pipelines: fix edges gathering

[pared]

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Fixes #3410

[efiop]

This is not a complaint or anything like that, but just wanted to say that this is actually a really good case where a PR/patch description would help understand the difference :) Just like a short note about "we are currently using dfs_edges, but it is not what we need because ... , so we need to use edge_dfs". Since you dove into researching this, your summary would have a great value 🙂

[pared]

Thats a valid point, Ill do that, it will probably be the best to include that in original issue, right?

[skshetry]

@pared, could this be related with this: #3353?

[pared]

@skshetry I don't think so. This issue was purely about dvc pipelines, and in #3353 throw happens upon reproduce. I would need to dive deeper into #3353 to say more.

[efiop]

@pared Great catch! 🙏 Let's add a test (probably unit test for _build_graph, if that makes sense to you) for it, so we are covered from regressions in the future.

[pared]

@efiop I will prepare test
Summary of bug reason can be found in the original issue.