He-Pin opened a new issue, #3094:
URL: https://github.com/apache/pekko/issues/3094
### Motivation
The `Source.queue(bufferSize, overflowStrategy)` overloads (both Scala and
Java DSLs) materialize a `SourceQueueWithComplete` whose asynchronous `offer`
future can hang indefinitely under `OverflowStrategy.backpressure` when the
buffer is full and downstream stalls. This is a long-standing trap that has
caused real-world deadlocks and OOM-like symptoms (the same class of issue was
documented upstream as akka/akka-core#29557, and more recently prompted
akka/akka.net#8248 to add a cancellation-aware offer overload).
Pekko already provides a safer replacement:
- `Source.queue[T](bufferSize)` materializes a `BoundedSourceQueue` with
synchronous feedback (drops the newest element when the buffer is full, never
hangs).
- For backpressure scenarios, `Source.actorRefWithBackpressure` and
`MergeHub.source` cover the common use cases.
The upstream Akka discussion in akka/akka-core#29801 ("Replace old
Source.queue") has been open since 2021 without converging on a new
backpressure primitive. We should not wait: the drop-new path
(`BoundedSourceQueue`) is already stable and production-proven, and the
backpressure path is covered by existing operators.
### Modification
Add `@deprecated(since = "2.0.0")` annotations to the four `Source.queue`
overloads that accept an `OverflowStrategy`:
- `scaladsl.Source.queue[T](bufferSize, overflowStrategy)`
- `scaladsl.Source.queue[T](bufferSize, overflowStrategy,
maxConcurrentOffers)`
- `javadsl.Source.queue[T](bufferSize, overflowStrategy)`
- `javadsl.Source.queue[T](bufferSize, overflowStrategy,
maxConcurrentOffers)`
The `Source.queue[T](bufferSize)` overloads that return a
`BoundedSourceQueue` are **not** deprecated and remain the recommended path.
The `SourceQueue` / `SourceQueueWithComplete` traits are not deprecated
(they are still referenced by the deprecated factory return types and by user
code); only the factory entry points are marked deprecated so users see the
signal at the call site without cascading warnings.
### Result
Users calling `Source.queue(...)` with an `OverflowStrategy` get a
compile-time deprecation warning pointing them to `Source.queue(bufferSize)`
(for drop-new) or `Source.actorRefWithBackpressure` / `MergeHub.source` (for
backpressure). This closes the documented hang trap at the API level without
breaking binary compatibility.
### Tests
Not run - API surface change only (annotation addition, no behavior change).
The deprecation annotations are verified by the compiler on every compilation
of user code.
A follow-up may add a `@nowarn` guard or a compile-time test to ensure the
warning surfaces, but that is out of scope for this PR.
### References
- Refs akka/akka-core#29801 (Replace old Source.queue - OPEN, stale since
2021)
- Refs akka/akka-core#29557 (Async callback tracking OOM in Source.queue)
- Refs akka/akka.net#8248 (Akka.NET's cancellation-aware Source.Queue offer,
motivating the same review)
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