python/private/toolchains_repo.bzl - third_party/github.com/bazelbuild/rules_python - Git at Google

 # Copyright 2022 The Bazel Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #    http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.

 """Create a repository to hold the toolchains.

 This follows guidance here:
 https://docs.bazel.build/versions/main/skylark/deploying.html#registering-toolchains

 The "complex computation" in our case is simply downloading large artifacts.
 This guidance tells us how to avoid that: we put the toolchain targets in the
 alias repository with only the toolchain attribute pointing into the
 platform-specific repositories.
 """

 load(
     "//python:versions.bzl",
     "LINUX_NAME",
     "MACOS_NAME",
     "PLATFORMS",
     "WINDOWS_NAME",
 )
 load("//python/private:repo_utils.bzl", "REPO_DEBUG_ENV_VAR", "repo_utils")

 def get_repository_name(repository_workspace):
     dummy_label = "//:_"
     return str(repository_workspace.relative(dummy_label))[:-len(dummy_label)] or "@"

 def python_toolchain_build_file_content(
         prefix,
         python_version,
         set_python_version_constraint,
         user_repository_name):
     """Creates the content for toolchain definitions for a build file.

     Args:
         prefix: Python toolchain name prefixes
         python_version: Python versions for the toolchains
         set_python_version_constraint: string, "True" if the toolchain should
             have the Python version constraint added as a requirement for
             matching the toolchain, "False" if not.
         user_repository_name: names for the user repos

     Returns:
         build_content: Text containing toolchain definitions
     """

     # We create a list of toolchain content from iterating over
     # the enumeration of PLATFORMS.  We enumerate PLATFORMS in
     # order to get us an index to increment the increment.
     return "\n\n".join([
         """\
 py_toolchain_suite(
     user_repository_name = "{user_repository_name}_{platform}",
     prefix = "{prefix}{platform}",
     target_compatible_with = {compatible_with},
     python_version = "{python_version}",
     set_python_version_constraint = "{set_python_version_constraint}",
 )""".format(
             compatible_with = meta.compatible_with,
             platform = platform,
             set_python_version_constraint = set_python_version_constraint,
             user_repository_name = user_repository_name,
             prefix = prefix,
             python_version = python_version,
         )
         for platform, meta in PLATFORMS.items()
     ])

 def _toolchains_repo_impl(rctx):
     build_content = """\
 # Generated by python/private/toolchains_repo.bzl
 #
 # These can be registered in the workspace file or passed to --extra_toolchains
 # flag. By default all these toolchains are registered by the
 # python_register_toolchains macro so you don't normally need to interact with
 # these targets.

 load("@{rules_python}//python/private:py_toolchain_suite.bzl", "py_toolchain_suite")

 """.format(
         rules_python = rctx.attr._rules_python_workspace.workspace_name,
     )

     toolchains = python_toolchain_build_file_content(
         prefix = "",
         python_version = rctx.attr.python_version,
         set_python_version_constraint = str(rctx.attr.set_python_version_constraint),
         user_repository_name = rctx.attr.user_repository_name,
     )

     rctx.file("BUILD.bazel", build_content + toolchains)

 toolchains_repo = repository_rule(
     _toolchains_repo_impl,
     doc = "Creates a repository with toolchain definitions for all known platforms " +
           "which can be registered or selected.",
     attrs = {
         "python_version": attr.string(doc = "The Python version."),
         "set_python_version_constraint": attr.bool(doc = "if target_compatible_with for the toolchain should set the version constraint"),
         "user_repository_name": attr.string(doc = "what the user chose for the base name"),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
     },
 )

 def _toolchain_aliases_impl(rctx):
     (os_name, arch) = get_host_os_arch(rctx)

     host_platform = get_host_platform(os_name, arch)

     is_windows = (os_name == WINDOWS_NAME)
     python3_binary_path = "python.exe" if is_windows else "bin/python3"

     # Base BUILD file for this repository.
     build_contents = """\
 # Generated by python/private/toolchains_repo.bzl
 package(default_visibility = ["//visibility:public"])
 load("@rules_python//python:versions.bzl", "gen_python_config_settings")
 gen_python_config_settings()
 exports_files(["defs.bzl"])

 PLATFORMS = [
 {loaded_platforms}
 ]
 alias(name = "files",           actual = select({{":" + item: "@{py_repository}_" + item + "//:files" for item in PLATFORMS}}))
 alias(name = "includes",        actual = select({{":" + item: "@{py_repository}_" + item + "//:includes" for item in PLATFORMS}}))
 alias(name = "libpython",       actual = select({{":" + item: "@{py_repository}_" + item + "//:libpython" for item in PLATFORMS}}))
 alias(name = "py3_runtime",     actual = select({{":" + item: "@{py_repository}_" + item + "//:py3_runtime" for item in PLATFORMS}}))
 alias(name = "python_headers",  actual = select({{":" + item: "@{py_repository}_" + item + "//:python_headers" for item in PLATFORMS}}))
 alias(name = "python_runtimes", actual = select({{":" + item: "@{py_repository}_" + item + "//:python_runtimes" for item in PLATFORMS}}))
 alias(name = "python3",         actual = select({{":" + item: "@{py_repository}_" + item + "//:" + ("python.exe" if "windows" in item else "bin/python3") for item in PLATFORMS}}))
 """.format(
         py_repository = rctx.attr.user_repository_name,
         loaded_platforms = "\n".join(["    \"{}\",".format(p) for p in rctx.attr.platforms]),
     )
     if not is_windows:
         build_contents += """\
 alias(name = "pip",             actual = select({{":" + item: "@{py_repository}_" + item + "//:python_runtimes" for item in PLATFORMS if "windows" not in item}}))
 """.format(
             py_repository = rctx.attr.user_repository_name,
             host_platform = host_platform,
         )
     rctx.file("BUILD.bazel", build_contents)

     # Expose a Starlark file so rules can know what host platform we used and where to find an interpreter
     # when using repository_ctx.path, which doesn't understand aliases.
     rctx.file("defs.bzl", content = """\
 # Generated by python/private/toolchains_repo.bzl

 load(
     "{rules_python}//python/config_settings:transition.bzl",
     _py_binary = "py_binary",
     _py_test = "py_test",
 )
 load(
     "{rules_python}//python/entry_points:py_console_script_binary.bzl",
     _py_console_script_binary = "py_console_script_binary",
 )
 load("{rules_python}//python:pip.bzl", _compile_pip_requirements = "compile_pip_requirements")

 host_platform = "{host_platform}"
 interpreter = "@{py_repository}_{host_platform}//:{python3_binary_path}"

 def py_binary(name, **kwargs):
     return _py_binary(
         name = name,
         python_version = "{python_version}",
         **kwargs
     )

 def py_console_script_binary(name, **kwargs):
     return _py_console_script_binary(
         name = name,
         binary_rule = py_binary,
         **kwargs
     )

 def py_test(name, **kwargs):
     return _py_test(
         name = name,
         python_version = "{python_version}",
         **kwargs
     )

 def compile_pip_requirements(name, **kwargs):
     return _compile_pip_requirements(
         name = name,
         py_binary = py_binary,
         py_test = py_test,
         **kwargs
     )

 """.format(
         host_platform = host_platform,
         py_repository = rctx.attr.user_repository_name,
         python_version = rctx.attr.python_version,
         python3_binary_path = python3_binary_path,
         rules_python = get_repository_name(rctx.attr._rules_python_workspace),
     ))

 toolchain_aliases = repository_rule(
     _toolchain_aliases_impl,
     doc = """\
 Creates a repository with a shorter name only referencing the python version,
 it contains a BUILD.bazel file declaring aliases to the host platform's targets
 and is a great fit for any usage related to setting up toolchains for build
 actions.""",
     attrs = {
         "platforms": attr.string_list(
             doc = "List of platforms for which aliases shall be created",
         ),
         "python_version": attr.string(doc = "The Python version."),
         "user_repository_name": attr.string(
             mandatory = True,
             doc = "The base name for all created repositories, like 'python38'.",
         ),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
     },
     environ = [REPO_DEBUG_ENV_VAR],
 )

 def _host_toolchain_impl(rctx):
     rctx.file("BUILD.bazel", """\
 # Generated by python/private/toolchains_repo.bzl

 exports_files(["python"], visibility = ["//visibility:public"])
 """)

     (os_name, arch) = get_host_os_arch(rctx)
     host_platform = get_host_platform(os_name, arch)
     repo = "@@{py_repository}_{host_platform}".format(
         py_repository = rctx.attr.name[:-len("_host")],
         host_platform = host_platform,
     )

     rctx.report_progress("Symlinking interpreter files to the target platform")
     host_python_repo = rctx.path(Label("{repo}//:BUILD.bazel".format(repo = repo)))

     # The interpreter might not work on platfroms that don't have symlink support if
     # we just symlink the interpreter itself. rctx.symlink does a copy in such cases
     # so we can just attempt to symlink all of the directories in the host interpreter
     # repo, which should be faster than re-downloading it.
     for p in host_python_repo.dirname.readdir():
         if p.basename in [
             # ignore special files created by the repo rule automatically
             "BUILD.bazel",
             "MODULE.bazel",
             "REPO.bazel",
             "WORKSPACE",
             "WORKSPACE.bazel",
             "WORKSPACE.bzlmod",
         ]:
             continue

         # symlink works on all platforms that bazel supports, so it should work on
         # UNIX and Windows with and without symlink support. For better performance
         # users should enable the symlink startup option, however that requires admin
         # privileges.
         rctx.symlink(p, p.basename)

     is_windows = (os_name == WINDOWS_NAME)
     python_binary = "python.exe" if is_windows else "python"

     # Ensure that we can run the interpreter and check that we are not
     # using the host interpreter.
     python_tester_contents = """\
 from pathlib import Path
 import sys

 python = Path(sys.executable)
 want_python = str(Path("{python}").resolve())
 got_python = str(Path(sys.executable).resolve())

 assert want_python == got_python, \
     "Expected to use a different interpreter:\\nwant: '{{}}'\\n got: '{{}}'".format(
         want_python,
         got_python,
     )
 """.format(repo = repo.strip("@"), python = python_binary)
     python_tester = rctx.path("python_tester.py")
     rctx.file(python_tester, python_tester_contents)
     repo_utils.execute_checked(
         rctx,
         op = "CheckHostInterpreter",
         arguments = [rctx.path(python_binary), python_tester],
     )
     if not rctx.delete(python_tester):
         fail("Failed to delete the python tester")

 host_toolchain = repository_rule(
     _host_toolchain_impl,
     doc = """\
 Creates a repository with a shorter name meant to be used in the repository_ctx,
 which needs to have `symlinks` for the interpreter. This is separate from the
 toolchain_aliases repo because referencing the `python` interpreter target from
 this repo causes an eager fetch of the toolchain for the host platform.
     """,
     attrs = {
         "platforms": attr.string_list(
             doc = "List of platforms for which aliases shall be created",
         ),
         "python_version": attr.string(doc = "The Python version."),
         "user_repository_name": attr.string(
             mandatory = True,
             doc = "The base name for all created repositories, like 'python38'.",
         ),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
     },
 )

 def _multi_toolchain_aliases_impl(rctx):
     rules_python = rctx.attr._rules_python_workspace.workspace_name

     for python_version, repository_name in rctx.attr.python_versions.items():
         file = "{}/defs.bzl".format(python_version)
         rctx.file(file, content = """\
 # Generated by python/private/toolchains_repo.bzl

 load(
     "@{repository_name}//:defs.bzl",
     _compile_pip_requirements = "compile_pip_requirements",
     _host_platform = "host_platform",
     _interpreter = "interpreter",
     _py_binary = "py_binary",
     _py_console_script_binary = "py_console_script_binary",
     _py_test = "py_test",
 )

 compile_pip_requirements = _compile_pip_requirements
 host_platform = _host_platform
 interpreter = _interpreter
 py_binary = _py_binary
 py_console_script_binary = _py_console_script_binary
 py_test = _py_test
 """.format(
             repository_name = repository_name,
         ))
         rctx.file("{}/BUILD.bazel".format(python_version), "")

     pip_bzl = """\
 # Generated by python/private/toolchains_repo.bzl

 load("@{rules_python}//python:pip.bzl", "pip_parse", _multi_pip_parse = "multi_pip_parse")

 def multi_pip_parse(name, requirements_lock, **kwargs):
     return _multi_pip_parse(
         name = name,
         python_versions = {python_versions},
         requirements_lock = requirements_lock,
         **kwargs
     )

 """.format(
         python_versions = rctx.attr.python_versions.keys(),
         rules_python = rules_python,
     )
     rctx.file("pip.bzl", content = pip_bzl)
     rctx.file("BUILD.bazel", "")

 multi_toolchain_aliases = repository_rule(
     _multi_toolchain_aliases_impl,
     attrs = {
         "python_versions": attr.string_dict(doc = "The Python versions."),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
     },
 )

 def sanitize_platform_name(platform):
     return platform.replace("-", "_")

 def get_host_platform(os_name, arch):
     """Gets the host platform.

     Args:
         os_name: the host OS name.
         arch: the host arch.
     Returns:
         The host platform.
     """
     host_platform = None
     for platform, meta in PLATFORMS.items():
         if meta.os_name == os_name and meta.arch == arch:
             host_platform = platform
     if not host_platform:
         fail("No platform declared for host OS {} on arch {}".format(os_name, arch))
     return host_platform

 def get_host_os_arch(rctx):
     """Infer the host OS name and arch from a repository context.

     Args:
         rctx: Bazel's repository_ctx.
     Returns:
         A tuple with the host OS name and arch.
     """
     os_name = rctx.os.name

     # We assume the arch for Windows is always x86_64.
     if "windows" in os_name.lower():
         arch = "x86_64"

         # Normalize the os_name. E.g. os_name could be "OS windows server 2019".
         os_name = WINDOWS_NAME
     else:
         # This is not ideal, but bazel doesn't directly expose arch.
         arch = repo_utils.execute_unchecked(
             rctx,
             op = "GetUname",
             arguments = [repo_utils.which_checked(rctx, "uname"), "-m"],
         ).stdout.strip()

         # Normalize the os_name.
         if "mac" in os_name.lower():
             os_name = MACOS_NAME
         elif "linux" in os_name.lower():
             os_name = LINUX_NAME

     return (os_name, arch)
	# Copyright 2022 The Bazel Authors. All rights reserved.
	#
	# Licensed under the Apache License, Version 2.0 (the "License");
	# you may not use this file except in compliance with the License.
	# You may obtain a copy of the License at
	#
	# http://www.apache.org/licenses/LICENSE-2.0
	#
	# Unless required by applicable law or agreed to in writing, software
	# distributed under the License is distributed on an "AS IS" BASIS,
	# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	# See the License for the specific language governing permissions and
	# limitations under the License.

	"""Create a repository to hold the toolchains.

	This follows guidance here:
	https://docs.bazel.build/versions/main/skylark/deploying.html#registering-toolchains

	The "complex computation" in our case is simply downloading large artifacts.
	This guidance tells us how to avoid that: we put the toolchain targets in the
	alias repository with only the toolchain attribute pointing into the
	platform-specific repositories.
	"""

	load(
	"//python:versions.bzl",
	"LINUX_NAME",
	"MACOS_NAME",
	"PLATFORMS",
	"WINDOWS_NAME",
	)
	load("//python/private:repo_utils.bzl", "REPO_DEBUG_ENV_VAR", "repo_utils")

	def get_repository_name(repository_workspace):
	dummy_label = "//:_"
	return str(repository_workspace.relative(dummy_label))[:-len(dummy_label)] or "@"

	def python_toolchain_build_file_content(
	prefix,
	python_version,
	set_python_version_constraint,
	user_repository_name):
	"""Creates the content for toolchain definitions for a build file.

	Args:
	prefix: Python toolchain name prefixes
	python_version: Python versions for the toolchains
	set_python_version_constraint: string, "True" if the toolchain should
	have the Python version constraint added as a requirement for
	matching the toolchain, "False" if not.
	user_repository_name: names for the user repos

	Returns:
	build_content: Text containing toolchain definitions
	"""

	# We create a list of toolchain content from iterating over
	# the enumeration of PLATFORMS. We enumerate PLATFORMS in
	# order to get us an index to increment the increment.
	return "\n\n".join([
	"""\
	py_toolchain_suite(
	user_repository_name = "{user_repository_name}_{platform}",
	prefix = "{prefix}{platform}",
	target_compatible_with = {compatible_with},
	python_version = "{python_version}",
	set_python_version_constraint = "{set_python_version_constraint}",
	)""".format(
	compatible_with = meta.compatible_with,
	platform = platform,
	set_python_version_constraint = set_python_version_constraint,
	user_repository_name = user_repository_name,
	prefix = prefix,
	python_version = python_version,
	)
	for platform, meta in PLATFORMS.items()
	])

	def _toolchains_repo_impl(rctx):
	build_content = """\
	# Generated by python/private/toolchains_repo.bzl
	#
	# These can be registered in the workspace file or passed to --extra_toolchains
	# flag. By default all these toolchains are registered by the
	# python_register_toolchains macro so you don't normally need to interact with
	# these targets.

	load("@{rules_python}//python/private:py_toolchain_suite.bzl", "py_toolchain_suite")

	""".format(
	rules_python = rctx.attr._rules_python_workspace.workspace_name,
	)

	toolchains = python_toolchain_build_file_content(
	prefix = "",
	python_version = rctx.attr.python_version,
	set_python_version_constraint = str(rctx.attr.set_python_version_constraint),
	user_repository_name = rctx.attr.user_repository_name,
	)

	rctx.file("BUILD.bazel", build_content + toolchains)

	toolchains_repo = repository_rule(
	_toolchains_repo_impl,
	doc = "Creates a repository with toolchain definitions for all known platforms " +
	"which can be registered or selected.",
	attrs = {
	"python_version": attr.string(doc = "The Python version."),
	"set_python_version_constraint": attr.bool(doc = "if target_compatible_with for the toolchain should set the version constraint"),
	"user_repository_name": attr.string(doc = "what the user chose for the base name"),
	"_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
	},
	)

	def _toolchain_aliases_impl(rctx):
	(os_name, arch) = get_host_os_arch(rctx)

	host_platform = get_host_platform(os_name, arch)

	is_windows = (os_name == WINDOWS_NAME)
	python3_binary_path = "python.exe" if is_windows else "bin/python3"

	# Base BUILD file for this repository.
	build_contents = """\
	# Generated by python/private/toolchains_repo.bzl
	package(default_visibility = ["//visibility:public"])
	load("@rules_python//python:versions.bzl", "gen_python_config_settings")
	gen_python_config_settings()
	exports_files(["defs.bzl"])

	PLATFORMS = [
	{loaded_platforms}
	]
	alias(name = "files", actual = select({{":" + item: "@{py_repository}_" + item + "//:files" for item in PLATFORMS}}))
	alias(name = "includes", actual = select({{":" + item: "@{py_repository}_" + item + "//:includes" for item in PLATFORMS}}))
	alias(name = "libpython", actual = select({{":" + item: "@{py_repository}_" + item + "//:libpython" for item in PLATFORMS}}))
	alias(name = "py3_runtime", actual = select({{":" + item: "@{py_repository}_" + item + "//:py3_runtime" for item in PLATFORMS}}))
	alias(name = "python_headers", actual = select({{":" + item: "@{py_repository}_" + item + "//:python_headers" for item in PLATFORMS}}))
	alias(name = "python_runtimes", actual = select({{":" + item: "@{py_repository}_" + item + "//:python_runtimes" for item in PLATFORMS}}))
	alias(name = "python3", actual = select({{":" + item: "@{py_repository}_" + item + "//:" + ("python.exe" if "windows" in item else "bin/python3") for item in PLATFORMS}}))
	""".format(
	py_repository = rctx.attr.user_repository_name,
	loaded_platforms = "\n".join([" \"{}\",".format(p) for p in rctx.attr.platforms]),
	)
	if not is_windows:
	build_contents += """\
	alias(name = "pip", actual = select({{":" + item: "@{py_repository}_" + item + "//:python_runtimes" for item in PLATFORMS if "windows" not in item}}))
	""".format(
	py_repository = rctx.attr.user_repository_name,
	host_platform = host_platform,
	)
	rctx.file("BUILD.bazel", build_contents)

	# Expose a Starlark file so rules can know what host platform we used and where to find an interpreter
	# when using repository_ctx.path, which doesn't understand aliases.
	rctx.file("defs.bzl", content = """\
	# Generated by python/private/toolchains_repo.bzl

	load(
	"{rules_python}//python/config_settings:transition.bzl",
	_py_binary = "py_binary",
	_py_test = "py_test",
	)
	load(
	"{rules_python}//python/entry_points:py_console_script_binary.bzl",
	_py_console_script_binary = "py_console_script_binary",
	)
	load("{rules_python}//python:pip.bzl", _compile_pip_requirements = "compile_pip_requirements")

	host_platform = "{host_platform}"
	interpreter = "@{py_repository}_{host_platform}//:{python3_binary_path}"

	def py_binary(name, **kwargs):
	return _py_binary(
	name = name,
	python_version = "{python_version}",
	**kwargs
	)

	def py_console_script_binary(name, **kwargs):
	return _py_console_script_binary(
	name = name,
	binary_rule = py_binary,
	**kwargs
	)

	def py_test(name, **kwargs):
	return _py_test(
	name = name,
	python_version = "{python_version}",
	**kwargs
	)

	def compile_pip_requirements(name, **kwargs):
	return _compile_pip_requirements(
	name = name,
	py_binary = py_binary,
	py_test = py_test,
	**kwargs
	)

	""".format(
	host_platform = host_platform,
	py_repository = rctx.attr.user_repository_name,
	python_version = rctx.attr.python_version,
	python3_binary_path = python3_binary_path,
	rules_python = get_repository_name(rctx.attr._rules_python_workspace),
	))

	toolchain_aliases = repository_rule(
	_toolchain_aliases_impl,
	doc = """\
	Creates a repository with a shorter name only referencing the python version,
	it contains a BUILD.bazel file declaring aliases to the host platform's targets
	and is a great fit for any usage related to setting up toolchains for build
	actions.""",
	attrs = {
	"platforms": attr.string_list(
	doc = "List of platforms for which aliases shall be created",
	),
	"python_version": attr.string(doc = "The Python version."),
	"user_repository_name": attr.string(
	mandatory = True,
	doc = "The base name for all created repositories, like 'python38'.",
	),
	"_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
	},
	environ = [REPO_DEBUG_ENV_VAR],
	)

	def _host_toolchain_impl(rctx):
	rctx.file("BUILD.bazel", """\
	# Generated by python/private/toolchains_repo.bzl

	exports_files(["python"], visibility = ["//visibility:public"])
	""")

	(os_name, arch) = get_host_os_arch(rctx)
	host_platform = get_host_platform(os_name, arch)
	repo = "@@{py_repository}_{host_platform}".format(
	py_repository = rctx.attr.name[:-len("_host")],
	host_platform = host_platform,
	)

	rctx.report_progress("Symlinking interpreter files to the target platform")
	host_python_repo = rctx.path(Label("{repo}//:BUILD.bazel".format(repo = repo)))

	# The interpreter might not work on platfroms that don't have symlink support if
	# we just symlink the interpreter itself. rctx.symlink does a copy in such cases
	# so we can just attempt to symlink all of the directories in the host interpreter
	# repo, which should be faster than re-downloading it.
	for p in host_python_repo.dirname.readdir():
	if p.basename in [
	# ignore special files created by the repo rule automatically
	"BUILD.bazel",
	"MODULE.bazel",
	"REPO.bazel",
	"WORKSPACE",
	"WORKSPACE.bazel",
	"WORKSPACE.bzlmod",
	]:
	continue

	# symlink works on all platforms that bazel supports, so it should work on
	# UNIX and Windows with and without symlink support. For better performance
	# users should enable the symlink startup option, however that requires admin
	# privileges.
	rctx.symlink(p, p.basename)

	is_windows = (os_name == WINDOWS_NAME)
	python_binary = "python.exe" if is_windows else "python"

	# Ensure that we can run the interpreter and check that we are not
	# using the host interpreter.
	python_tester_contents = """\
	from pathlib import Path
	import sys

	python = Path(sys.executable)
	want_python = str(Path("{python}").resolve())
	got_python = str(Path(sys.executable).resolve())

	assert want_python == got_python, \
	"Expected to use a different interpreter:\\nwant: '{{}}'\\n got: '{{}}'".format(
	want_python,
	got_python,
	)
	""".format(repo = repo.strip("@"), python = python_binary)
	python_tester = rctx.path("python_tester.py")
	rctx.file(python_tester, python_tester_contents)
	repo_utils.execute_checked(
	rctx,
	op = "CheckHostInterpreter",
	arguments = [rctx.path(python_binary), python_tester],
	)
	if not rctx.delete(python_tester):
	fail("Failed to delete the python tester")

	host_toolchain = repository_rule(
	_host_toolchain_impl,
	doc = """\
	Creates a repository with a shorter name meant to be used in the repository_ctx,
	which needs to have `symlinks` for the interpreter. This is separate from the
	toolchain_aliases repo because referencing the `python` interpreter target from
	this repo causes an eager fetch of the toolchain for the host platform.
	""",
	attrs = {
	"platforms": attr.string_list(
	doc = "List of platforms for which aliases shall be created",
	),
	"python_version": attr.string(doc = "The Python version."),
	"user_repository_name": attr.string(
	mandatory = True,
	doc = "The base name for all created repositories, like 'python38'.",
	),
	"_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
	},
	)

	def _multi_toolchain_aliases_impl(rctx):
	rules_python = rctx.attr._rules_python_workspace.workspace_name

	for python_version, repository_name in rctx.attr.python_versions.items():
	file = "{}/defs.bzl".format(python_version)
	rctx.file(file, content = """\
	# Generated by python/private/toolchains_repo.bzl

	load(
	"@{repository_name}//:defs.bzl",
	_compile_pip_requirements = "compile_pip_requirements",
	_host_platform = "host_platform",
	_interpreter = "interpreter",
	_py_binary = "py_binary",
	_py_console_script_binary = "py_console_script_binary",
	_py_test = "py_test",
	)

	compile_pip_requirements = _compile_pip_requirements
	host_platform = _host_platform
	interpreter = _interpreter
	py_binary = _py_binary
	py_console_script_binary = _py_console_script_binary
	py_test = _py_test
	""".format(
	repository_name = repository_name,
	))
	rctx.file("{}/BUILD.bazel".format(python_version), "")

	pip_bzl = """\
	# Generated by python/private/toolchains_repo.bzl

	load("@{rules_python}//python:pip.bzl", "pip_parse", _multi_pip_parse = "multi_pip_parse")

	def multi_pip_parse(name, requirements_lock, **kwargs):
	return _multi_pip_parse(
	name = name,
	python_versions = {python_versions},
	requirements_lock = requirements_lock,
	**kwargs
	)

	""".format(
	python_versions = rctx.attr.python_versions.keys(),
	rules_python = rules_python,
	)
	rctx.file("pip.bzl", content = pip_bzl)
	rctx.file("BUILD.bazel", "")

	multi_toolchain_aliases = repository_rule(
	_multi_toolchain_aliases_impl,
	attrs = {
	"python_versions": attr.string_dict(doc = "The Python versions."),
	"_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
	},
	)

	def sanitize_platform_name(platform):
	return platform.replace("-", "_")

	def get_host_platform(os_name, arch):
	"""Gets the host platform.

	Args:
	os_name: the host OS name.
	arch: the host arch.
	Returns:
	The host platform.
	"""
	host_platform = None
	for platform, meta in PLATFORMS.items():
	if meta.os_name == os_name and meta.arch == arch:
	host_platform = platform
	if not host_platform:
	fail("No platform declared for host OS {} on arch {}".format(os_name, arch))
	return host_platform

	def get_host_os_arch(rctx):
	"""Infer the host OS name and arch from a repository context.

	Args:
	rctx: Bazel's repository_ctx.
	Returns:
	A tuple with the host OS name and arch.
	"""
	os_name = rctx.os.name

	# We assume the arch for Windows is always x86_64.
	if "windows" in os_name.lower():
	arch = "x86_64"

	# Normalize the os_name. E.g. os_name could be "OS windows server 2019".
	os_name = WINDOWS_NAME
	else:
	# This is not ideal, but bazel doesn't directly expose arch.
	arch = repo_utils.execute_unchecked(
	rctx,
	op = "GetUname",
	arguments = [repo_utils.which_checked(rctx, "uname"), "-m"],
	).stdout.strip()

	# Normalize the os_name.
	if "mac" in os_name.lower():
	os_name = MACOS_NAME
	elif "linux" in os_name.lower():
	os_name = LINUX_NAME

	return (os_name, arch)