Affects Version/s: None
Fix Version/s: None
translations.sort_stages() sorts stages in topological order. However, calling translations.pipeline_from_stages() on sorted stages can result in a pipeline that is not topologically ordered. This is because of how it constructs the tree of parent to subtransforms.
- Leaf transforms are A, B and C.
- Composite transform D has subtransforms B and C.
- Root transform E has subtransforms A and D.
- A produces an output that is an input to C, and B produces an output that is an input to C.
- After optimizations and sort stages, the order of leaf stages is B, A, C (this is a valid ordering)
Under the current implementation of translations.pipeline_from_stages():
- B is added to the pipeline first, which also adds its parent D and its grandparent E to the pipeline. D is added as the first subtransform of E and B is added as the first subtransform of D.
- A is added to the pipeline second. A is added as the second subtransform of E.
- C is added to the pipeline third. C is added as the second subtransform of D.
The order is now E(D(B, C), A) which is invalid because C must follow A. A valid order would be E(A, D(B, C)).
The easiest fix is to change translations.pipeline_from_stages() such that whenever a leaf transform is added to the pipeline, all its ancestors are moved to the last position of the subtransforms of their respective parent.