Fix corruption in templates that use title function (prometheus#3278)

This commit fixes data corruption in templates that use the title
function as it used a shared cases.Title when casers should not
be shared between goroutines. When templates that used the title
function were executed at the same time, data corruption would occur
in the text that was being cased. This is fixed using a separate
caser for each function call.

Add tests to assertDefaultFuncs are thread-safe

Signed-off-by: George Robinson <[email protected]>

template/template.go

@@ -168,11 +168,14 @@ func (t *Template) ExecuteHTMLString(html string, data interface{}) (string, err
 type FuncMap map[string]interface{}
 
 var DefaultFuncs = FuncMap{
-	"toUpper":   strings.ToUpper,
-	"toLower":   strings.ToLower,
+	"toUpper": strings.ToUpper,
+	"toLower": strings.ToLower,
+	"title": func(text string) string {
+		// casers should not be shared between goroutines, instead
+		// create a new caser each time this function is called
+		return cases.Title(language.AmericanEnglish).String(text)
+	},
 	"trimSpace": strings.TrimSpace,
-
-	"title": cases.Title(language.AmericanEnglish).String,
 	// join is equal to strings.Join but inverts the argument order
 	// for easier pipelining in templates.
 	"join": func(sep string, s []string) string {

template/template_test.go

@@ -16,6 +16,7 @@ package template
 import (
 	tmplhtml "html/template"
 	"net/url"
+	"sync"
 	"testing"
 	tmpltext "text/template"
 	"time"
@@ -465,3 +466,60 @@ func TestTemplateExpansionWithOptions(t *testing.T) {
 		})
 	}
 }
+
+// This test asserts that template functions are thread-safe.
+func TestTemplateFuncs(t *testing.T) {
+	tmpl, err := FromGlobs([]string{})
+	require.NoError(t, err)
+
+	for _, tc := range []struct {
+		title string
+		in    string
+		data  interface{}
+		exp   string
+	}{{
+		title: "Template using toUpper",
+		in:    `{{ "abc" | toUpper }}`,
+		exp:   "ABC",
+	}, {
+		title: "Template using toLower",
+		in:    `{{ "ABC" | toLower }}`,
+		exp:   "abc",
+	}, {
+		title: "Template using title",
+		in:    `{{ "abc" | title }}`,
+		exp:   "Abc",
+	}, {
+		title: "Template using trimSpace",
+		in:    `{{ " abc " | trimSpace }}`,
+		exp:   "abc",
+	}, {
+		title: "Template using join",
+		in:    `{{ . | join "," }}`,
+		data:  []string{"abc", "def"},
+		exp:   "abc,def",
+	}, {
+		title: "Template using match",
+		in:    `{{ match "[a-z]+" "abc" }}`,
+		exp:   "true",
+	}, {
+		title: "Template using reReplaceAll",
+		in:    `{{ reReplaceAll "ab" "AB" "abc" }}`,
+		exp:   "ABc",
+	}} {
+		tc := tc
+		t.Run(tc.title, func(t *testing.T) {
+			wg := sync.WaitGroup{}
+			for i := 0; i < 10; i++ {
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					got, err := tmpl.ExecuteTextString(tc.in, tc.data)
+					require.NoError(t, err)
+					require.Equal(t, tc.exp, got)
+				}()
+			}
+			wg.Wait()
+		})
+	}
+}