util/codegen: permit running in directories without copyright headers

It broke in our corp repo that lacks copyright headers.

Change-Id: Iafc433e6b6affe83b45477899455527658dc4f12
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/codegen/codegen.go

@@ -58,11 +58,13 @@ func HasNoClone(structTag string) bool {
 	return false
 }
 
-const header = `// Copyright (c) %d Tailscale Inc & AUTHORS All rights reserved.
+const copyrightHeader = `// Copyright (c) %d 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.
 
-// Code generated by %v; DO NOT EDIT.
+`
+
+const genAndPackageHeader = `// Code generated by %v; DO NOT EDIT.
 
 package %s
 `
@@ -109,7 +111,10 @@ func (it *ImportTracker) Write(w io.Writer) {
 }
 
 func writeHeader(w io.Writer, tool, pkg string, copyrightYear int) {
-	fmt.Fprintf(w, header, copyrightYear, tool, pkg)
+	if copyrightYear != 0 {
+		fmt.Fprintf(w, copyrightHeader, copyrightYear)
+	}
+	fmt.Fprintf(w, genAndPackageHeader, tool, pkg)
 }
 
 // WritePackageFile adds a file with the provided imports and contents to package.
@@ -287,7 +292,8 @@ func CopyrightYear(dir string) (year int) {
 Files:
 	for _, f := range files {
 		name := f.Name()
-		if f.IsDir() ||
+		if !f.Type().IsRegular() ||
+			strings.HasPrefix(name, ".") || // includes emacs noise
 			!strings.HasSuffix(name, ".go") ||
 			strings.HasSuffix(name, "_clone.go") ||
 			strings.HasSuffix(name, "_view.go") ||
@@ -312,8 +318,5 @@ Files:
 			}
 		}
 	}
-	if year == 0 {
-		panic("no Copyright line found in any *.go file in " + dir)
-	}
 	return year
 }