He-Pin opened a new issue, #3106:
URL: https://github.com/apache/pekko/issues/3106
### Motivation
`SourceRefImpl.source` and `SinkRefImpl.sink()` create a new
`SourceRefStageImpl` / `SinkRefStageImpl` instance on every access. If a user
accidentally materializes the returned Source/Sink more than once, multiple
stage instances compete for the same partner handshake, causing the second
materialization to fail with a confusing error.
### Current behavior
`SourceRefImpl.scala:32-33`:
```scala
def source: Source[T, NotUsed] =
Source.fromGraph(new SourceRefStageImpl(OptionVal.Some(initialPartnerRef)))
```
`SinkRefImpl.scala:33-34`:
```scala
override def sink(): Sink[In, NotUsed] =
Sink.fromGraph(new SinkRefStageImpl[In](OptionVal.Some(initialPartnerRef)))
```
### Proposed fix
Option A: Cache with `lazy val` (requires changing from `case class` to
`class`):
```scala
private[stream] final class SourceRefImpl[T](initialPartnerRef: ActorRef)
extends SourceRef[T] {
lazy val source: Source[T, NotUsed] =
Source.fromGraph(new
SourceRefStageImpl(OptionVal.Some(initialPartnerRef))).mapMaterializedValue(_
=> NotUsed)
// ... equals/hashCode/toString to preserve case class semantics
}
```
Option B: Add a clear error message when double-materialization is
attempted, rather than a confusing handshake failure.
### References
- Inspired by Akka.NET
[#7905](https://github.com/akkadotnet/akka.net/pull/7905)
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