template: all `text_template` settings before parsing (bugfix "nil-pointer dereference") (#3854)

template/template.go

@@ -265,8 +265,7 @@ func (te Expander) Expand() (result string, resultErr error) {
 		}
 	}()
 
-	tmpl, err := text_template.New(te.name).Funcs(te.funcMap).Parse(te.text)
-	tmpl.Option("missingkey=zero")
+	tmpl, err := text_template.New(te.name).Funcs(te.funcMap).Option("missingkey=zero").Parse(te.text)
 	if err != nil {
 		return "", fmt.Errorf("error parsing template %v: %v", te.name, err)
 	}

template/template_test.go

@@ -31,6 +31,7 @@ type testTemplatesScenario struct {
 	queryResult promql.Vector
 	shouldFail  bool
 	html        bool
+	errorMsg    string
 }
 
 func TestTemplateExpansion(t *testing.T) {
@@ -45,6 +46,12 @@ func TestTemplateExpansion(t *testing.T) {
 			text:   "{{ 1 }}",
 			output: "1",
 		},
+		{
+			// Non-ASCII space (not allowed in text/template, see https://github.com/golang/go/blob/master/src/text/template/parse/lex.go#L98)
+			text:       "{{ }}",
+			shouldFail: true,
+			errorMsg:   "error parsing template test: template: test:1: unexpected unrecognized character in action: U+00A0 in command",
+		},
 		{
 			// HTML escaping.
 			text:   "{{ \"<b>\" }}",
@@ -140,18 +147,21 @@ func TestTemplateExpansion(t *testing.T) {
 			// Unparsable template.
 			text:       "{{",
 			shouldFail: true,
+			errorMsg:   "error parsing template test: template: test:1: unexpected unclosed action in command",
 		},
 		{
 			// Error in function.
 			text:        "{{ query \"missing\" | first }}",
 			queryResult: promql.Vector{},
 			shouldFail:  true,
+			errorMsg:    "error executing template test: template: test:1:21: executing \"test\" at <first>: error calling first: first() called on vector with no elements",
 		},
 		{
 			// Panic.
 			text:        "{{ (query \"missing\").banana }}",
 			queryResult: promql.Vector{},
 			shouldFail:  true,
+			errorMsg:    "error executing template test: template: test:1:10: executing \"test\" at <\"missing\">: can't evaluate field banana in type template.queryResult",
 		},
 		{
 			// Regex replacement.
@@ -262,6 +272,9 @@ func TestTemplateExpansion(t *testing.T) {
 			if err == nil {
 				t.Fatalf("%d. Error not returned from %v", i, s.text)
 			}
+			if err.Error() != s.errorMsg {
+				t.Fatalf("%d. Error message returned is wrong:\n returned: %v\n expected: %v", i, err.Error(), s.errorMsg)
+			}
 			continue
 		}
 		if err != nil {