reactor/reactor-core v3.2.0.M2

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MILESTONE 2 for Reactor 3.2.0, which will be the first release in the CALIFORNIUM Release Train.

This milestone contains a lot of fixes and improvements, as well as new features we'd really like your feedback on:

📢 Feedback Wanted! New features

• Add metrics() to Flux (#1183, #1123, 34e0d4b)
  • This operator does nothing if Micrometer is not on the classpath
  • It exposes metrics from upstream signals visible at the operator's position in the chain
  • 📢 Feeback wanted on the type of metrics exposed
• Add exponential backoff retry with jitter to core: retryWithBackoff (#1122)
  • This version of retry reflects what we think is the industry best practice in terms of retries.
  • It is a good middle ground between the too simple retry(n), the complex retryWhen(Function) and the more configurable RetryFunction from reactor-addons
• New operators for transactional reactive use cases: usingWhen (#1220, b87ea6d)
  • Like using, but the resource is provided asynchronously through a Publisher
  • Can have separate async "cleanup" for complete, error and cancel terminations
  • Cleanups are asynchronous as well (Function<Resource, Publisher>) and only delay the propagation of the terminal signals, NOT the onNext signals.

⚠️ Update considerations and deprecations

• windowUntil and windowWhile don't have a final empty window at the end of the sequence (#1033)
• Default behavior for Operators.onErrorDropped is now to both bubble and throw the error
  • Mono.fromCompletionStage now drops fatal exceptions rather than hanging (#1118)
• Blocking APIs (like blockLast(), block(), iterator()) called inside a parallel or single Scheduler trigger an exception (#1102)
  • This kind of blocking call are harmful as they impact limited resources, with a high risk of freezing the application
• concatMapDelayError default behavior is now to delay the errors until the END (#1101)
  • This aligns with other *DelayError operators
• StepVerifier.withVirtualTime now use a global lock, making virtual time verifications mutually exclusive (#648)
  • Virtual time impacts schedulers at the application level. Parallelisation of tests could lead to unforeseen side effects due to this (initializing operators with a VTS and have the StepVerifier see and manipulate the wrong VTS)
• create uses Operators.onOperatorError hook instead of onErrorDropped (#1078)
• (deprecation) Scannable#operatorName() is deprecated in favor of stepName() (#1115, #1140)
• (internal) Switch to a simplified implementation for Mono.publish(Function) (#437)
• (internal) Simplify MonoProcessor internals in preparation for #1114 and further 3.2 changes

✨ New features and improvements

• cache(Duration) now have an overload that takes a Scheduler, added Mono.cache TTL-generator variant (#1189, #1125)
• Operators.lift now has an alternative that exposes the raw Publisher: liftPublisher (#1205)
• Operators.lift now supports Fuseable (#1206)
• Added zip variants with 7 and 8 arguments (#1210
• Support 0 delay/period in Flux.interval (#1178)
• Add immutable empty()`` queue Supplier to Queues, support empty and one cases in Queues.capacity()` (#1161, d9d76ab)
• doOnEach now support Fuseable and ConditionalSubscriber (#1003)
• Flux.take(0) now eagerly cancels on subscription (#1158)
• Lazy Mono#fromFuture and Mono#fromCompletionStage added, with Supplier param (#1131)
• Add an `error(Supplier)`` variant to Flux and Mono (#1100)
• Add Scannable#steps(), which produces a Stream<String> of all the stepNames both upstream and downstream (including the current Scannable) (#1115, #1140)
• All operators now implement basic Scannable (this avoids problems for users that use instanceof checks in hooks, #1136)
• [kotlin] Add Flux.split extension to convert Flux<List<T>> to Flux<T> (#1089)
• [reactor-test] Add verifyThenAssertThat(Duration) to allow assertions and verification timeout (#1124)
• [reactor-test] Allow naming of a whole StepVerifier scenario through options (#1077)
• [reactor-test] Allow to change the LoggerFactory with a custom implementation, provide a TestLogger one that can be used to assert log contents (8f3a8fa)
• [reactor-test] Add a cold version of the TestPublisher than can be set up before eg. subscribing in a StepVerifier (#1236)

🪲 Bug fixes

• Flux.last() now correctly throws a NoSuchElementException on empty source even when said source is a Callable Mono(#1226)
• delayUntil correctly request Long.MAX_VALUE and not Integer.MAX_VALUE on subscribe (#1214)
• Operators.lift() properly maintain GroupedFlux/ConnectableFlux interfaces (#1204)
• Request is now accounted for in `FluxOnBackpressureBufferTimeout (#1194)
• onSubscribe should be done before add in MonoCacheTime.subscribe (#1190)
• Multi-subscriber operators Context resolution resolves to 1st context (#1114)
• Fix dangling thread when calling WorkQueueProcessor.forceShutdown (#1142)
• Let VirtualTimeScheduler defer advanceTime if queue is empty (#783)
  • This avoids a situation where, due to a subscribeOn, the operators delaying values end up reading the clock AFTER the virtual time has been moved forward by thenAwait, hanging the test.
• Tweak FluxPublish dispose to clear the existing connection (#1173)
• Avoid interrupting WorkerTask Future if cancelled race (#1107)
• FluxRefCount DisconnectException on cancellation race (#1088)
• (internal cleanup) use passed queue in FluxGroupBy.checkTerminated (#1094)
• Only terminate ExecutorScheduler if underlying is shut down (#1080)
• Mono.fromCompletionStage now drops fatal exceptions rather than hanging (#1118)

📖 Documentation, Tests and Build

• Improve various marble diagrams (#1043, #1008, #1167)
• Document (im)mutability of Tuple2 toList() vs iterator() (#1135)
• Point to reference doc operator matrix in Flux/Mono javadoc (f2bc74f)
• Warn on doOnSubscribe usage (#1090)
• various javadoc typos and minor rephrasing (#1222, 89a8e0e, #1149, #1199, 51dff9f, 4a1f691, 4e55122, #1095, 7a6a8a1, 35cb6fd,
• Various test fixes (2f74309, 0254f45, #1139, 1c27a15, 44dc73c, 8d942e1, 38f8edb)
• Add meaningful toString to FluxSink/MonoSink implementations (#1130)

👍 Thanks to the following contributors that also participated to this release

@alex-diez, @baptistemesta, @charlesmuchene, @igoperikov, @madgnome, @OlegDokuka, @smiklos, @utwyko, @yamkazy