build(python): align toolchain with monorepo pattern

MODULE.bazel

@@ -7,29 +7,11 @@ bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "rules_python", version = "1.6.3")
 bazel_dep(name = "rules_rust", version = "0.74.0")
 
-local_runtime_repo = use_repo_rule(
-    "@rules_python//python/local_toolchains:repos.bzl",
-    "local_runtime_repo",
-)
-local_runtime_toolchains_repo = use_repo_rule(
-    "@rules_python//python/local_toolchains:repos.bzl",
-    "local_runtime_toolchains_repo",
-)
-
-local_runtime_repo(
-    name = "local_python3",
-    interpreter_path = "python3",
-    on_failure = "fail",
-    dev_dependency = True,
-)
-local_runtime_toolchains_repo(
-    name = "local_python_toolchains",
-    runtimes = ["local_python3"],
-    dev_dependency = True,
-)
+python = use_extension("//bazel:python.bzl", "python")
+use_repo(python, "python")
 
 rust = use_extension("//bazel:rust.bzl", "rust")
 use_repo(rust, "rust")
 
-register_toolchains("@local_python_toolchains//:all", dev_dependency = True)
+register_toolchains("@python//:python")
 register_toolchains("@rust//:rust")

MODULE.bazel.lock

@@ -146,6 +146,23 @@
   },
   "selectedYankedVersions": {},
   "moduleExtensions": {
+    "//bazel:python.bzl%python": {
+      "general": {
+        "bzlTransitiveDigest": "CbGiMYJXjcea8dmoNxUWe/lWOjqRWfQPTXFHUUMheIY=",
+        "usagesDigest": "bcY43VsTrhTJ8N4Dj5FtsgMhZxLyKKjHP+wWK4ntJQ4=",
+        "recordedFileInputs": {},
+        "recordedDirentsInputs": {},
+        "envVariables": {},
+        "generatedRepoSpecs": {
+          "python": {
+            "bzlFile": "@@//bazel:python.bzl",
+            "ruleClassName": "python_repository",
+            "attributes": {}
+          }
+        },
+        "recordedRepoMappingEntries": []
+      }
+    },
     "//bazel:rust.bzl%rust": {
       "general": {
         "bzlTransitiveDigest": "4kUzQyjRjzchWIBZJKkNs8LuO4F+g/DwuwtTa0RTPcU=",

bazel/BUILD

@@ -1,4 +1,6 @@
 exports_files([
+    "python.BUILD.template",
+    "python.bzl",
     "rust.BUILD.template",
     "rust.bzl",
     "utils.bzl",

bazel/python.BUILD.template

@@ -0,0 +1,25 @@
+load("@rules_python//python:py_runtime.bzl", "py_runtime")
+load("@rules_python//python:py_runtime_pair.bzl", "py_runtime_pair")
+
+py_runtime(
+    name = "runtime",
+    implementation_name = "cpython",
+    interpreter_path = "{interpreter_path}",
+    interpreter_version_info = {
+        "major": "{major}",
+        "minor": "{minor}",
+        "micro": "{micro}",
+    },
+    python_version = "PY3",
+)
+
+py_runtime_pair(
+    name = "runtime_pair",
+    py3_runtime = ":runtime",
+)
+
+toolchain(
+    name = "python",
+    toolchain = ":runtime_pair",
+    toolchain_type = "@rules_python//python:toolchain_type",
+)

bazel/python.bzl

@@ -0,0 +1,46 @@
+load("//bazel:utils.bzl", "execute", "require_env")
+
+
+def _python_repository_impl(repository_ctx):
+    root = repository_ctx.path(require_env(repository_ctx, "PYTHON_TOOLCHAIN_ROOT"))
+    python3 = root.get_child("bin/python3")
+
+    version = execute(
+        repository_ctx,
+        [
+            str(python3),
+            "-c",
+            "import sys; print('{}.{}.{}'.format(*sys.version_info[:3]))",
+        ],
+    )
+    parts = version.split(".")
+    if len(parts) != 3:
+        fail("python3 did not report a major.minor.micro version")
+
+    repository_ctx.file("WORKSPACE.bazel", "")
+    repository_ctx.template(
+        "BUILD.bazel",
+        Label("//bazel:python.BUILD.template"),
+        {
+            "{interpreter_path}": str(python3),
+            "{major}": parts[0],
+            "{minor}": parts[1],
+            "{micro}": parts[2],
+        },
+    )
+
+
+python_repository = repository_rule(
+    implementation = _python_repository_impl,
+    local = True,
+    environ = ["PYTHON_TOOLCHAIN_ROOT"],
+)
+
+
+def _python_impl(module_ctx):
+    python_repository(name = "python")
+
+
+python = module_extension(
+    implementation = _python_impl,
+)

configs/nixos/modules/shell/python.nix

@@ -13,6 +13,8 @@ in
   options.modules.shell.python.enable = lib.mkEnableOption "Python tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
+    registry.env-var.${config.networking.hostName}.PYTHON_TOOLCHAIN_ROOT = "${pkgs.python3}";
+
     home-manager.users.${user}.home.packages = [ pkgs.python3 ];
   };
 }

third_party/python/README.md

@@ -1,7 +1,7 @@
 # Vendored Python packages
 
 This package exposes vendored Python dependencies through stable public Bazel targets.
-Bazel runs Python targets with the local `python3` runtime.
+Bazel uses `PYTHON_TOOLCHAIN_ROOT` to find the monorepo `python3` runtime.
 
 Project code depends on aliases in `//third_party/python`.
 It does not depend on versioned package directories directly.
@@ -22,7 +22,7 @@ Use the upstream package name for public aliases.
 For command-line tools, add a `_bin` suffix to the binary target.
 
 Preserve upstream license files and package metadata.
-Keep local Bazel files separate from upstream source when possible.
+Keep repository Bazel files separate from upstream source when possible.
 
 Add optional dependencies as separate public targets.
 Do not bundle optional dependencies into the primary package target.