tpl: avoid panic on too few args to apply

Fixes #1089

tpl/template_funcs.go

@@ -653,6 +653,16 @@ func applyFnToThis(fn, this reflect.Value, args ...interface{}) (reflect.Value,
 		}
 	}
 
+	num := fn.Type().NumIn()
+
+	if fn.Type().IsVariadic() {
+		num--
+	}
+
+	if len(args) < num {
+		return reflect.ValueOf(nil), errors.New("Too few arguments")
+	}
+
 	res := fn.Call(n)
 
 	if len(res) == 1 || res[1].IsNil() {

tpl/template_test.go

@@ -1,3 +1,74 @@
 package tpl
 
-// TODO(bep) test it
+import (
+	"errors"
+	"io/ioutil"
+	"testing"
+)
+
+// Test for bugs discovered by https://github.com/dvyukov/go-fuzz
+func TestTplGoFuzzReports(t *testing.T) {
+	for i, this := range []struct {
+		data      string
+		expectErr int
+	}{{"{{apply .C \"first\" }}", 2}} {
+		templ := New()
+
+		d := &Data{
+			A: 42,
+			B: "foo",
+			C: []int{1, 2, 3},
+			D: map[int]string{1: "foo", 2: "bar"},
+			E: Data1{42, "foo"},
+		}
+
+		err := templ.AddTemplate("fuzz", this.data)
+
+		if err != nil && this.expectErr == 0 {
+			t.Fatalf("Test %d errored: %s", i, err)
+		} else if err == nil && this.expectErr == 1 {
+			t.Fatalf("#1 Test %d should have errored", i)
+		}
+
+		err = templ.ExecuteTemplate(ioutil.Discard, "fuzz", d)
+
+		if err != nil && this.expectErr == 0 {
+			t.Fatalf("Test %d errored: %s", i, err)
+		} else if err == nil && this.expectErr == 2 {
+			t.Fatalf("#2 Test %d should have errored", i)
+		}
+	}
+}
+
+type Data struct {
+	A int
+	B string
+	C []int
+	D map[int]string
+	E Data1
+}
+
+type Data1 struct {
+	A int
+	B string
+}
+
+func (Data1) Q() string {
+	return "foo"
+}
+
+func (Data1) W() (string, error) {
+	return "foo", nil
+}
+
+func (Data1) E() (string, error) {
+	return "foo", errors.New("Data.E error")
+}
+
+func (Data1) R(v int) (string, error) {
+	return "foo", nil
+}
+
+func (Data1) T(s string) (string, error) {
+	return s, nil
+}