AsyncSearch - Handle released finalResponse with tryIncRef() to avoid race assertions

[drempapis]

The current implementation of MutableSearchResponse.toAsyncSearchResponse() assumes that if finalResponse != null, the SearchResponse can always be retained. It enforces this by calling mustIncRef().

However, under concurrent execution, this assumption breaks down. One thread may be serializing a response while another (via AsyncSearchTask.close()) decrements the reference count. If decRef() drops the count to 0, the object is released. A later mustIncRef() will then fail, leading to assertion errors or use-after-release. This creates a race condition that can cause sporadic failures when fetching async search results, especially in the narrow window between task closure and document deletion.

This change replaces mustIncRef() with tryIncRef(). If the reference can no longer be retained, we safely set searchResponse = null and return a clear GONE error to the client. This avoids unsafe access to released resources and provides deterministic behavior in race scenarios.

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @drempapis, I've created a changelog YAML for you.

[drempapis]

Closing, a different approach was implemented #134359