[BUG] Cache loading of nested list fails

[fellhorn]

Describe the bug

Flytepropeller fails loading the cache for a nested list if one sub-list is empty.

Minimal example workflow:

from flytekit import workflow, task

@task(
    cache=True,
    cache_version="0"
)
def my_task() -> list[list[str]]:
    return [["foo"], []]

@workflow
def my_workflow():
    my_task()

Flytepropeller logs show this error:

Error when trying to reconcile workflow. Error [Failed to check Catalog for previous results: unexpected artifactData: [o0] type: [collection_type:<union_type:<variants:<collection_type:<simple:NONE > > variants:<collection_type:<simple:STRING > > > > ]

which looks like that it interprets the type of

{
	"o0": [
		["foo"],
		[]
	]
}

as

list[Union[list[None],list[str]]

instead of

list[list[str]]

User experience

For the user the task is just stuck in Queued and they need access to the flytepropeller logs to do any debugging.

Expected behavior

• Caching should also work for these nested lists with empty sub-lists
• Ideally flytepropeller error logs are present in the frontend and the task fails for caching issues instead of being stuck in Queued

Additional context to reproduce

1. Execute the workflow from above once to write the cache
2. Execute it a second time and the task should be stuck in Queued

Screenshots

No response

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[fellhorn]

It seems like the underlying issue is connected to different types in the sublists. E.g. this workflow will hang too:

from typing import Union

from flytekit import workflow, task

@task(
    cache=True,
    cache_version="union_list"
)
def my_task() -> list[list[Union[str,int]]]:
    return [["foo"], [2]]

@workflow
def my_workflow():
    my_task()

[hamersaw]

This is related to flyteorg/flytepropeller#599, but the problem seems to be slightly different. In this scenario, the validation between union in collection is causing issues with validation. Will explore further.

[hamersaw]

closed as completed

[fellhorn]

Thank you :)