ihji commented on pull request #12086:
URL: https://github.com/apache/beam/pull/12086#issuecomment-653377848
Here's a concrete example that can be fixed by this PR:
```
Stage A:
(Input PCollection i - PTransform 1 - Output PCollection j)
Stage B:
(Input PCollection j - PTransform 2 - Output PCollection k)
Stage C:
(Input PCollection j for side input - PTransform 3)
```
We want to find `Stage A` for emitting side input for `Stage C`. However,
some synthetic PTransforms are inserted during pipeline optimization phase:
```
Stage A:
(Input PCollection i - PTransform 1 - Output PCollection j)
(Input PCollection j - Synthetic Write - Data Sink)
Stage B:
(Data source - Synthetic Read - Output PCollection j)
(Input PCollection j - PTransform 2 - Output PCollection k)
Stage C:
(Input PCollection j for side input - PTransform 3)
```
If we allow multiple assignments to `producing_stages_by_pcoll`, `Stage B`
will emit side input for `Stage C` (topologically `Stage B` comes after `Stage
A`). Since `Stage B` and `Stage C` have no dependencies, the pipeline will
succeed when `Stage B` is executed first and fail when `Stage C` is executed
before `Stage B`.
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