feat: Provide a host platform alias (#635)

* feat: Provide a host platform alias

This lets users and repository rules access the interpreter for whatever
host the repository is running on.

* Apply suggestions from code review

Co-authored-by: Thulio Ferraz Assis <3149049+f0rmiga@users.noreply.github.com>

Author: alexeagle

python/private/toolchains_repo.bzl

@@ -29,27 +29,64 @@ PLATFORMS = {
             "@platforms//os:macos",
             "@platforms//cpu:aarch64",
         ],
+        # Matches the value returned from:
+        # repository_ctx.os.name.lower()
+        os_name = "mac os",
+        # Matches the value returned from:
+        # repository_ctx.execute(["uname", "-m"]).stdout.strip()
+        arch = "arm64",
     ),
     "x86_64-apple-darwin": struct(
         compatible_with = [
             "@platforms//os:macos",
             "@platforms//cpu:x86_64",
         ],
+        # See comments above.
+        os_name = "mac os",
+        arch = "x86_64",
     ),
     "x86_64-pc-windows-msvc": struct(
         compatible_with = [
             "@platforms//os:windows",
             "@platforms//cpu:x86_64",
         ],
+        # See comments above.
+        os_name = "windows",
+        arch = "x86_64",
     ),
     "x86_64-unknown-linux-gnu": struct(
         compatible_with = [
             "@platforms//os:linux",
             "@platforms//cpu:x86_64",
         ],
+        # See comments above.
+        os_name = "linux",
+        arch = "x86_64",
     ),
 }
 
+def host_platform(rctx):
+    """Infer the host platform from a repository context.
+
+    Args:
+        rctx: Bazel's repository_ctx
+    Returns:
+        a key from the PLATFORMS dictionary
+    """
+    os_name = rctx.os.name
+
+    # We assume the arch for Windows is always x86_64.
+    if "windows" in os_name:
+        arch = "x86_64"
+    else:
+        # This is not ideal, but bazel doesn't directly expose arch.
+        arch = rctx.execute(["uname", "-m"]).stdout.strip()
+
+    for platform, meta in PLATFORMS.items():
+        if meta.os_name == os_name and meta.arch == arch:
+            return platform
+    fail("No platform declared for host OS {} on arch {}".format(os_name, arch))
+
 def _toolchains_repo_impl(repository_ctx):
     build_content = """\
 # Generated by toolchains_repo.bzl

python/repositories.bzl

@@ -17,11 +17,12 @@
 For historic reasons, pip_repositories() is defined in //python:pip.bzl.
 """
 
-load("//python/private:toolchains_repo.bzl", "PLATFORMS", "toolchains_repo")
-load("//python/private:versions.bzl",
-    "get_release_url",
+load("//python/private:toolchains_repo.bzl", "PLATFORMS", "host_platform", "toolchains_repo")
+load(
+    "//python/private:versions.bzl",
     "MINOR_MAPPING",
     "TOOL_VERSIONS",
+    "get_release_url",
 )
 
 def py_repositories():
@@ -117,20 +118,16 @@ py_runtime_pair(
     rctx.file("BUILD.bazel", build_content)
 
     return {
-        "sha256": download_result.sha256,
         "name": rctx.attr.name,
         "platform": platform,
         "python_version": python_version,
+        "sha256": download_result.sha256,
     }
 
 python_repository = repository_rule(
     _python_repository_impl,
     doc = "Fetches the external tools needed for the Python toolchain.",
     attrs = {
-        "sha256": attr.string(
-            doc = "The SHA256 integrity hash for the Python interpreter tarball.",
-            mandatory = True,
-        ),
         "platform": attr.string(
             doc = "The platform name for the Python interpreter tarball.",
             mandatory = True,
@@ -141,6 +138,48 @@ python_repository = repository_rule(
             mandatory = True,
             values = TOOL_VERSIONS.keys() + MINOR_MAPPING.keys(),
         ),
+        "sha256": attr.string(
+            doc = "The SHA256 integrity hash for the Python interpreter tarball.",
+            mandatory = True,
+        ),
+    },
+)
+
+def _host_os_alias_impl(repository_ctx):
+    # Base BUILD file for this repository.
+    repository_ctx.file("BUILD.bazel", """\
+# Generated by python/repositories.bzl
+package(default_visibility = ["//visibility:public"])
+alias(name = "files",           actual = "@{py_repository}_{host_platform}//:files")
+alias(name = "py3_runtime",     actual = "@{py_repository}_{host_platform}//:py3_runtime")
+alias(name = "python_runtimes", actual = "@{py_repository}_{host_platform}//:python_runtimes")
+alias(name = "python3",         actual = "@{py_repository}_{host_platform}//:bin/python3")
+""".format(
+        py_repository = repository_ctx.attr.user_repository_name,
+        host_platform = host_platform(repository_ctx),
+    ))
+
+    # 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.
+    repository_ctx.file("defs.bzl", content = """\
+# Generated by python/repositories.bzl
+host_platform="{host_platform}"
+interpreter="@{py_repository}_{host_platform}//:bin/python3"
+""".format(
+        py_repository = repository_ctx.attr.user_repository_name,
+        host_platform = host_platform(repository_ctx),
+    ))
+
+_host_os_alias = repository_rule(
+    _host_os_alias_impl,
+    doc = """Creates a repository with a shorter name meant for the host platform, which contains
+    a BUILD.bazel file declaring aliases to the host platform's targets.
+    """,
+    attrs = {
+        "user_repository_name": attr.string(
+            mandatory = True,
+            doc = "The base name for all created repositories, like 'python38'.",
+        ),
     },
 )
 
@@ -150,8 +189,6 @@ def python_register_toolchains(name, python_version, **kwargs):
 
     - Create a repository for each built-in platform like "python_linux_amd64" -
       this repository is lazily fetched when Python is needed for that platform.
-    - TODO(f0rmiga): create a convenience repository for the host platform like
-      "python_host".
     - Create a repository exposing toolchains for each platform like
       "python_platforms".
     - Register a toolchain pointing at each platform.
@@ -185,6 +222,10 @@ def python_register_toolchains(name, python_version, **kwargs):
             platform = platform,
         ))
 
+    _host_os_alias(
+        name = "{name}_host".format(name = name),
+        user_repository_name = name,
+    )
     toolchains_repo(
         name = "{name}_toolchains".format(name = name),
         user_repository_name = name,