GROOVY-9381: Add native async/await support

[daniellansun]

https://issues.apache.org/jira/browse/GROOVY-9381

See also:

1. https://github.com/apache/groovy/blob/GROOVY-9381_3/src/spec/doc/core-async-await.adoc
2. https://github.com/apache/groovy/actions/runs/22914058057/artifacts/5854362878

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.16563% with 87 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.9030%. Comparing base (a2a7da6) to head (5df6414).

Files with missing lines	Patch %	Lines
.../org/apache/groovy/runtime/async/AsyncSupport.java	88.3895%	17 Missing and 14 partials ⚠️
...va/groovy/concurrent/AwaitableAdapterRegistry.java	84.8921%	7 Missing and 14 partials ⚠️
...ehaus/groovy/transform/AsyncASTTransformation.java	82.2917%	6 Missing and 11 partials ⚠️
...che/groovy/runtime/async/AsyncStreamGenerator.java	88.3721%	3 Missing and 7 partials ⚠️
...odehaus/groovy/transform/AsyncTransformHelper.java	93.9394%	0 Missing and 4 partials ⚠️
...va/org/apache/groovy/parser/antlr4/AstBuilder.java	95.8333%	3 Missing ⚠️
...org/apache/groovy/runtime/async/GroovyPromise.java	96.2963%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2387        +/-   ##
==================================================
+ Coverage     66.7519%   66.9030%   +0.1511%     
- Complexity      29867      30138       +271     
==================================================
  Files            1382       1392        +10     
  Lines          116106     116908       +802     
  Branches        20472      20607       +135     
==================================================
+ Hits            77503      78215       +712     
- Misses          32272      32312        +40     
- Partials         6331       6381        +50     

Files with missing lines	Coverage Δ
src/main/java/groovy/concurrent/AsyncStream.java	100.0000% <100.0000%> (ø)
src/main/java/groovy/concurrent/AwaitResult.java	100.0000% <100.0000%> (ø)
src/main/java/groovy/concurrent/Awaitable.java	100.0000% <100.0000%> (ø)
.../main/java/groovy/concurrent/AwaitableAdapter.java	100.0000% <100.0000%> (ø)
...g/apache/groovy/parser/antlr4/ModifierManager.java	93.4210% <100.0000%> (ø)
...ain/java/org/codehaus/groovy/ast/ModifierNode.java	77.7778% <100.0000%> (+0.4193%)	⬆️
...org/apache/groovy/runtime/async/GroovyPromise.java	96.2963% <96.2963%> (ø)
...va/org/apache/groovy/parser/antlr4/AstBuilder.java	86.8007% <95.8333%> (+0.2935%)	⬆️
...odehaus/groovy/transform/AsyncTransformHelper.java	93.9394% <93.9394%> (ø)
...che/groovy/runtime/async/AsyncStreamGenerator.java	88.3721% <88.3721%> (ø)
... and 3 more

... and 6 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR implements GROOVY-9381 by adding native async/await syntax to the Groovy language (including for await, yield return async generators, and defer), plus runtime support and extensive documentation/tests.

Changes:

• Extend the Groovy grammar + AST builder to parse/compile async, await, for await, yield return, and defer.
• Add/extend runtime primitives (Awaitable, AsyncStream, adapters/registry, promise + generator implementation) and @Async AST transformation support.
• Add comprehensive tests and new spec documentation; add Reactor/RxJava test dependencies for adapter interop coverage.

Reviewed changes

Copilot reviewed 29 out of 30 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
versions.properties	Adds version pins for Reactor and RxJava3 used by new integration tests.
build.gradle	Adds Reactor/RxJava3 as testImplementation dependencies.
gradle/verification-metadata.xml	Adds verification metadata entries for new test dependencies.
src/antlr/GroovyLexer.g4	Introduces lexer tokens for async, await, defer.
src/antlr/GroovyParser.g4	Adds grammar support for async closure/lambda, await expressions, for await, yield return, and defer.
src/main/java/org/apache/groovy/parser/antlr4/AstBuilder.java	Lowers new syntax into runtime calls (e.g., AsyncSupport.await, stream conversion/cleanup, defer, yieldReturn).
src/main/java/org/codehaus/groovy/ast/ModifierNode.java	Treats ASYNC as a non-bytecode modifier (opcode 0).
src/main/java/org/codehaus/groovy/transform/AsyncTransformHelper.java	Centralizes AST construction + scanning/rewriting for async constructs.
src/main/java/org/codehaus/groovy/transform/AsyncASTTransformation.java	Implements @Async transformation using shared helper logic (await rewrite, generator/defer handling).
src/main/java/groovy/transform/Async.java	Adds @Async annotation and documentation.
src/main/java/groovy/concurrent/*	Adds new async public API (Awaitable, AsyncStream, adapters/registry, AwaitResult).
src/main/java/org/apache/groovy/runtime/async/*	Adds runtime implementations (GroovyPromise, AsyncStreamGenerator) used by compiler output.
src/spec/doc/core-async-await.adoc	Adds user-facing language documentation/spec for async/await.
src/test/groovy/org/codehaus/groovy/transform/*	Adds extensive test coverage for async/await, generators, defer, adapters, and virtual threads.

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[Copilot]

Adding ASYNC to the general modifier rule means async can now appear anywhere modifiers are allowed (e.g., on fields, variables, classes) and will likely be silently ignored (since it maps to opcode 0). If async is intended to be a method-only modifier, consider restricting it in the grammar or adding a parse/semantic error when async is used in unsupported contexts, to avoid misleading code compiling without effect.

Suggested change

| ASYNC

[Copilot]

The custom fixed thread pool created here is never shut down. Since the thread factory creates non-daemon threads, this can leak threads and potentially hang the test JVM. Please ensure the executor is shut down (preferably in the existing finally block) after restoring the previous executor.

[Copilot]

This test sets the global executor to a newSingleThreadExecutor but never shuts that executor down. Even though the executor is reset to null later, the underlying thread can remain alive and leak across the suite. Please shut down the created executor (e.g., keep a reference and shutdown in a finally block).

Suggested change

	Awaitable.setExecutor(Executors.newSingleThreadExecutor())
	assert Awaitable.getExecutor() != originalExecutor
	// Reset to null — should restore default
	Awaitable.setExecutor(null)
	def restored = Awaitable.getExecutor()
	assert restored != null
	// Verify it works
	def task = async { 42 }; def awaitable = task()
	assert await(awaitable) == 42
	// Restore original
	Awaitable.setExecutor(originalExecutor)
	def customExecutor = Executors.newSingleThreadExecutor()
	try {
	Awaitable.setExecutor(customExecutor)
	assert Awaitable.getExecutor() != originalExecutor
	// Reset to null — should restore default
	Awaitable.setExecutor(null)
	def restored = Awaitable.getExecutor()
	assert restored != null
	// Verify it works
	def task = async { 42 }; def awaitable = task()
	assert await(awaitable) == 42
	} finally {
	// Restore original and shut down custom executor to avoid thread leaks
	Awaitable.setExecutor(originalExecutor)
	customExecutor.shutdown()
	}

[Copilot]

The static Executor created via Executors.newFixedThreadPool uses non-daemon threads and is never shut down. This can leak threads for the remainder of the test run and may prevent the JVM from exiting. Consider using daemon threads (as other tests do) and/or explicitly shutting down the pool at the end of the script.