feat: add template.FromGlobsWithAdditionalFuncs (#3174)

* refactor: add Options to the template.FromGlob function to allow customizing the Template

Signed-off-by: Martin Chodur <m.chodur@seznam.cz>

cli/check_config.go

@@ -84,7 +84,7 @@ func CheckConfig(args []string) error {
 			fmt.Printf(" - %d receivers\n", len(cfg.Receivers))
 			fmt.Printf(" - %d templates\n", len(cfg.Templates))
 			if len(cfg.Templates) > 0 {
-				_, err = template.FromGlobs(cfg.Templates...)
+				_, err = template.FromGlobs(cfg.Templates)
 				if err != nil {
 					fmt.Printf("  FAILED: %s\n", err)
 					failed++

cli/template_render.go

@@ -107,7 +107,7 @@ func configureTemplateRenderCmd(cc *kingpin.CmdClause) {
 }
 
 func (c *templateRenderCmd) render(ctx context.Context, _ *kingpin.ParseContext) error {
-	tmpl, err := template.FromGlobs(c.templateFilesGlobs...)
+	tmpl, err := template.FromGlobs(c.templateFilesGlobs)
 	if err != nil {
 		return err
 	}

cmd/alertmanager/main.go

@@ -419,7 +419,7 @@ func run() int {
 		configLogger,
 	)
 	configCoordinator.Subscribe(func(conf *config.Config) error {
-		tmpl, err = template.FromGlobs(conf.Templates...)
+		tmpl, err = template.FromGlobs(conf.Templates)
 		if err != nil {
 			return errors.Wrap(err, "failed to parse templates")
 		}

notify/email/email_test.go

@@ -183,7 +183,7 @@ func notifyEmailWithContext(ctx context.Context, cfg *config.EmailConfig, server
 		return nil, false, err
 	}
 
-	tmpl, err := template.FromGlobs()
+	tmpl, err := template.FromGlobs([]string{})
 	if err != nil {
 		return nil, false, err
 	}

notify/test/test.go

@@ -128,7 +128,7 @@ func DefaultRetryCodes() []int {
 
 // CreateTmpl returns a ready-to-use template.
 func CreateTmpl(t *testing.T) *template.Template {
-	tmpl, err := template.FromGlobs()
+	tmpl, err := template.FromGlobs([]string{})
 	require.NoError(t, err)
 	tmpl.ExternalURL, _ = url.Parse("http://am")
 	return tmpl

template/template.go

@@ -42,16 +42,24 @@ type Template struct {
 	ExternalURL *url.URL
 }
 
+// Option is generic modifier of the text and html templates used by a Template.
+type Option func(text *tmpltext.Template, html *tmplhtml.Template)
+
 // FromGlobs calls ParseGlob on all path globs provided and returns the
-// resulting Template.
+// resulting Template. Options allows customization of the text and html templates in given order.
-func FromGlobs(paths ...string) (*Template, error) {
+// The DefaultFuncs have precedence over any added custom functions.
+func FromGlobs(paths []string, options ...Option) (*Template, error) {
 	t := &Template{
 		text: tmpltext.New("").Option("missingkey=zero"),
 		html: tmplhtml.New("").Option("missingkey=zero"),
 	}
 
-	t.text = t.text.Funcs(tmpltext.FuncMap(DefaultFuncs))
+	for _, o := range options {
-	t.html = t.html.Funcs(tmplhtml.FuncMap(DefaultFuncs))
+		o(t.text, t.html)
+	}
+
+	t.text.Funcs(tmpltext.FuncMap(DefaultFuncs))
+	t.html.Funcs(tmplhtml.FuncMap(DefaultFuncs))
 
 	defaultTemplates := []string{"default.tmpl", "email.tmpl"}
 

template/template_test.go

@@ -14,8 +14,10 @@
 package template
 
 import (
+	tmplhtml "html/template"
 	"net/url"
 	"testing"
+	tmpltext "text/template"
 	"time"
 
 	"github.com/prometheus/common/model"
@@ -277,7 +279,7 @@ func TestData(t *testing.T) {
 }
 
 func TestTemplateExpansion(t *testing.T) {
-	tmpl, err := FromGlobs()
+	tmpl, err := FromGlobs([]string{})
 	require.NoError(t, err)
 
 	for _, tc := range []struct {
@@ -387,3 +389,67 @@ func TestTemplateExpansion(t *testing.T) {
 		})
 	}
 }
+
+func TestTemplateExpansionWithOptions(t *testing.T) {
+	testOptionWithAdditionalFuncs := func(funcs FuncMap) Option {
+		return func(text *tmpltext.Template, html *tmplhtml.Template) {
+			text.Funcs(tmpltext.FuncMap(funcs))
+			html.Funcs(tmplhtml.FuncMap(funcs))
+		}
+	}
+	for _, tc := range []struct {
+		options []Option
+		title   string
+		in      string
+		data    interface{}
+		html    bool
+
+		exp  string
+		fail bool
+	}{
+		{
+			title:   "Test custom function",
+			options: []Option{testOptionWithAdditionalFuncs(FuncMap{"printFoo": func() string { return "foo" }})},
+			in:      `{{ printFoo }}`,
+			exp:     "foo",
+		},
+		{
+			title:   "Test Default function with additional function added",
+			options: []Option{testOptionWithAdditionalFuncs(FuncMap{"printFoo": func() string { return "foo" }})},
+			in:      `{{ toUpper "test" }}`,
+			exp:     "TEST",
+		},
+		{
+			title:   "Test custom function is overridden by the DefaultFuncs",
+			options: []Option{testOptionWithAdditionalFuncs(FuncMap{"toUpper": func(s string) string { return "foo" }})},
+			in:      `{{ toUpper "test" }}`,
+			exp:     "TEST",
+		},
+		{
+			title: "Test later Option overrides the previous",
+			options: []Option{
+				testOptionWithAdditionalFuncs(FuncMap{"printFoo": func() string { return "foo" }}),
+				testOptionWithAdditionalFuncs(FuncMap{"printFoo": func() string { return "bar" }}),
+			},
+			in:  `{{ printFoo }}`,
+			exp: "bar",
+		},
+	} {
+		tc := tc
+		t.Run(tc.title, func(t *testing.T) {
+			tmpl, err := FromGlobs([]string{}, tc.options...)
+			require.NoError(t, err)
+			f := tmpl.ExecuteTextString
+			if tc.html {
+				f = tmpl.ExecuteHTMLString
+			}
+			got, err := f(tc.in, tc.data)
+			if tc.fail {
+				require.NotNil(t, err)
+				return
+			}
+			require.NoError(t, err)
+			require.Equal(t, tc.exp, got)
+		})
+	}
+}