Source.combine single source with type-transforming fan-in strategies#2746
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…in strategies (apache#2723) (apache#2726) Motivation: Source.combine with a single source bypassed the fan-in strategy using an unsafe asInstanceOf cast. This worked for type-preserving strategies like Merge (T → T), but silently produced incorrect results for type-transforming strategies like MergeLatest (T → List[T]). For example: Source.combine(Seq(Source.single(1)))(MergeLatest(_)) emitted 1 instead of List(1) Modification: - Introduce TypePreservingFanIn marker trait for fan-in stages where T == U AND single-input behavior is a no-op pass-through (Merge, Concat, Interleave, MergePrioritized, OrElse) - MergeSequence intentionally NOT marked: despite being T → T, it validates sequence ordering (not a pure pass-through) - Source.combine single-source case: check TypePreservingFanIn trait before bypassing. Strategies without the trait are routed through the fan-in graph. - Relax Concat, Interleave, MergeSequence to accept inputPorts >= 1 (was > 1). This eliminates the need for a try-catch fallback in Source.combine and allows these stages to be used directly with a single input. - Use Source.fromGraph for non-Source Graph inputs safety - Add 14 regression tests (12 Scala + 1 Java + MergeSequence validation) Result: - MergeLatest/ZipWithN correctly apply their transformation even for single source - Merge/Concat/Interleave correctly bypass (type-preserving optimization) - MergeSequence correctly validates sequences even for single source - Unknown/third-party strategies default to routing through the fan-in graph (safe default for strategies that may transform types) - Binary compatibility maintained (verified via MiMa) References: - apache#2723 - apache#2726
He-Pin
approved these changes
Mar 16, 2026
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cherry pick 566a347 #2723 #2726