Allow registering a custom `Predicate` for determining non-blocking threads

[trustin]

Related issue: #3833 Motivation:

It is currently not possible to create a non-blocking threads without implementing the reactor.core.scheduler.NonBlocking interface. Some third-party libraries and frameworks don't directly depend on reactor-core, yet they want to mark the threads they manage as non-blocking.

Modifications:

• Added a new method Schedulers.registerNonBlockingThreadPredicate() so that a user can register their own Predicate that determines whether a given thread is non-blocking or not
  • Also added Schedulers.resetNonBlockingThreadPredicate() so that a user can unregister all previous Predicates
• Fixed an incorrectly implemented test that doesn't really test anything:
  • SchedulersTest.isInNonBlockingThreadTrue()

Result:

• A user can now mark their own Thread classes as non-blocking without depending on reactor-core or implementing the NonBlocking marker interface.

[chemicL]

Please run ./gradlew spotlessApply and push the license header updates.

[trustin]

@chemicL It looks like 3.6.x doesn't have Spotless. I can update the license headers of the modified files, but could you point me to the right license header?

[trustin]

By the way, I ran ./gradlew spotlessApply against the main branch and it gives me this error:

> Task :spotlessJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':spotlessJava'.
> org.eclipse.jgit.errors.CorruptObjectException: DIRC checksum mismatch

Environment:

❯ ./gradlew --version     
------------------------------------------------------------
Gradle 8.5
------------------------------------------------------------

Build time:   2023-11-29 14:08:57 UTC
Revision:     28aca86a7180baa17117e0e5ba01d8ea9feca598

Kotlin:       1.9.20
Groovy:       3.0.17
Ant:          Apache Ant(TM) version 1.10.13 compiled on January 4 2023
JVM:          21.0.3 (Eclipse Adoptium 21.0.3+9-LTS)
OS:           Linux 6.9.10-arch1-1 amd64

[chemicL]

I'm not sure what's up with your local setup, but there's nothing fishy on our side I think. I will commit my updates and we can merge.

Here's what I did:

➜  reactor-core git:(3.6.x) gh pr checkout 3854
[...]
Switched to branch 'custom_nonblocking_thread_predicate_3.6'
➜  reactor-core git:(custom_nonblocking_thread_predicate_3.6) ./gradlew spotlessApply
[...]
BUILD SUCCESSFUL in 8s
7 actionable tasks: 2 executed, 5 up-to-date

And running git diff gives:

diff --git a/reactor-core/src/main/java/reactor/core/scheduler/Schedulers.java b/reactor-core/src/main/java/reactor/core/scheduler/Schedulers.java
index 02dfa31c5..7617cb180 100644
--- a/reactor-core/src/main/java/reactor/core/scheduler/Schedulers.java
+++ b/reactor-core/src/main/java/reactor/core/scheduler/Schedulers.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2023 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.
diff --git a/reactor-core/src/test/java/reactor/core/scheduler/SchedulersTest.java b/reactor-core/src/test/java/reactor/core/scheduler/SchedulersTest.java
index 821fbdb5a..7232a0248 100644
--- a/reactor-core/src/test/java/reactor/core/scheduler/SchedulersTest.java
+++ b/reactor-core/src/test/java/reactor/core/scheduler/SchedulersTest.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.