util/codegen: add package

This is a package for shared utilities used in doing codegen programs.
The inaugural API is for writing gofmt'd code to a file.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

util/codegen/codegen.go

@@ -0,0 +1,40 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package codegen contains shared utilities for generating code.
+package codegen
+
+import (
+	"fmt"
+	"go/format"
+	"os"
+)
+
+// WriteFormatted writes code to path.
+// It runs gofmt on it before writing;
+// if gofmt fails, it writes code unchanged.
+// Errors can include I/O errors and gofmt errors.
+//
+// The advantage of always writing code to path,
+// even if gofmt fails, is that it makes debugging easier.
+// The code can be long, but you need it in order to debug.
+// It is nicer to work with it in a file than a terminal.
+// It is also easier to interpret gofmt errors
+// with an editor providing file and line numbers.
+func WriteFormatted(code []byte, path string) error {
+	out, fmterr := format.Source(code)
+	if fmterr != nil {
+		out = code
+	}
+	ioerr := os.WriteFile(path, out, 0644)
+	// Prefer I/O errors. They're usually easier to fix,
+	// and until they're fixed you can't do much else.
+	if ioerr != nil {
+		return ioerr
+	}
+	if fmterr != nil {
+		return fmt.Errorf("%s:%v", path, fmterr)
+	}
+	return nil
+}