He-Pin opened a new pull request, #3006:
URL: https://github.com/apache/pekko/pull/3006
### Motivation
`SourceWithContextSpec` and `FlowWithContextSpec` fail on JDK 25 nightly
with a **real assertion error (not a timeout)**:
```
Vector(Message(A,1), Message(B,2), Message(D,3)) was not equal to
Vector(Message(A,1), Message(B,2), Message(D,3), Message(C,4))
(SourceWithContextSpec.scala:94) *** FAILED *** (15 milliseconds)
```
`alsoTo` / `alsoToContext` / `wireTap` / `wireTapContext` feed an
**asynchronous** side `Sink`. The tests collected the side stream output into a
buffer and asserted on it immediately after the main stream's
`expectComplete()`. There is no happens-before between the main stream
completing and the side `Sink` draining its last element, so the assertion can
run before the side `Sink` has observed every element. The enclosing
`within(10.seconds) { ... }` only bounds how long the block may take — it does
**not** retry, so the buffer was read exactly once. On JDK 17 the side `Sink`
usually drained first by luck; JDK 25's different scheduling surfaces the race
as a hard failure in ~15ms.
The shared `scala.collection.mutable.ListBuffer` was also written from the
side-stream dispatcher thread and read from the test thread without
synchronization — a second latent data race.
### Modification
- Collect side-stream output into a
`java.util.concurrent.ConcurrentLinkedQueue` (thread-safe, preserves arrival
order).
- Replace the single `within(10.seconds) { assert }` with
`awaitAssert(assert, 10.seconds)`, which polls until the async side `Sink` has
caught up (or the deadline elapses).
### Result
The assertions now wait for the asynchronous side `Sink` to finish instead
of sampling it once, removing both the timing race and the cross-thread buffer
race. The checks still fail fast if the side `Sink` genuinely drops data, since
`awaitAssert` rethrows the last assertion error at the deadline.
### Tests
- `sbt "stream-tests/testOnly
org.apache.pekko.stream.scaladsl.SourceWithContextSpec
org.apache.pekko.stream.scaladsl.FlowWithContextSpec"` — 24 tests pass (Scala
2.13).
### References
- Failing nightly: https://github.com/apache/pekko/actions/runs/26571377673
- Tracking issue: #2573
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