Build broken with Bazel 2.0.0

[laszlocsomor]

Describe the bug

Build is broken on Bazel CI.
Last green: https://buildkite.com/bazel/rules-haskell-haskell/builds/231
First red: https://buildkite.com/bazel/rules-haskell-haskell/builds/232

To Reproduce

Not sure. Build broke on Bazel CI, see log links above.
I couldn't set up the build locally.

Expected behavior
A clear and concise description of what you expected to happen.

Environment

• OS name + version: Debian testing
• Bazel version: 2.0.0
• Version of the rules: see above

Additional context
Add any other context about the problem here.

[aherrmann]

The error looks like it's caused by a Python version issue.

Use --sandbox_debug to see verbose messages from the sandbox
  | Traceback (most recent call last):
  | File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/sandbox/linux-sandbox/392/execroot/rules_haskell/bazel-out/host/bin/haskell/cabal_wrapper.runfiles/rules_haskell/haskell/cabal_wrapper.py", line 110, in <module>
  | recache_db()
  | File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/sandbox/linux-sandbox/392/execroot/rules_haskell/bazel-out/host/bin/haskell/cabal_wrapper.runfiles/rules_haskell/haskell/cabal_wrapper.py", line 108, in recache_db
  | run([ghc_pkg, "recache", "--package-db=" + package_database])
  | File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/sandbox/linux-sandbox/392/execroot/rules_haskell/bazel-out/host/bin/haskell/cabal_wrapper.runfiles/rules_haskell/haskell/cabal_wrapper.py", line 39, in run
  | subprocess.run(cmd, check=True, *args, **kwargs)
  | AttributeError: 'module' object has no attribute 'run'

cabal_wrapper.py is a Python script that requires Python version 3 due to the use of subprocess.run (added in Python 3.5). The py_binary target specifies python_version = "PY3". With Nix we use nixpkgs_python_configure to configure a Python 3 toolchain. Without Nix we rely on the autodetected Python toolchain. Could this be an instance of bazelbuild/bazel#4815?

[laszlocsomor]

Could this be an instance of bazelbuild/bazel#4815?

I don't know. @brandjon?

[brandjon]

bazelbuild/bazel#4815 is fixed except for on Windows. However, note that if you register a Python toolchain having python_version = "PY3", but it points to a Python interpreter path that is really Python 2, then you'll end up running Python 2 for PY3 targets.

I'd double check that nixpkgs_python_configure's python3_bin_path is actually referring to a Python 3 interpreter in your environment. And if it is, verify that the Python 3 interpreter it's getting is specifically >= 3.5.

[aherrmann]

@brandjon Thanks for clarifying. I tried to reproduce the issue on our own CI and noticed that the Bazel CI config still disables the python toolchains. Sorry, I overlooked that before. Removing that flag should fix the issue. I've opened #1224 and confirmed on Bazel CI.