[#3821] Support Set for exact buffers in Flux.buffer (#3822)

When a `Set` was returned from the `Supplier` provided to `Flux.buffer`,
the stream would hang if/when there were duplicates in a given buffer.
This change allows using `Set` for exact buffers - buffers where
`maxSize` and `skip` arguments are equal. It also guarantees in this
case the duplicate items are discarded and the upstream demand is
replenished.

Resolves #3821

---------

Co-authored-by: spierce <[email protected]>

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

@@ -2883,6 +2883,9 @@ public final Flux<List<T>> buffer(int maxSize) {
 	 * or once this Flux completes.
 	 * <p>
 	 * <img class="marble" src="doc-files/marbles/bufferWithMaxSize.svg" alt="">
+	 * <p>
+	 * Note that if buffers provided by the bufferSupplier return {@literal false} upon invocation
+	 * of {@link Collection#add(Object)} for a given element, that element will be discarded.
 	 *
 	 * <p><strong>Discard Support:</strong> This operator discards the currently open buffer upon cancellation or error triggered by a data signal,
 	 * as well as latest unbuffered element if the bufferSupplier fails.
@@ -2947,6 +2950,10 @@ public final Flux<List<T>> buffer(int maxSize, int skip) {
 	 * <p>
 	 * <img class="marble" src="doc-files/marbles/bufferWithMaxSizeEqualsSkipSize.svg" alt="">
 	 *
+	 * <p>
+	 * Note for exact buffers: If buffers provided by the bufferSupplier return {@literal false} upon invocation
+	 * of {@link Collection#add(Object)} for a given element, that element will be discarded.
+	 *
 	 * <p><strong>Discard Support:</strong> This operator discards elements in between buffers (in the case of
 	 * dropping buffers). It also discards the currently open buffer upon cancellation or error triggered by a data signal.
 	 * Note however that overlapping buffer variant DOES NOT discard, as this might result in an element

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2021 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2016-2024 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.
@@ -153,11 +153,14 @@ public void onNext(T t) {
 				buffer = b;
 			}
 
-			b.add(t);
-
-			if (b.size() == size) {
-				buffer = null;
-				actual.onNext(b);
+			if (b.add(t)) {
+				if (b.size() == size) {
+					buffer = null;
+					actual.onNext(b);
+				}
+			} else {
+				Operators.onDiscard(t, actual.currentContext());
+				s.request(1);
 			}
 		}
 

reactor-core/src/test/java/reactor/core/publisher/FluxBufferTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2022 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2016-2024 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.
@@ -19,17 +19,24 @@
 import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.function.Function;
 import java.util.function.Supplier;
+import java.util.stream.Collectors;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.provider.CsvSource;
 import org.reactivestreams.Subscription;
 import reactor.core.CoreSubscriber;
 import reactor.core.Scannable;
+import reactor.test.ParameterizedTestWithName;
 import reactor.test.StepVerifier;
 import reactor.test.publisher.FluxOperatorTest;
 import reactor.test.subscriber.AssertSubscriber;
+import reactor.util.annotation.Nullable;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -661,4 +668,33 @@ public void discardOnErrorOverlap() {
 		            .verifyThenAssertThat()
 		            .hasDiscardedExactly(1, 2, 3, 3); //we already opened a 2nd buffer
 	}
+
+    @ParameterizedTestWithName
+    @CsvSource({
+        "1|2,     1|2,        ",
+        "1|1|1,   1,       1|1",
+        "1|1|2,   1|2,       1",
+        "1|2|1,   1|2;1,      ",
+        "1|2|1|3, 1|2;1|3,    ",
+        "1|1|2|3, 1|2;3,     1",
+        "2|1|1|3, 2|1;1|3,    "
+    })
+    public void bufferExactSupplierUsesSet(String input, String output, @Nullable String discard) {
+        List<Set<Object>> outputs = Arrays.stream(output.split(";"))
+            .map(it -> Arrays.<Object>stream(it.split("\\|")).collect(Collectors.toSet()))
+            .collect(Collectors.toList());
+
+        StepVerifier.Assertions assertions = Flux.just(input.split("\\|"))
+            .<Collection<Object>>buffer(2, HashSet::new)
+            .as(it -> StepVerifier.create(it, outputs.size()))
+            .expectNextSequence(outputs)
+            .expectComplete()
+            .verifyThenAssertThat(Duration.ofSeconds(2));
+
+        if (discard == null) {
+            assertions.hasNotDiscardedElements();
+        } else {
+            assertions.hasDiscardedExactly((Object[]) discard.split("\\|"));
+        }
+    }
 }