Make some Mono sources and aggregators lazier (#3081)

This commit changes 4 broad categories of Mono operators to be lazier
and only start requesting from their upstream once they've been
themselves requested.

Previously, the request to upstream would happen immediately upon
receiving the Subscription (in `onSubscribe`).

The first category of Monos is the one where the upstream is a Flux
that gets aggregated into a Mono, such as `MonoCollectList`, `MonoCount`
but also `MonoAll` or `MonoElementAt`...
These Mono now all inherit a common `BaseFluxToMonoOperator` which track
two states as bits in a single volatile: request status (true/false) and
completed status (true/false). It uses a non-volatile boolean hasRequest
as a shortcut to ignore any request past the first one.

Note that for now these implementations do declare a `Fuseable` trait,
but they always negotiate fusion `NONE`. It is kept as a future possible
improvement that they would all support SYNC fusion.

The second category is Monos that combine multiple sources, like
`MonoZip` or `MonoWhen`.
A single volatile long-based state machine is used to coordinate the
multiple inners and track which inner has made the first request.

The third category is ParallelFlux-to-Mono operators, mixing the above
two aspects: aggregating values and combining multiple sources (rails).
Some of these can benefit from the `BaseFluxToMonoOperator` approach.

The last category is sources like `MonoSupplier` or `MonoCallable`.
Often the functional interface representing the logic would be run in
`onSubscribe` directly, but making it lazier implies the creation of
dedicated `Subscription` implementations now.

Fixes #2913.

Signed-off-by: Oleh Dokuka <odokuka@vmware.com>
Co-authored-by: Simon Baslé <sbasle@vmware.com>

benchmarks/src/main/java/reactor/core/publisher/MonoAllBenchmark.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2022 VMware Inc. or its affiliates, All Rights Reserved.
+ *
+ * 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
+ *
+ *   https://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 reactor.core.publisher;
+
+import java.util.concurrent.TimeUnit;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+
+@BenchmarkMode({Mode.AverageTime})
+@Warmup(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
+@Fork(value = 1)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+public class MonoAllBenchmark {
+
+	@Param({"0", "10", "1000", "100000"})
+	int rangeSize;
+
+	public static void main(String[] args) throws Exception {
+		reactor.core.scrabble.ShakespearePlaysScrabbleOpt
+				s = new reactor.core.scrabble.ShakespearePlaysScrabbleOpt();
+		s.init();
+		System.out.println(s.measureThroughput());
+	}
+
+	@SuppressWarnings("unused")
+	@Benchmark
+	public void measureThroughput() {
+		Flux.range(0, rangeSize)
+			.all(i -> i < Integer.MAX_VALUE)
+			.block();
+	}
+}

benchmarks/src/main/java/reactor/core/publisher/MonoCallableBenchmark.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2022 VMware Inc. or its affiliates, All Rights Reserved.
+ *
+ * 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
+ *
+ *   https://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 reactor.core.publisher;
+
+import java.util.concurrent.TimeUnit;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+
+@BenchmarkMode({Mode.AverageTime})
+@Warmup(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
+@Fork(value = 1)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+public class MonoCallableBenchmark {
+
+	@Param({"10", "1000", "100000"})
+	int rangeSize;
+
+	public static void main(String[] args) throws Exception {
+		reactor.core.scrabble.ShakespearePlaysScrabbleOpt
+				s = new reactor.core.scrabble.ShakespearePlaysScrabbleOpt();
+		s.init();
+		System.out.println(s.measureThroughput());
+	}
+
+	@SuppressWarnings("unused")
+	@Benchmark
+	public void measureThroughput() {
+		Flux.range(0, rangeSize)
+			.all(i -> i < Integer.MAX_VALUE)
+			.block();
+	}
+}

reactor-core/src/main/java/reactor/core/publisher/FluxCallable.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2021 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2016-2022 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,21 +37,7 @@ final class FluxCallable<T> extends Flux<T> implements Callable<T>, Fuseable, So
 
 	@Override
 	public void subscribe(CoreSubscriber<? super T> actual) {
-		Operators.MonoSubscriber<T, T> wrapper = new Operators.MonoSubscriber<>(actual);
-		actual.onSubscribe(wrapper);
-
-		try {
-			T v = callable.call();
-			if (v == null) {
-				wrapper.onComplete();
-			}
-			else {
-				wrapper.complete(v);
-			}
-		}
-		catch (Throwable ex) {
-			actual.onError(Operators.onOperatorError(ex, actual.currentContext()));
-		}
+		actual.onSubscribe(new MonoCallable.MonoCallableSubscription<>(actual, callable));
 	}
 
 	@Override

reactor-core/src/main/java/reactor/core/publisher/FluxDefaultIfEmpty.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2021 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2016-2022 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,10 +17,9 @@
 package reactor.core.publisher;
 
 import java.util.Objects;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
-import org.reactivestreams.Subscription;
 import reactor.core.CoreSubscriber;
-import reactor.core.Fuseable;
 import reactor.util.annotation.Nullable;
 
 /**
@@ -50,75 +49,120 @@ public Object scanUnsafe(Attr key) {
 	}
 
 	static final class DefaultIfEmptySubscriber<T>
-			extends Operators.MonoSubscriber<T, T> {
+			extends Operators.BaseFluxToMonoOperator<T, T> {
 
-		Subscription s;
+		boolean done;
 
 		boolean hasValue;
 
-		DefaultIfEmptySubscriber(CoreSubscriber<? super T> actual, T value) {
+		volatile T fallbackValue;
+		@SuppressWarnings("rawtypes")
+		static final AtomicReferenceFieldUpdater<DefaultIfEmptySubscriber, Object> FALLBACK_VALUE =
+				AtomicReferenceFieldUpdater.newUpdater(DefaultIfEmptySubscriber.class, Object.class, "fallbackValue");
+
+		DefaultIfEmptySubscriber(CoreSubscriber<? super T> actual, T fallbackValue) {
 			super(actual);
-			//noinspection deprecation
-			this.value = value; //we write once, setValue() is NO-OP
+			FALLBACK_VALUE.lazySet(this, fallbackValue);
 		}
 
 		@Override
 		@Nullable
 		public Object scanUnsafe(Attr key) {
-			if (key == Attr.PARENT) return s;
-			if (key == Attr.RUN_STYLE) return Attr.RunStyle.SYNC;
+			if (key == Attr.TERMINATED) return done;
 
 			return super.scanUnsafe(key);
 		}
 
 		@Override
 		public void request(long n) {
-			super.request(n);
+			if (!hasRequest) {
+				hasRequest = true;
+
+				final int state = this.state;
+
+				if (state != 1 && STATE.compareAndSet(this, state, state | 1)) {
+					if (state > 1) {
+						final T fallbackValue = this.fallbackValue;
+						if (fallbackValue != null && FALLBACK_VALUE.compareAndSet(this,
+								fallbackValue,
+								null)) {
+							// completed before request means source was empty
+							actual.onNext(fallbackValue);
+							actual.onComplete();
+ 						}
+						return;
+					}
+				}
+			}
+
 			s.request(n);
 		}
 
 		@Override
 		public void cancel() {
 			super.cancel();
-			s.cancel();
-		}
-
-		@Override
-		public void onSubscribe(Subscription s) {
-			if (Operators.validate(this.s, s)) {
-				this.s = s;
 
-				actual.onSubscribe(this);
+			final T fallbackValue = this.fallbackValue;
+			if (fallbackValue != null && FALLBACK_VALUE.compareAndSet(this, fallbackValue, null)) {
+				Operators.onDiscard(fallbackValue, actual.currentContext());
 			}
 		}
 
 		@Override
 		public void onNext(T t) {
 			if (!hasValue) {
 				hasValue = true;
+
+				final T fallbackValue = this.fallbackValue;
+				if (fallbackValue != null && FALLBACK_VALUE.compareAndSet(this, fallbackValue, null)) {
+					Operators.onDiscard(fallbackValue, actual.currentContext());
+				}
 			}
 
 			actual.onNext(t);
 		}
 
 		@Override
 		public void onComplete() {
-			if (hasValue) {
-				actual.onComplete();
-			} else {
-				complete(this.value);
+			if (done) {
+				return;
 			}
+
+			done = true;
+
+			if (!hasValue) {
+				completePossiblyEmpty();
+
+				return;
+			}
+
+			actual.onComplete();
 		}
 
 		@Override
-		public void setValue(T value) {
-			// value is constant. writes from the base class are redundant, and the constant
-			// would always be visible in cancel(), so it will safely be discarded.
+		public void onError(Throwable t) {
+			if (done) {
+				return;
+			}
+
+			done = true;
+			if (!hasValue) {
+				final T fallbackValue = this.fallbackValue;
+				if (fallbackValue != null && FALLBACK_VALUE.compareAndSet(this, fallbackValue, null)) {
+					Operators.onDiscard(t, actual.currentContext());
+				}
+			}
+
+			actual.onError(t);
 		}
 
 		@Override
-		public int requestFusion(int requestedMode) {
-			return Fuseable.NONE; // prevent fusion because of the upstream
+		T accumulatedValue() {
+			final T fallbackValue = this.fallbackValue;
+			if (fallbackValue != null && FALLBACK_VALUE.compareAndSet(this, fallbackValue, null)) {
+				return fallbackValue;
+			}
+			return null;
 		}
 	}
 }