Python 2 interpreter used for PY3 binaries that are implicit dependencies of rules

[apt-itude]

Description of the problem

A rule that depends on a py_binary with python_version = "PY3" uses Python 2 interpreter when executing that binary.

Example

BUILD:

load("//:rules.bzl", "touchfile")

touchfile(name = "test")

rules.bzl:

def _touchfile_impl(ctx):
    ctx.actions.run(
        outputs = [ctx.outputs.touchfile],
        executable = ctx.executable._binary,
        arguments = [ctx.outputs.touchfile.path],
    )


touchfile = rule(
    implementation = _touchfile_impl,
    attrs = {
        "_binary": attr.label(
            default = "//src:touch",
            executable = True,
            cfg = "host",
        )
    },
    outputs = {
        "touchfile": "%{name}.touch",
    },
)

src/BUILD:

py_binary(
    name = "touch",
    srcs = ["touch.py"],
    python_version = "PY3",
    srcs_version = "PY3",
    visibility = ["//visibility:public"],
)

Build result:

$ bazel build --incompatible_allow_python_version_transitions :test
INFO: Build option --python_top has changed, discarding analysis cache.
INFO: Analysed target //:test (1 packages loaded, 103 targets configured).
INFO: Found 1 target...
ERROR: /Users/athompson/code/bazel-testing/bazel-py3-binary-as-implicit-rule-dep/src/BUILD:1:1: This target is being built for Python 2 but (transitively) includes Python 3-only sources. You can get diagnostic information about which dependencies introduce this version requirement by running the `find_requirements` aspect. For more info see the documentation for the `srcs_version` attribute.
Target //:test failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.481s, Critical Path: 0.01s
INFO: 0 processes.
FAILED: Build did NOT complete successfully

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

1. git clone https://github.com/apt-itude/bazel-py3-binary-as-implicit-rule-dep.git
2. cd bazel-py3-binary-as-implicit-rule-dep/
3. bazel build //:test

What operating system are you running Bazel on?

CentOS Linux release 7.5.1804

What's the output of bazel info release?

release 0.24.0- (@non-git)

If bazel info release returns "development version" or "(@Non-Git)", tell us how you built Bazel.

Installed from vbatts-bazel COPR repository.

Have you found anything relevant by searching the web?

Issue #6443 might be related, just based on the comment that cfg = "host" "prevents switching the PY2/PY3 mode".

[brandjon]

Yes, this is the generalization of #6443: there's no way to have multiple configurations in host mode, so the plan is to replace host with a host-like mode that allows different configurations. Closing as duplicate.

Note that for non-host configured implicit dependencies, this should work.