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Reactive Extensions for Java
/**
* 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.internal.operators.flowable;
import java.util.concurrent.atomic.*;
import org.reactivestreams.*;
import io.reactivex.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.exceptions.*;
import io.reactivex.flowables.ConnectableFlowable;
import io.reactivex.functions.Consumer;
import io.reactivex.internal.fuseable.*;
import io.reactivex.internal.queue.SpscArrayQueue;
import io.reactivex.internal.subscriptions.SubscriptionHelper;
import io.reactivex.internal.util.*;
import io.reactivex.plugins.RxJavaPlugins;
/**
* A connectable observable which shares an underlying source and dispatches source values to subscribers in a backpressure-aware
* manner.
* @param the value type
*/
public final class FlowablePublish extends ConnectableFlowable implements HasUpstreamPublisher {
/**
* Indicates this child has been cancelled: the state is swapped in atomically and
* will prevent the dispatch() to emit (too many) values to a terminated child subscriber.
*/
static final long CANCELLED = Long.MIN_VALUE;
/** The source observable. */
final Flowable source;
/** Holds the current subscriber that is, will be or just was subscribed to the source observable. */
final AtomicReference> current;
/** The size of the prefetch buffer. */
final int bufferSize;
final Publisher onSubscribe;
/**
* Creates a OperatorPublish instance to publish values of the given source observable.
* @param the source value type
* @param source the source observable
* @param bufferSize the size of the prefetch buffer
* @return the connectable observable
*/
public static ConnectableFlowable create(Flowable source, final int bufferSize) {
// the current connection to source needs to be shared between the operator and its onSubscribe call
final AtomicReference> curr = new AtomicReference>();
Publisher onSubscribe = new FlowablePublisher(curr, bufferSize);
return RxJavaPlugins.onAssembly(new FlowablePublish(onSubscribe, source, curr, bufferSize));
}
private FlowablePublish(Publisher onSubscribe, Flowable source,
final AtomicReference> current, int bufferSize) {
this.onSubscribe = onSubscribe;
this.source = source;
this.current = current;
this.bufferSize = bufferSize;
}
@Override
public Publisher source() {
return source;
}
@Override
protected void subscribeActual(Subscriber s) {
onSubscribe.subscribe(s);
}
@Override
public void connect(Consumer connection) {
boolean doConnect;
PublishSubscriber ps;
// we loop because concurrent connect/disconnect and termination may change the state
for (;;) {
// retrieve the current subscriber-to-source instance
ps = current.get();
// if there is none yet or the current has been disposed
if (ps == null || ps.isDisposed()) {
// create a new subscriber-to-source
PublishSubscriber u = new PublishSubscriber(current, bufferSize);
// try setting it as the current subscriber-to-source
if (!current.compareAndSet(ps, u)) {
// did not work, perhaps a new subscriber arrived
// and created a new subscriber-to-source as well, retry
continue;
}
ps = u;
}
// if connect() was called concurrently, only one of them should actually
// connect to the source
doConnect = !ps.shouldConnect.get() && ps.shouldConnect.compareAndSet(false, true);
break; // NOPMD
}
/*
* Notify the callback that we have a (new) connection which it can dispose
* but since ps is unique to a connection, multiple calls to connect() will return the
* same Subscription and even if there was a connect-disconnect-connect pair, the older
* references won't disconnect the newer connection.
* Synchronous source consumers have the opportunity to disconnect via dispose on the
* Disposable as subscribe() may never return on its own.
*
* Note however, that asynchronously disconnecting a running source might leave
* child subscribers without any terminal event; PublishProcessor does not have this
* issue because the cancellation was always triggered by the child subscribers
* themselves.
*/
try {
connection.accept(ps);
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
throw ExceptionHelper.wrapOrThrow(ex);
}
if (doConnect) {
source.subscribe(ps);
}
}
@SuppressWarnings("rawtypes")
static final class PublishSubscriber
extends AtomicInteger
implements FlowableSubscriber, Disposable {
private static final long serialVersionUID = -202316842419149694L;
/** Indicates an empty array of inner subscribers. */
static final InnerSubscriber[] EMPTY = new InnerSubscriber[0];
/** Indicates a terminated PublishSubscriber. */
static final InnerSubscriber[] TERMINATED = new InnerSubscriber[0];
/** Holds onto the current connected PublishSubscriber. */
final AtomicReference> current;
/** The prefetch buffer size. */
final int bufferSize;
/** Tracks the subscribed InnerSubscribers. */
final AtomicReference[]> subscribers;
/**
* Atomically changed from false to true by connect to make sure the
* connection is only performed by one thread.
*/
final AtomicBoolean shouldConnect;
final AtomicReference upstream = new AtomicReference();
/** Contains either an onComplete or an onError token from upstream. */
volatile Object terminalEvent;
int sourceMode;
/** Holds notifications from upstream. */
volatile SimpleQueue queue;
@SuppressWarnings("unchecked")
PublishSubscriber(AtomicReference> current, int bufferSize) {
this.subscribers = new AtomicReference[]>(EMPTY);
this.current = current;
this.shouldConnect = new AtomicBoolean();
this.bufferSize = bufferSize;
}
@Override
public void dispose() {
if (subscribers.get() != TERMINATED) {
@SuppressWarnings("unchecked")
InnerSubscriber[] ps = subscribers.getAndSet(TERMINATED);
if (ps != TERMINATED) {
current.compareAndSet(PublishSubscriber.this, null);
SubscriptionHelper.cancel(upstream);
}
}
}
@Override
public boolean isDisposed() {
return subscribers.get() == TERMINATED;
}
@Override
public void onSubscribe(Subscription s) {
if (SubscriptionHelper.setOnce(this.upstream, s)) {
if (s instanceof QueueSubscription) {
@SuppressWarnings("unchecked")
QueueSubscription qs = (QueueSubscription) s;
int m = qs.requestFusion(QueueSubscription.ANY | QueueSubscription.BOUNDARY);
if (m == QueueSubscription.SYNC) {
sourceMode = m;
queue = qs;
terminalEvent = NotificationLite.complete();
dispatch();
return;
}
if (m == QueueSubscription.ASYNC) {
sourceMode = m;
queue = qs;
s.request(bufferSize);
return;
}
}
queue = new SpscArrayQueue(bufferSize);
s.request(bufferSize);
}
}
@Override
public void onNext(T t) {
// we expect upstream to honor backpressure requests
if (sourceMode == QueueSubscription.NONE && !queue.offer(t)) {
onError(new MissingBackpressureException("Prefetch queue is full?!"));
return;
}
// since many things can happen concurrently, we have a common dispatch
// loop to act on the current state serially
dispatch();
}
@Override
public void onError(Throwable e) {
// The observer front is accessed serially as required by spec so
// no need to CAS in the terminal value
if (terminalEvent == null) {
terminalEvent = NotificationLite.error(e);
// since many things can happen concurrently, we have a common dispatch
// loop to act on the current state serially
dispatch();
} else {
RxJavaPlugins.onError(e);
}
}
@Override
public void onComplete() {
// The observer front is accessed serially as required by spec so
// no need to CAS in the terminal value
if (terminalEvent == null) {
terminalEvent = NotificationLite.complete();
// since many things can happen concurrently, we have a common dispatch loop
// to act on the current state serially
dispatch();
}
}
/**
* Atomically try adding a new InnerSubscriber to this Subscriber or return false if this
* Subscriber was terminated.
* @param producer the producer to add
* @return true if succeeded, false otherwise
*/
boolean add(InnerSubscriber producer) {
// the state can change so we do a CAS loop to achieve atomicity
for (;;) {
// get the current producer array
InnerSubscriber[] c = subscribers.get();
// if this subscriber-to-source reached a terminal state by receiving
// an onError or onComplete, just refuse to add the new producer
if (c == TERMINATED) {
return false;
}
// we perform a copy-on-write logic
int len = c.length;
@SuppressWarnings("unchecked")
InnerSubscriber[] u = new InnerSubscriber[len + 1];
System.arraycopy(c, 0, u, 0, len);
u[len] = producer;
// try setting the subscribers array
if (subscribers.compareAndSet(c, u)) {
return true;
}
// if failed, some other operation succeeded (another add, remove or termination)
// so retry
}
}
/**
* Atomically removes the given InnerSubscriber from the subscribers array.
* @param producer the producer to remove
*/
@SuppressWarnings("unchecked")
void remove(InnerSubscriber producer) {
// the state can change so we do a CAS loop to achieve atomicity
for (;;) {
// let's read the current subscribers array
InnerSubscriber[] c = subscribers.get();
int len = c.length;
// if it is either empty or terminated, there is nothing to remove so we quit
if (len == 0) {
break;
}
// let's find the supplied producer in the array
// although this is O(n), we don't expect too many child subscribers in general
int j = -1;
for (int i = 0; i < len; i++) {
if (c[i].equals(producer)) {
j = i;
break;
}
}
// we didn't find it so just quit
if (j < 0) {
return;
}
// we do copy-on-write logic here
InnerSubscriber[] u;
// we don't create a new empty array if producer was the single inhabitant
// but rather reuse an empty array
if (len == 1) {
u = EMPTY;
} else {
// otherwise, create a new array one less in size
u = new InnerSubscriber[len - 1];
// copy elements being before the given producer
System.arraycopy(c, 0, u, 0, j);
// copy elements being after the given producer
System.arraycopy(c, j + 1, u, j, len - j - 1);
}
// try setting this new array as
if (subscribers.compareAndSet(c, u)) {
break;
}
// if we failed, it means something else happened
// (a concurrent add/remove or termination), we need to retry
}
}
/**
* Perform termination actions in case the source has terminated in some way and
* the queue has also become empty.
* @param term the terminal event (a NotificationLite.error or completed)
* @param empty set to true if the queue is empty
* @return true if there is indeed a terminal condition
*/
@SuppressWarnings("unchecked")
boolean checkTerminated(Object term, boolean empty) {
// first of all, check if there is actually a terminal event
if (term != null) {
// is it a completion event (impl. note, this is much cheaper than checking for isError)
if (NotificationLite.isComplete(term)) {
// but we also need to have an empty queue
if (empty) {
// this will prevent OnSubscribe spinning on a terminated but
// not yet cancelled PublishSubscriber
current.compareAndSet(this, null);
/*
* This will swap in a terminated array so add() in OnSubscribe will reject
* child subscribers to associate themselves with a terminated and thus
* never again emitting chain.
*
* Since we atomically change the contents of 'subscribers' only one
* operation wins at a time. If an add() wins before this getAndSet,
* its value will be part of the returned array by getAndSet and thus
* will receive the terminal notification. Otherwise, if getAndSet wins,
* add() will refuse to add the child producer and will trigger the
* creation of subscriber-to-source.
*/
for (InnerSubscriber ip : subscribers.getAndSet(TERMINATED)) {
ip.child.onComplete();
}
// indicate we reached the terminal state
return true;
}
} else {
Throwable t = NotificationLite.getError(term);
// this will prevent OnSubscribe spinning on a terminated
// but not yet cancelled PublishSubscriber
current.compareAndSet(this, null);
// this will swap in a terminated array so add() in OnSubscribe will reject
// child subscribers to associate themselves with a terminated and thus
// never again emitting chain
InnerSubscriber[] a = subscribers.getAndSet(TERMINATED);
if (a.length != 0) {
for (InnerSubscriber ip : a) {
ip.child.onError(t);
}
} else {
RxJavaPlugins.onError(t);
}
// indicate we reached the terminal state
return true;
}
}
// there is still work to be done
return false;
}
/**
* The common serialization point of events arriving from upstream and child subscribers
* requesting more.
*/
void dispatch() {
// standard construct of queue-drain
// if there is an emission going on, indicate that more work needs to be done
// the exact nature of this work needs to be determined from other data structures
if (getAndIncrement() != 0) {
return;
}
int missed = 1;
// saving a local copy because this will be accessed after every item
// delivered to detect changes in the subscribers due to an onNext
// and thus not dropping items
AtomicReference[]> subscribers = this.subscribers;
// We take a snapshot of the current child subscribers.
// Concurrent subscribers may miss this iteration, but it is to be expected
InnerSubscriber[] ps = subscribers.get();
outer:
for (;;) {
/*
* We need to read terminalEvent before checking the queue for emptiness because
* all enqueue happens before setting the terminal event.
* If it were the other way around, when the emission is paused between
* checking isEmpty and checking terminalEvent, some other thread might
* have produced elements and set the terminalEvent and we'd quit emitting
* prematurely.
*/
Object term = terminalEvent;
/*
* See if the queue is empty; since we need this information multiple
* times later on, we read it one.
* Although the queue can become non-empty in the mean time, we will
* detect it through the missing flag and will do another iteration.
*/
SimpleQueue q = queue;
boolean empty = q == null || q.isEmpty();
// if the queue is empty and the terminal event was received, quit
// and don't bother restoring emitting to false: no further activity is
// possible at this point
if (checkTerminated(term, empty)) {
return;
}
// We have elements queued. Note that due to the serialization nature of dispatch()
// this loop is the only one which can turn a non-empty queue into an empty one
// and as such, no need to ask the queue itself again for that.
if (!empty) {
int len = ps.length;
// Let's assume everyone requested the maximum value.
long maxRequested = Long.MAX_VALUE;
// count how many have triggered cancellation
int cancelled = 0;
// Now find the minimum amount each child-subscriber requested
// since we can only emit that much to all of them without violating
// backpressure constraints
for (InnerSubscriber ip : ps) {
long r = ip.get();
// if there is one child subscriber that hasn't requested yet
// we can't emit anything to anyone
if (r != CANCELLED) {
maxRequested = Math.min(maxRequested, r - ip.emitted);
} else {
cancelled++;
}
}
// it may happen everyone has cancelled between here and subscribers.get()
// or we have no subscribers at all to begin with
if (len == cancelled) {
term = terminalEvent;
// so let's consume a value from the queue
T v;
try {
v = q.poll();
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
upstream.get().cancel();
term = NotificationLite.error(ex);
terminalEvent = term;
v = null;
}
// or terminate if there was a terminal event and the queue is empty
if (checkTerminated(term, v == null)) {
return;
}
// otherwise, just ask for a new value
if (sourceMode != QueueSubscription.SYNC) {
upstream.get().request(1);
}
// and retry emitting to potential new child subscribers
continue;
}
// if we get here, it means there are non-cancelled child subscribers
// and we count the number of emitted values because the queue
// may contain less than requested
int d = 0;
while (d < maxRequested) {
term = terminalEvent;
T v;
try {
v = q.poll();
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
upstream.get().cancel();
term = NotificationLite.error(ex);
terminalEvent = term;
v = null;
}
empty = v == null;
// let's check if there is a terminal event and the queue became empty just now
if (checkTerminated(term, empty)) {
return;
}
// the queue is empty but we aren't terminated yet, finish this emission loop
if (empty) {
break;
}
// we need to unwrap potential nulls
T value = NotificationLite.getValue(v);
boolean subscribersChanged = false;
// let's emit this value to all child subscribers
for (InnerSubscriber ip : ps) {
// if ip.get() is negative, the child has either cancelled in the
// meantime or hasn't requested anything yet
// this eager behavior will skip cancelled children in case
// multiple values are available in the queue
long ipr = ip.get();
if (ipr != CANCELLED) {
if (ipr != Long.MAX_VALUE) {
// indicate this child has received 1 element
ip.emitted++;
}
ip.child.onNext(value);
} else {
subscribersChanged = true;
}
}
// indicate we emitted one element
d++;
// see if the array of subscribers changed as a consequence
// of emission or concurrent activity
InnerSubscriber[] freshArray = subscribers.get();
if (subscribersChanged || freshArray != ps) {
ps = freshArray;
// if we did emit at least one element, request more to replenish the queue
if (d != 0) {
if (sourceMode != QueueSubscription.SYNC) {
upstream.get().request(d);
}
}
continue outer;
}
}
// if we did emit at least one element, request more to replenish the queue
if (d != 0) {
if (sourceMode != QueueSubscription.SYNC) {
upstream.get().request(d);
}
}
// if we have requests but not an empty queue after emission
// let's try again to see if more requests/child subscribers are
// ready to receive more
if (maxRequested != 0L && !empty) {
continue;
}
}
missed = addAndGet(-missed);
if (missed == 0) {
break;
}
// get a fresh copy of the current subscribers
ps = subscribers.get();
}
}
}
/**
* A Subscription that manages the request and cancellation state of a
* child subscriber in thread-safe manner.
* @param the value type
*/
static final class InnerSubscriber extends AtomicLong implements Subscription {
private static final long serialVersionUID = -4453897557930727610L;
/** The actual child subscriber. */
final Subscriber child;
/**
* The parent subscriber-to-source used to allow removing the child in case of
* child cancellation.
*/
volatile PublishSubscriber parent;
/** Track the number of emitted items (avoids decrementing the request counter). */
long emitted;
InnerSubscriber(Subscriber child) {
this.child = child;
}
@Override
public void request(long n) {
if (SubscriptionHelper.validate(n)) {
BackpressureHelper.addCancel(this, n);
PublishSubscriber p = parent;
if (p != null) {
p.dispatch();
}
}
}
@Override
public void cancel() {
long r = get();
// let's see if we are cancelled
if (r != CANCELLED) {
// if not, swap in the terminal state, this is idempotent
// because other methods using CAS won't overwrite this value,
// concurrent calls to cancel will atomically swap in the same
// terminal value
r = getAndSet(CANCELLED);
// and only one of them will see a non-terminated value before the swap
if (r != CANCELLED) {
PublishSubscriber p = parent;
if (p != null) {
// remove this from the parent
p.remove(this);
// After removal, we might have unblocked the other child subscribers:
// let's assume this child had 0 requested before the cancellation while
// the others had non-zero. By removing this 'blocking' child, the others
// are now free to receive events
p.dispatch();
}
}
}
}
}
static final class FlowablePublisher implements Publisher {
private final AtomicReference> curr;
private final int bufferSize;
FlowablePublisher(AtomicReference> curr, int bufferSize) {
this.curr = curr;
this.bufferSize = bufferSize;
}
@Override
public void subscribe(Subscriber child) {
// create the backpressure-managing producer for this child
InnerSubscriber inner = new InnerSubscriber(child);
child.onSubscribe(inner);
// concurrent connection/disconnection may change the state,
// we loop to be atomic while the child subscribes
for (;;) {
// get the current subscriber-to-source
PublishSubscriber r = curr.get();
// if there isn't one or it is cancelled/disposed
if (r == null || r.isDisposed()) {
// create a new subscriber to source
PublishSubscriber u = new PublishSubscriber(curr, bufferSize);
// let's try setting it as the current subscriber-to-source
if (!curr.compareAndSet(r, u)) {
// didn't work, maybe someone else did it or the current subscriber
// to source has just finished
continue;
}
// we won, let's use it going onwards
r = u;
}
/*
* Try adding it to the current subscriber-to-source, add is atomic in respect
* to other adds and the termination of the subscriber-to-source.
*/
if (r.add(inner)) {
if (inner.get() == CANCELLED) {
r.remove(inner);
} else {
inner.parent = r;
}
r.dispatch();
break; // NOPMD
}
/*
* The current PublishSubscriber has been terminated, try with a newer one.
*/
/*
* Note: although technically correct, concurrent disconnects can cause
* unexpected behavior such as child subscribers never receiving anything
* (unless connected again). An alternative approach, similar to
* PublishProcessor would be to immediately terminate such child
* subscribers as well:
*
* Object term = r.terminalEvent;
* if (r.nl.isCompleted(term)) {
* child.onComplete();
* } else {
* child.onError(r.nl.getError(term));
* }
* return;
*
* The original concurrent behavior was non-deterministic in this regard as well.
* Allowing this behavior, however, may introduce another unexpected behavior:
* after disconnecting a previous connection, one might not be able to prepare
* a new connection right after a previous termination by subscribing new child
* subscribers asynchronously before a connect call.
*/
}
}
}
}
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