Python providers loaded during workspace fail due to stub provider

[sgowroji]

https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/2967#01879289-b1c2-48af-a43c-0e8961a8618d

"py2_runtime": attr.label(
Error in label: Illegal argument: element in 'providers' is of unexpected type. Either all elements should be providers, or all elements should be lists of providers, but got an element of type Provider. 

Culprit finder: https://buildkite.com/bazel/culprit-finder/builds/5297#0187928b-2541-4ce6-9235-e674cb8b496e
Bisect shows 18aeb52 is the culprit

@rickeylev
Could you please guide whether this is a bug to be fixed in Bazel or the downstream project (Bazel bench) should adapt this change.

CC @meteorcloudy

[meteorcloudy]

For downstream breakage, we can prioritize them as P1 since there should be ongoing actions to fix the breakage. Either by fixing Bazel, fixing the downstream project or just disabling the project in downstream pipeline in case of no response from the project owner.

[meteorcloudy]

/cc @joeleba @tjgq as maintainers of Bazel bench

[fmeum]

Could be worth trying whether #18022 fixes this breakage.

[meteorcloudy]

Indeed, this looks like the same issue as #17713

[rickeylev]

Does that build have this commit? 4392eb6

When rewriting the first couple providers in Starlark, I initially put None as their value. A couple days later, #17713 was pointed out to me, so I changed None to new ProviderStub(), as done in e2d406c for ProtoLibrary

The error message says "but got an element of type Provider" instead of "NoneType". So this would lead me to believe "fix" commit is present, but that work around isn't working for this case for some reason. Maybe because it's used as part of toolchain rules? Which usually have more setup/usage during WORKSPACE evaluation.

[meteorcloudy]

https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/2967 indicates Bazel was built at 5848524 which is after 4392eb6

[meteorcloudy]

@sgowroji Maybe it's also help to give a few commands to reproduce this failure (including fetching Bazel at a given commit, fetching the project at a given commit, and a Bazel command)? I know it's not hard to figure it out from the build log, but having that directly in the issue would help people to diagnose the problem.

[sgowroji]

Please find the below steps to reproduce the above error :

1. git clone https://github.com/bazelbuild/bazel-bench.git
2. cd bazel-bench
3. git reset f525eb78939c139df53c8e77b8f680f1ea96a2ab --hard
4. export USE_BAZEL_VERSION=5848524ec9133e11f90cea13702dfdad813ae7a6
5. bazel build //...