Introduce NixOS support (download proper kustomize binary)

Author: AleksanderGondek

skylib/kustomize/kustomize.bzl

@@ -13,6 +13,11 @@ load(
     "@io_bazel_rules_docker//skylib:path.bzl",
     _get_runfile_path = "runfile",
 )
+load(
+    "//skylib/kustomize:kustomize_nixos.bzl",
+    "is_running_on_nixos",
+    "copy_kustomize_bin_from_nix_store"
+)
 load("//skylib:push.bzl", "K8sPushInfo")
 load("//skylib:stamp.bzl", "stamp")
 
@@ -37,9 +42,11 @@ sh_binary(
     visibility = ["//visibility:public"],
 )
 """)
-
-    filename, sha256 = _binaries[platform]
-    ctx.download_and_extract(filename, "bin/", sha256 = sha256)
+    if is_running_on_nixos(ctx):
+        copy_kustomize_bin_from_nix_store(ctx, path)
+    else:
+        filename, sha256 = _binaries[platform]
+        ctx.download_and_extract(filename, "bin/", sha256 = sha256)
 
 _download_binary = repository_rule(
     _download_binary_impl,

skylib/kustomize/kustomize_nixos.bzl

@@ -0,0 +1,74 @@
+# Copyright 2020 Adobe. All rights reserved.
+# This file is licensed to you 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 REPRESENTATIONS
+# OF ANY KIND, either express or implied. See the License for the specific language
+# governing permissions and limitations under the License.
+
+def _get_path_to_kustomize_on_nixos(repo_ctx):
+    """Ephemeral download of kustomize binary."""
+    command = "nix-shell --pure -p bash busybox kustomize --run 'which kustomize'"
+    result = repo_ctx.execute([
+        "sh",
+        "-c",
+        command,
+    ],
+    environment = {
+        "NIX_PATH": "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
+    })
+    kustomize_downloaded = result.return_code == 0
+    if not kustomize_downloaded:
+        fail("Failed to run '%s'" % command)
+
+    path_to_kustomize = result.stdout.strip()
+    return path_to_kustomize
+
+def is_running_on_nixos(repo_ctx):
+    """Check if Bazel is executed on NixOS.
+
+    Args:
+        repo_ctx: context of the repository rule,
+            containing helper functions and information about attributes
+
+    Returns:
+        boolean: indicating if Bazel is executed on NixOS.
+    """
+    result = repo_ctx.execute([
+        "sh",
+        "-c",
+        "test -f /etc/os-release && cat /etc/os-release | head -n1",
+    ])
+    os_release_file_read_success = result.return_code == 0
+    if not os_release_file_read_success:
+        return False
+
+    os_release_first_line = result.stdout
+    host_is_nixos = os_release_first_line.strip() == 'NAME=NixOS'
+    return host_is_nixos
+
+def copy_kustomize_bin_from_nix_store(repo_ctx, path):
+    """Copy kustomize binary from nix_store to given path.
+
+    Downloads kustomize binary via nix package manger,
+    then copies the binary to Bazel cache. This operation
+    does not impact host configuration in any way.
+
+    Args:
+        repo_ctx: context of the repository rule,
+            containing helper functions and information about attributes
+        path: path under which the kustomize should be copied
+
+    Returns:
+        None
+    """
+    repo_ctx.file("%s/kustomize" % path)
+    result = repo_ctx.execute([
+        "sh",
+        "-c",
+        "cp -f %s %s/kustomize" % (_get_path_to_kustomize_on_nixos(repo_ctx), path),
+    ])
+    if result.return_code != 0:
+        fail("Failed to copy kustomize bin from nix_store")