He-Pin opened a new pull request, #2824:
URL: https://github.com/apache/pekko/pull/2824
## Motivation
Add the `gather` operator across the Scala and Java DSLs, tighten its
execution semantics, and broaden its statefulMap-equivalent regression coverage.
This PR also includes the hot-path and backpressure fixes found during
review, plus benchmark and documentation support for the new operator.
## Modification
- add `gather` API support in the Scala and Java DSLs
- implement the `Gather` stage and supporting `Gatherer` / `GatherCollector`
APIs
- add Scala, Java, and docs examples for the operator
- add `FlowGatherSpec` plus Java parity coverage
- add JMH coverage for `zipWithIndex`-style workloads
- optimize the public single-output gather hot path
- fix the `OneToOneGatherer` backpressure issue found during review
- expand tests toward statefulMap-equivalent scenarios such as happy-path
state retention, restart, stop, and backpressure
## Result
- `gather` is documented and available end-to-end in Scala and Java DSLs
- correctness coverage is materially stronger, especially around supervision
and backpressure
- local verification passed:
- `sbt --no-colors 'scalafmtAll' 'stream-tests/testOnly
org.apache.pekko.stream.scaladsl.FlowGatherSpec' 'stream-tests/testOnly
org.apache.pekko.stream.javadsl.FlowTest' 'stream-tests/testOnly
org.apache.pekko.stream.javadsl.SourceTest' 'docs/test:compile'
'bench-jmh/Jmh/compile'`
- corrected local JMH indicates `gather`-based `zipWithIndex` is now near
parity with `statefulMap`
## References
- `stream/src/main/scala/org/apache/pekko/stream/impl/fusing/Ops.scala`
-
`stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowGatherSpec.scala`
- `stream/src/main/scala/org/apache/pekko/stream/scaladsl/Gather.scala`
- `stream/src/main/scala/org/apache/pekko/stream/javadsl/Gather.scala`
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