[llvm] [libc][bazel] Add a repo with linux kernel UAPI headers (PR #216867)

[Jordan Rupprecht via llvm-commits]

================
@@ -0,0 +1,55 @@
+# This file is licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+"""bzlmod extension for making Linux kernel UAPI headers available in Bazel."""
+
+_SYSTEM_HEADERS_PATH_ENV_VAR = "LINUX_UAPI_INCLUDE_DIR"
+
+_ERROR_MESSAGE_BZL = """\
+def error_message(name, message):
+    fail(message)
+"""
+
+_ERROR_BUILD = """\
+load(":error_message.bzl", "error_message")
+
+error_message(
+    name = "linux_uapi_headers",
+    message = \"""System linux UAPI headers not found.
+
+Pass --repo_env={var_name}=/path/to/your/linux/include to bazel build.
+\"""
+)
+""".format(var_name = _SYSTEM_HEADERS_PATH_ENV_VAR)
+
+_SYSTEM_BUILD = """\
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["include/**"]),
+)
+
+cc_library(
+    name = "linux_uapi_headers",
+    hdrs = [":srcs"],
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)
+"""
+
+def _linux_uapi_setup_impl(repository_ctx):
+    """Sets up a repository of UAPI headers from a system directory"""
+    include_dir = repository_ctx.os.environ.get(_SYSTEM_HEADERS_PATH_ENV_VAR, "")
+    if include_dir:
+        repository_ctx.symlink(include_dir, "include")
+        repository_ctx.file("BUILD.bazel", _SYSTEM_BUILD)
+    else:
+        repository_ctx.file("error_message.bzl", _ERROR_MESSAGE_BZL)
+        repository_ctx.file("BUILD.bazel", _ERROR_BUILD)
+
+linux_uapi_setup = repository_rule(
+    implementation = _linux_uapi_setup_impl,
+    environ = [_SYSTEM_HEADERS_PATH_ENV_VAR],
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rupprecht wrote:

This is deprecated; `repository_ctx.getenv()` is suggested instead: https://bazel.build/rules/lib/globals/bzl#parameters-11

https://github.com/llvm/llvm-project/pull/216867