[Bug]: "unbound variable" error in py_test, py_binary when using py_runtime with interpreter_path set

[wade-arista]

What happened?

I declared a custom toolchain that points at the system python interpreter.

load("@rules_python//python:defs.bzl", "py_runtime", "py_runtime_pair")

py_runtime(
    name = "my_py3_runtime",
    interpreter_path = "/usr/bin/python",
    interpreter_version_info = {
        "major": "3",
        "minor": "10",
        "micro": "8",
    },
    python_version = "PY3",
)

py_runtime_pair(
    name = "my_py_runtime_pair",
    py2_runtime = None,
    py3_runtime = ":my_py3_runtime",
)

toolchain(
    name = "default",
    toolchain = ":my_py_runtime_pair",
    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
)

I registered it in MODULE.bazel:

register_toolchains("//toolchain:default")

When I tried to run a py_test, the sh wrapper failed:

$ bazel test --action_env=RUNFILES_LIB_DEBUG=1 src_test
==================== Test output for //:src_test:
INFO[runfiles.bash]: rlocation(aspect_rules_py~override/py/tools/venv_bin/venv_bin): start
INFO[runfiles.bash]: runfiles_current_repository(2): caller's path is (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test)
INFO[runfiles.bash]: runfiles_current_repository(2): (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test) has path (_main/src_test) relative to the runfiles directory (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles)
INFO[runfiles.bash]: runfiles_current_repository(2): (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test) corresponds to rlocation path (_main/src_test)
INFO[runfiles.bash]: runfiles_current_repository(2): (_main/src_test) lies in the main repository
INFO[runfiles.bash]: rlocation(aspect_rules_py~override/py/tools/venv_bin/venv_bin): looking up canonical name for (aspect_rules_py~override) from () in (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_repo_mapping)
INFO[runfiles.bash]: rlocation(aspect_rules_py~override/py/tools/venv_bin/venv_bin): canonical name of target repo is ()
INFO[runfiles.bash]: rlocation(aspect_rules_py~override/py/tools/venv_bin/venv_bin): found under RUNFILES_DIR (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles), return
INFO[runfiles.bash]: rlocation(_main//usr/bin/python): start
ERROR[runfiles.bash]: rlocation(_main//usr/bin/python): path is not normalized
/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test: line 25: $1: unbound variable
INFO[runfiles.bash]: rlocation(_main/src_test.venv.pth): start
INFO[runfiles.bash]: runfiles_current_repository(2): caller's path is (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test)
INFO[runfiles.bash]: runfiles_current_repository(2): (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test) has path (_main/src_test) relative to the runfiles directory (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles)
INFO[runfiles.bash]: runfiles_current_repository(2): (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test) corresponds to rlocation path (_main/src_test)
INFO[runfiles.bash]: runfiles_current_repository(2): (_main/src_test) lies in the main repository
INFO[runfiles.bash]: rlocation(_main/src_test.venv.pth): looking up canonical name for (_main) from () in (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_repo_mapping)
INFO[runfiles.bash]: rlocation(_main/src_test.venv.pth): canonical name of target repo is ()
INFO[runfiles.bash]: rlocation(_main/src_test.venv.pth): found under RUNFILES_DIR (/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles), return
error: a value is required for '--python <PYTHON>' but none was supplied

Version

Development (host) and target OS/architectures: linux x86-64

Output of bazel --version: bazel 7.0.2

Version of the Aspect rules: rules_py 6962d6a (also v0.7.1)

Language(s) and/or frameworks involved: Python 3.10.8 (happens with 3.9, 3.11 as well)

How to reproduce

After defining the python toolchain as shown above, trying to run any py_test or py_binary target will reproduce the problem.

Min repro:
https://github.com/wade-arista/bazel-demo/tree/23ff2cebe6058a5e99535bd3f76c72b1b2b188ce

Test run:
https://github.com/wade-arista/bazel-demo/actions/runs/8209925408/job/22456408401#step:4:615

Any other information?

ERROR[runfiles.bash]: rlocation(_main//usr/bin/python): path is not normalized
/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/src_test.runfiles/_main/src_test: line 25: $1: unbound variable

It's trying to resolve {{ARG_PYTHON}} in

"${VENV_TOOL}" \
    --location "${VIRTUAL_ENV}" \
    --python "$(alocation $(rlocation {{ARG_PYTHON}}))" \
    --python-version "{{ARG_VENV_PYTHON_VERSION}}" \
    --pth-file "$(rlocation {{ARG_PTH_FILE}})"

See

rules_py/py/private/run.tmpl.sh

Lines 27 to 31 in 734070b

	"${VENV_TOOL}" \
	--location "${VIRTUAL_ENV}" \
	--python "$(alocation $(rlocation {{ARG_PYTHON}}))" \
	--python-version "{{ARG_VENV_PYTHON_VERSION}}" \
	--pth-file "$(rlocation {{ARG_PTH_FILE}})"

and

rules_py/py/private/py_binary.bzl

Line 72 in 734070b

"{{ARG_PYTHON}}": to_rlocation_path(ctx, py_toolchain.python),