s.onError(err);
} else {
s.onComplete();
}
return true;
}
} else {
Throwable err = qd.error();
if (err != null) {
q.clear();
s.onError(err);
return true;
} else
if (empty) {
s.onComplete();
return true;
}
}
}
return false;
}
/**
* Creates a queue: spsc-array if capacityHint is positive and
* spsc-linked-array if capacityHint is negative; in both cases, the
* capacity is the absolute value of prefetch.
* @param the value type of the queue
* @param capacityHint the capacity hint, negative value will create an array-based SPSC queue
* @return the queue instance
*/
public static FusedQueue createQueue(int capacityHint) {
if (capacityHint < 0) {
return new SpscLinkedArrayQueue(-capacityHint);
}
return new SpscArrayQueue(capacityHint);
}
/**
* Requests Long.MAX_VALUE if prefetch is negative or the exact
* amount if prefetch is positive.
* @param s the Subscription to request from
* @param prefetch the prefetch value
*/
public static void request(Subscription s, int prefetch) {
s.request(prefetch < 0 ? Long.MAX_VALUE : prefetch);
}
static final long COMPLETED_MASK = 0x8000000000000000L;
static final long REQUESTED_MASK = 0x7FFFFFFFFFFFFFFFL;
/**
* Accumulates requests (not validated) and handles the completed mode draining of the queue based on the requests.
*
*
* 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 onComplete() 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 the value type emitted
* @param n the request amount, positive (not validated)
* @param actual the target Subscriber to send events to
* @param queue the queue to drain if in the post-complete state
* @param state holds the request amount and the post-completed flag
* @param isCancelled a supplier that returns true if the drain has been cancelled
* @return true if the state indicates a completion state.
*/
public static boolean postCompleteRequest(long n,
Subscriber actual,
Queue queue,
AtomicLong state,
BooleanSupplier isCancelled) {
for (; ; ) {
long r = state.get();
// extract the current request amount
long r0 = r & REQUESTED_MASK;
// preserve COMPLETED_MASK and calculate new requested amount
long u = (r & COMPLETED_MASK) | BackpressureHelper.addCap(r0, n);
if (state.compareAndSet(r, u)) {
// (complete, 0) -> (complete, n) transition then replay
if (r == COMPLETED_MASK) {
postCompleteDrain(n | COMPLETED_MASK, actual, queue, state, isCancelled);
return true;
}
// (active, r) -> (active, r + n) transition then continue with requesting from upstream
return false;
}
}
}
static boolean isCancelled(BooleanSupplier cancelled) {
try {
return cancelled.getAsBoolean();
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
return true;
}
}
/**
* Drains the queue based on the outstanding requests in post-completed mode (only!).
*
* @param n the current request amount
* @param actual the target Subscriber to send events to
* @param queue the queue to drain if in the post-complete state
* @param state holds the request amount and the post-completed flag
* @param isCancelled a supplier that returns true if the drain has been cancelled
* @return true if the queue was completely drained or the drain process was cancelled
*/
static boolean postCompleteDrain(long n,
Subscriber actual,
Queue queue,
AtomicLong state,
BooleanSupplier isCancelled) {
// TODO enable fast-path
// if (n == -1 || n == Long.MAX_VALUE) {
// for (;;) {
// if (isCancelled.getAsBoolean()) {
// break;
// }
//
// T v = queue.poll();
//
// if (v == null) {
// actual.onComplete();
// break;
// }
//
// actual.onNext(v);
// }
//
// return true;
// }
long e = n & COMPLETED_MASK;
for (; ; ) {
while (e != n) {
if (isCancelled(isCancelled)) {
return true;
}
T t = queue.poll();
if (t == null) {
actual.onComplete();
return true;
}
actual.onNext(t);
e++;
}
if (isCancelled(isCancelled)) {
return true;
}
if (queue.isEmpty()) {
actual.onComplete();
return true;
}
n = state.get();
if (n == e) {
n = state.addAndGet(-(e & REQUESTED_MASK));
if ((n & REQUESTED_MASK) == 0L) {
return false;
}
e = n & COMPLETED_MASK;
}
}
}
/**
* Signals the completion of the main sequence and switches to post-completion replay mode.
*
*
* Don't modify the queue after calling this method!
*
*
* 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 onComplete() 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.
*
* 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 the value type emitted
* @param actual the target Subscriber to send events to
* @param queue the queue to drain if in the post-complete state
* @param state holds the request amount and the post-completed flag
* @param isCancelled a supplier that returns true if the drain has been cancelled
*/
public static void postComplete(Subscriber actual,
Queue queue,
AtomicLong state,
BooleanSupplier isCancelled) {
if (queue.isEmpty()) {
actual.onComplete();
return;
}
if (postCompleteDrain(state.get(), actual, queue, state, isCancelled)) {
return;
}
for (; ; ) {
long r = state.get();
if ((r & COMPLETED_MASK) != 0L) {
return;
}
long u = r | COMPLETED_MASK;
// (active, r) -> (complete, r) transition
if (state.compareAndSet(r, u)) {
// if the requested amount was non-zero, drain the queue
if (r != 0L) {
postCompleteDrain(u, actual, queue, state, isCancelled);
}
return;
}
}
}
}