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>

cmd/cloner/cloner.go

@@ -18,15 +18,14 @@
 	"flag"
 	"fmt"
 	"go/ast"
-	"go/format"
 	"go/token"
 	"go/types"
-	"io/ioutil"
 	"log"
 	"os"
 	"strings"
 
 	"golang.org/x/tools/go/packages"
+	"tailscale.com/util/codegen"
 )
 
 var (
@@ -130,17 +129,12 @@ func main() {
 	fmt.Fprintf(contents, ")\n\n")
 	contents.Write(buf.Bytes())
 
-	out, err := format.Source(contents.Bytes())
-	if err != nil {
-		log.Fatalf("%s, in source:\n%s", err, contents.Bytes())
-	}
-
 	output := *flagOutput
 	if output == "" {
 		flag.Usage()
 		os.Exit(2)
 	}
-	if err := ioutil.WriteFile(output, out, 0644); err != nil {
+	if err := codegen.WriteFormatted(contents.Bytes(), output); err != nil {
 		log.Fatal(err)
 	}
 }

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
+}