text/template: fix variadic function call corner case

Executing a template involving variadic functions featuring
a []interface{} slice (such as printf) could result in a
panic in reflect.Value.Call, due to incorrect type checking.

The following expressions failed (with a panic):
{{true|printf}}
{{1|printf}}
{{1.1|printf}}
{{'x'|printf}}
{{1+2i|printf}}

Implemented proper type checks for the fixed parameters of the
variadic functions.

Fixes #10946

Change-Id: Ia75333f651f73b3d2e024cb0c47cc30d90cb6852
Reviewed-on: https://go-review.googlesource.com/10403
Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec.go

@@ -581,7 +581,15 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 	if final.IsValid() {
 		t := typ.In(typ.NumIn() - 1)
 		if typ.IsVariadic() {
-			t = t.Elem()
+			if numIn-1 < numFixed {
+				// The added final argument corresponds to a fixed parameter of the function.
+				// Validate against the type of the actual parameter.
+				t = typ.In(numIn - 1)
+			} else {
+				// The added final argument corresponds to the variadic part.
+				// Validate against the type of the elements of the variadic slice.
+				t = t.Elem()
+			}
 		}
 		argv[i] = s.validateType(final, t)
 	}

src/text/template/exec_test.go

@@ -533,6 +533,18 @@ var execTests = []execTest{
 	{"bug14c", `{{$x := (1.0)}}{{$y := ("hello")}}{{$x.anything}}{{$y.true}}`, "", tVal, false},
 	// Didn't call validateType on function results. Issue 10800.
 	{"bug15", "{{valueString returnInt}}", "", tVal, false},
+	// Variadic function corner cases. Issue 10946.
+	{"bug16a", "{{true|printf}}", "", tVal, false},
+	{"bug16b", "{{1|printf}}", "", tVal, false},
+	{"bug16c", "{{1.1|printf}}", "", tVal, false},
+	{"bug16d", "{{'x'|printf}}", "", tVal, false},
+	{"bug16e", "{{0i|printf}}", "", tVal, false},
+	{"bug16f", "{{true|twoArgs \"xxx\"}}", "", tVal, false},
+	{"bug16g", "{{\"aaa\" |twoArgs \"bbb\"}}", "twoArgs=bbbaaa", tVal, true},
+	{"bug16h", "{{1|oneArg}}", "", tVal, false},
+	{"bug16i", "{{\"aaa\"|oneArg}}", "oneArg=aaa", tVal, true},
+	{"bug16j", "{{1+2i|printf \"%v\"}}", "(1+2i)", tVal, true},
+	{"bug16k", "{{\"aaa\"|printf }}", "aaa", tVal, true},
 }
 
 func zeroArgs() string {
@@ -543,6 +555,10 @@ func oneArg(a string) string {
 	return "oneArg=" + a
 }
 
+func twoArgs(a, b string) string {
+	return "twoArgs=" + a + b
+}
+
 func dddArg(a int, b ...string) string {
 	return fmt.Sprintln(a, b)
 }
@@ -620,6 +636,7 @@ func testExecute(execTests []execTest, template *Template, t *testing.T) {
 		"oneArg":      oneArg,
 		"returnInt":   returnInt,
 		"stringer":    stringer,
+		"twoArgs":     twoArgs,
 		"typeOf":      typeOf,
 		"valueString": valueString,
 		"vfunc":       vfunc,