Make keyed Compose HTML EAP rendering thread-safe

[Clemens Koprolin (clemenskoprolin)]

This PR makes keyed Compose HTML EAP string and streaming rendering thread-safe. Concurrent and nested calls, including calls with the same key, borrow separate renderers from a shared pool. Rendering storage can be reused across threads.

The JVM pool uses preallocated atomic slots to avoid allocating queue nodes when returning renderers. Attribute builders and logical output contents are cleared after each use. Pool retention is limited by the number of keys and idle renderers and oversized JVM output buffers are not retained.

The PR also:

• Adds keyed overloads for renderHydratedDocument and both renderHydratedDocumentToStream variants.
• Adds JMH benchmarks comparing fresh renderer creation, thread-local reuse and shared pooling.

Fixes CMP-10919

Testing

Run from the html directory:

./gradlew -Pcompose.html.eap.enabled=true \
  :html-core-eap:jvmTest \
  :html-core-eap:jsBrowserTest \
  :html-core-eap:wasmJsBrowserTest

Run the JVM benchmarks:

./gradlew -Pcompose.html.eap.enabled=true \
  :html-eap-benchmarks:run

Release Notes

N/A

[Oleksandr Karpovich (eymar)]

Can this benchmark be written in Kotlin rather than Java?

[Clemens Koprolin (clemenskoprolin)]

Yes, I migrated the benchmark to Kotlin using kotlinx-benchmark.

[Oleksandr Karpovich (eymar)]

Let's use a more straightforward syntax:

val pool = getHtmlRendererPool(key)

internal fun getHtmlRendererPool(key: String?): HtmlRendererPool? = if (key == null) null else renderers[key]