refactor template parsing logic

Move the logic for selecting a license template based on user input into
a standalone func (fetchTemplate), and add test cases for all code
paths.

Delay parsing predefined license templates. This allows the new
fetchTemplate method to modify these templates before returning in the
future (to add SPDX license information).  Add tests to ensure that
these templates must always parse properly.

Rename copyrightData type to licenseData, since we will soon begin to
add more than just copyright data here (SPDX ID).

Rename prefix func to executeTemplate, since this better describes what
the function is doing.

These are all refactoring and cleanup changes; no behavioral changes.

main.go

@@ -58,6 +58,14 @@ var (
 	checkonly = flag.Bool("check", false, "check only mode: verify presence of license headers and exit with non-zero code if missing")
 )
 
+func init() {
+	flag.Usage = func() {
+		fmt.Fprintln(os.Stderr, helpText)
+		flag.PrintDefaults()
+	}
+	flag.Var(&skipExtensionFlags, "skip", "To skip files to check/add the header file, for example: -skip rb -skip go")
+}
+
 type skipExtensionFlag []string
 
 func (i *skipExtensionFlag) String() string {
@@ -70,40 +78,24 @@ func (i *skipExtensionFlag) Set(value string) error {
 }
 
 func main() {
-	flag.Usage = func() {
-		fmt.Fprintln(os.Stderr, helpText)
-		flag.PrintDefaults()
-	}
-	flag.Var(&skipExtensionFlags, "skip", "To skip files to check/add the header file, for example: -skip rb -skip go")
 	flag.Parse()
 	if flag.NArg() == 0 {
 		flag.Usage()
 		os.Exit(1)
 	}
 
-	data := &copyrightData{
+	data := licenseData{
 		Year:   *year,
 		Holder: *holder,
 	}
 
-	var t *template.Template
-	if *licensef != "" {
-		d, err := ioutil.ReadFile(*licensef)
-		if err != nil {
-			log.Printf("license file: %v", err)
-			os.Exit(1)
-		}
-		t, err = template.New("").Parse(string(d))
-		if err != nil {
-			log.Printf("license file: %v", err)
-			os.Exit(1)
-		}
-	} else {
-		t = licenseTemplate[*license]
-		if t == nil {
-			log.Printf("unknown license: %s", *license)
-			os.Exit(1)
-		}
+	tpl, err := fetchTemplate(*license, *licensef)
+	if err != nil {
+		log.Fatal(err)
+	}
+	t, err := template.New("").Parse(tpl)
+	if err != nil {
+		log.Fatal(err)
 	}
 
 	// process at most 1000 files in parallel
@@ -189,7 +181,7 @@ func walk(ch chan<- *file, start string) {
 // addLicense add a license to the file if missing.
 //
 // It returns true if the file was updated.
-func addLicense(path string, fmode os.FileMode, tmpl *template.Template, data *copyrightData) (bool, error) {
+func addLicense(path string, fmode os.FileMode, tmpl *template.Template, data licenseData) (bool, error) {
 	var lic []byte
 	var err error
 	lic, err = licenseHeader(path, tmpl, data)
@@ -227,32 +219,32 @@ func fileHasLicense(path string) (bool, error) {
 	return hasLicense(b) || isGenerated(b), nil
 }
 
-func licenseHeader(path string, tmpl *template.Template, data *copyrightData) ([]byte, error) {
+func licenseHeader(path string, tmpl *template.Template, data licenseData) ([]byte, error) {
 	var lic []byte
 	var err error
 	switch fileExtension(path) {
 	default:
 		return nil, nil
 	case ".c", ".h", ".gv":
-		lic, err = prefix(tmpl, data, "/*", " * ", " */")
+		lic, err = executeTemplate(tmpl, data, "/*", " * ", " */")
 	case ".js", ".mjs", ".cjs", ".jsx", ".tsx", ".css", ".scss", ".sass", ".tf", ".ts":
-		lic, err = prefix(tmpl, data, "/**", " * ", " */")
+		lic, err = executeTemplate(tmpl, data, "/**", " * ", " */")
 	case ".cc", ".cpp", ".cs", ".go", ".hh", ".hpp", ".java", ".m", ".mm", ".proto", ".rs", ".scala", ".swift", ".dart", ".groovy", ".kt", ".kts", ".v", ".sv":
-		lic, err = prefix(tmpl, data, "", "// ", "")
+		lic, err = executeTemplate(tmpl, data, "", "// ", "")
 	case ".py", ".sh", ".yaml", ".yml", ".dockerfile", "dockerfile", ".rb", "gemfile", ".tcl", ".bzl":
-		lic, err = prefix(tmpl, data, "", "# ", "")
+		lic, err = executeTemplate(tmpl, data, "", "# ", "")
 	case ".el", ".lisp":
-		lic, err = prefix(tmpl, data, "", ";; ", "")
+		lic, err = executeTemplate(tmpl, data, "", ";; ", "")
 	case ".erl":
-		lic, err = prefix(tmpl, data, "", "% ", "")
+		lic, err = executeTemplate(tmpl, data, "", "% ", "")
 	case ".hs", ".sql", ".sdl":
-		lic, err = prefix(tmpl, data, "", "-- ", "")
+		lic, err = executeTemplate(tmpl, data, "", "-- ", "")
 	case ".html", ".xml", ".vue":
-		lic, err = prefix(tmpl, data, "<!--", " ", "-->")
+		lic, err = executeTemplate(tmpl, data, "<!--", " ", "-->")
 	case ".php":
-		lic, err = prefix(tmpl, data, "", "// ", "")
+		lic, err = executeTemplate(tmpl, data, "", "// ", "")
 	case ".ml", ".mli", ".mll", ".mly":
-		lic, err = prefix(tmpl, data, "(**", "   ", "*)")
+		lic, err = executeTemplate(tmpl, data, "(**", "   ", "*)")
 	}
 	return lic, err
 }

testdata/custom.tpl

@@ -0,0 +1,3 @@
+Copyright {{.Year}} {{.Holder}}
+
+Custom License Template

tmpl.go

@@ -19,27 +19,50 @@ import (
 	"bytes"
 	"fmt"
 	"html/template"
+	"io/ioutil"
 	"strings"
 	"unicode"
 )
 
-var licenseTemplate = make(map[string]*template.Template)
-
-func init() {
-	licenseTemplate["apache"] = template.Must(template.New("").Parse(tmplApache))
-	licenseTemplate["mit"] = template.Must(template.New("").Parse(tmplMIT))
-	licenseTemplate["bsd"] = template.Must(template.New("").Parse(tmplBSD))
-	licenseTemplate["mpl"] = template.Must(template.New("").Parse(tmplMPL))
+var licenseTemplate = map[string]string{
+	"apache": tmplApache,
+	"mit":    tmplMIT,
+	"bsd":    tmplBSD,
+	"mpl":    tmplMPL,
 }
 
-type copyrightData struct {
-	Year   string
-	Holder string
+// licenseData specifies the data used to fill out a license template.
+type licenseData struct {
+	Year   string // Copyright year(s).
+	Holder string // Name of the copyright holder.
 }
 
-// prefix will execute a license template t with data d
+// fetchTemplate returns the license template for the specified license and
+// optional templateFile. If templateFile is provided, the license is read
+// from the specified file. Otherwise, a template is loaded for the specified
+// license, if recognized.
+func fetchTemplate(license string, templateFile string) (string, error) {
+	var t string
+	if templateFile != "" {
+		d, err := ioutil.ReadFile(templateFile)
+		if err != nil {
+			return "", fmt.Errorf("license file: %w", err)
+		}
+
+		t = string(d)
+	} else {
+		t = licenseTemplate[license]
+		if t == "" {
+			return "", fmt.Errorf("unknown license: %q", license)
+		}
+	}
+
+	return t, nil
+}
+
+// executeTemplate will execute a license template t with data d
 // and prefix the result with top, middle and bottom.
-func prefix(t *template.Template, d *copyrightData, top, mid, bot string) ([]byte, error) {
+func executeTemplate(t *template.Template, d licenseData, top, mid, bot string) ([]byte, error) {
 	var buf bytes.Buffer
 	if err := t.Execute(&buf, d); err != nil {
 		return nil, err

tmpl_test.go

@@ -0,0 +1,102 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+package main
+
+import (
+	"errors"
+	"html/template"
+	"os"
+	"testing"
+)
+
+func init() {
+	// ensure that pre-defined templates must parse
+	template.Must(template.New("").Parse(tmplApache))
+	template.Must(template.New("").Parse(tmplMIT))
+	template.Must(template.New("").Parse(tmplBSD))
+	template.Must(template.New("").Parse(tmplMPL))
+}
+
+func TestFetchTemplate(t *testing.T) {
+	tests := []struct {
+		description  string // test case description
+		license      string // license passed to fetchTemplate
+		templateFile string // templatefile passed to fetchTemplate
+		wantTemplate string // expected returned template
+		wantErr      error  // expected returned error
+	}{
+		{
+			"non-existant template file",
+			"",
+			"/does/not/exist",
+			"",
+			os.ErrNotExist,
+		},
+		{
+			"custom template file",
+			"",
+			"testdata/custom.tpl",
+			"Copyright {{.Year}} {{.Holder}}\n\nCustom License Template\n",
+			nil,
+		},
+		{
+			"unknown license",
+			"unknown",
+			"",
+			"",
+			errors.New(`unknown license: "unknown"`),
+		},
+		{
+			"apache license template",
+			"apache",
+			"",
+			tmplApache,
+			nil,
+		},
+		{
+			"mit license template",
+			"mit",
+			"",
+			tmplMIT,
+			nil,
+		},
+		{
+			"bsd license template",
+			"bsd",
+			"",
+			tmplBSD,
+			nil,
+		},
+		{
+			"mpl license template",
+			"mpl",
+			"",
+			tmplMPL,
+			nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			tpl, err := fetchTemplate(tt.license, tt.templateFile)
+			if tt.wantErr != nil && (err == nil || (!errors.Is(err, tt.wantErr) && err.Error() != tt.wantErr.Error())) {
+				t.Fatalf("fetchTemplate(%q, %q) returned error: %#v, want %#v", tt.license, tt.templateFile, err, tt.wantErr)
+			}
+			if tpl != tt.wantTemplate {
+				t.Errorf("fetchTemplate(%q, %q) returned template: %q, want %q", tt.license, tt.templateFile, tpl, tt.wantTemplate)
+			}
+		})
+	}
+}