treewide: move build helper to more fitting places

Change-Id: I3d0cfe9283222d403ae369ec9db09201ad511e15
Reviewed-on: https://review.monogon.dev/c/monogon/+/3327
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/osbase/build/genosrelease/BUILD.bazel b/osbase/build/genosrelease/BUILD.bazel
new file mode 100644
index 0000000..f845cae
--- /dev/null
+++ b/osbase/build/genosrelease/BUILD.bazel
@@ -0,0 +1,18 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+    name = "genosrelease_lib",
+    srcs = ["main.go"],
+    importpath = "source.monogon.dev/osbase/build/genosrelease",
+    visibility = ["//visibility:private"],
+    deps = ["@com_github_joho_godotenv//:godotenv"],
+)
+
+go_binary(
+    name = "genosrelease",
+    embed = [":genosrelease_lib"],
+    visibility = [
+        "//metropolis/installer:__subpackages__",
+        "//metropolis/node:__subpackages__",
+    ],
+)
diff --git a/osbase/build/genosrelease/defs.bzl b/osbase/build/genosrelease/defs.bzl
new file mode 100644
index 0000000..6fed483
--- /dev/null
+++ b/osbase/build/genosrelease/defs.bzl
@@ -0,0 +1,54 @@
+#  Copyright 2020 The Monogon Project Authors.
+#
+#  SPDX-License-Identifier: Apache-2.0
+#
+#  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.
+
+def _os_release_impl(ctx):
+    ctx.actions.run(
+        mnemonic = "GenOSRelease",
+        progress_message = "Generating os-release",
+        inputs = [ctx.info_file],
+        outputs = [ctx.outputs.out],
+        executable = ctx.executable._genosrelease,
+        arguments = [
+            "-status_file",
+            ctx.info_file.path,
+            "-out_file",
+            ctx.outputs.out.path,
+            "-stamp_var",
+            ctx.attr.stamp_var,
+            "-name",
+            ctx.attr.os_name,
+            "-id",
+            ctx.attr.os_id,
+        ],
+    )
+
+os_release = rule(
+    implementation = _os_release_impl,
+    attrs = {
+        "os_name": attr.string(mandatory = True),
+        "os_id": attr.string(mandatory = True),
+        "stamp_var": attr.string(mandatory = True),
+        "_genosrelease": attr.label(
+            default = Label("//osbase/build/genosrelease"),
+            cfg = "host",
+            executable = True,
+            allow_files = True,
+        ),
+    },
+    outputs = {
+        "out": "os-release",
+    },
+)
diff --git a/osbase/build/genosrelease/main.go b/osbase/build/genosrelease/main.go
new file mode 100644
index 0000000..adb8202
--- /dev/null
+++ b/osbase/build/genosrelease/main.go
@@ -0,0 +1,79 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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.
+
+// genosrelease provides rudimentary support to generate os-release files
+// following the freedesktop spec from arguments and stamping
+//
+// https://www.freedesktop.org/software/systemd/man/os-release.html
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/joho/godotenv"
+)
+
+var (
+	flagStatusFile = flag.String("status_file", "", "path to bazel workspace status file")
+	flagOutFile    = flag.String("out_file", "os-release", "path to os-release output file")
+	flagStampVar   = flag.String("stamp_var", "", "variable to use as version from the workspace status file")
+	flagName       = flag.String("name", "", "name parameter (see freedesktop spec)")
+	flagID         = flag.String("id", "", "id parameter (see freedesktop spec)")
+)
+
+func main() {
+	flag.Parse()
+	statusFileContent, err := os.ReadFile(*flagStatusFile)
+	if err != nil {
+		fmt.Printf("Failed to open bazel workspace status file: %v\n", err)
+		os.Exit(1)
+	}
+	statusVars := make(map[string]string)
+	for _, line := range strings.Split(string(statusFileContent), "\n") {
+		line = strings.TrimSpace(line)
+		parts := strings.Fields(line)
+		if len(parts) != 2 {
+			continue
+		}
+		statusVars[parts[0]] = parts[1]
+	}
+
+	version, ok := statusVars[*flagStampVar]
+	if !ok {
+		fmt.Printf("%v key not set in bazel workspace status file\n", *flagStampVar)
+		os.Exit(1)
+	}
+	// As specified by https://www.freedesktop.org/software/systemd/man/os-release.html
+	osReleaseVars := map[string]string{
+		"NAME":        *flagName,
+		"ID":          *flagID,
+		"VERSION":     version,
+		"VERSION_ID":  version,
+		"PRETTY_NAME": *flagName + " " + version,
+	}
+	osReleaseContent, err := godotenv.Marshal(osReleaseVars)
+	if err != nil {
+		fmt.Printf("Failed to encode os-release file: %v\n", err)
+		os.Exit(1)
+	}
+	if err := os.WriteFile(*flagOutFile, []byte(osReleaseContent), 0644); err != nil {
+		fmt.Printf("Failed to write os-release file: %v\n", err)
+		os.Exit(1)
+	}
+}