text/template: make reflect.Value indirections more robust

Always shadow or modify the original parameter name. With code like:

	func index(item reflect.Value, ... {
		v := indirectInterface(item)

It was possible to incorrectly use 'item' and 'v' later in the function,
which could result in subtle bugs. This is precisely the kind of mistake
that led to #36199.

Instead, don't keep both the old and new reflect.Value variables in
scope. Always shadow or modify the original variable.

While at it, simplify the signature of 'length', to receive a
reflect.Value directly and save a few redundant lines.

Change-Id: I01416636a9d49f81246d28b91aca6413b1ba1aa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/212117
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Roberto Clapis <robclap8@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>

Author: mvdan

src/text/template/funcs.go

@@ -185,41 +185,41 @@ func indexArg(index reflect.Value, cap int) (int, error) {
 // arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
 // indexed item must be a map, slice, or array.
 func index(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
-	v := indirectInterface(item)
-	if !v.IsValid() {
+	item = indirectInterface(item)
+	if !item.IsValid() {
 		return reflect.Value{}, fmt.Errorf("index of untyped nil")
 	}
-	for _, i := range indexes {
-		index := indirectInterface(i)
+	for _, index := range indexes {
+		index = indirectInterface(index)
 		var isNil bool
-		if v, isNil = indirect(v); isNil {
+		if item, isNil = indirect(item); isNil {
 			return reflect.Value{}, fmt.Errorf("index of nil pointer")
 		}
-		switch v.Kind() {
+		switch item.Kind() {
 		case reflect.Array, reflect.Slice, reflect.String:
-			x, err := indexArg(index, v.Len())
+			x, err := indexArg(index, item.Len())
 			if err != nil {
 				return reflect.Value{}, err
 			}
-			v = v.Index(x)
+			item = item.Index(x)
 		case reflect.Map:
-			index, err := prepareArg(index, v.Type().Key())
+			index, err := prepareArg(index, item.Type().Key())
 			if err != nil {
 				return reflect.Value{}, err
 			}
-			if x := v.MapIndex(index); x.IsValid() {
-				v = x
+			if x := item.MapIndex(index); x.IsValid() {
+				item = x
 			} else {
-				v = reflect.Zero(v.Type().Elem())
+				item = reflect.Zero(item.Type().Elem())
 			}
 		case reflect.Invalid:
-			// the loop holds invariant: v.IsValid()
+			// the loop holds invariant: item.IsValid()
 			panic("unreachable")
 		default:
-			return reflect.Value{}, fmt.Errorf("can't index item of type %s", v.Type())
+			return reflect.Value{}, fmt.Errorf("can't index item of type %s", item.Type())
 		}
 	}
-	return v, nil
+	return item, nil
 }
 
 // Slicing.
@@ -229,29 +229,27 @@ func index(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error)
 // is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" is x[1:2:3]. The first
 // argument must be a string, slice, or array.
 func slice(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
-	var (
-		cap int
-		v   = indirectInterface(item)
-	)
-	if !v.IsValid() {
+	item = indirectInterface(item)
+	if !item.IsValid() {
 		return reflect.Value{}, fmt.Errorf("slice of untyped nil")
 	}
 	if len(indexes) > 3 {
 		return reflect.Value{}, fmt.Errorf("too many slice indexes: %d", len(indexes))
 	}
-	switch v.Kind() {
+	var cap int
+	switch item.Kind() {
 	case reflect.String:
 		if len(indexes) == 3 {
 			return reflect.Value{}, fmt.Errorf("cannot 3-index slice a string")
 		}
-		cap = v.Len()
+		cap = item.Len()
 	case reflect.Array, reflect.Slice:
-		cap = v.Cap()
+		cap = item.Cap()
 	default:
-		return reflect.Value{}, fmt.Errorf("can't slice item of type %s", v.Type())
+		return reflect.Value{}, fmt.Errorf("can't slice item of type %s", item.Type())
 	}
 	// set default values for cases item[:], item[i:].
-	idx := [3]int{0, v.Len()}
+	idx := [3]int{0, item.Len()}
 	for i, index := range indexes {
 		x, err := indexArg(index, cap)
 		if err != nil {
@@ -264,44 +262,40 @@ func slice(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error)
 		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[0], idx[1])
 	}
 	if len(indexes) < 3 {
-		return v.Slice(idx[0], idx[1]), nil
+		return item.Slice(idx[0], idx[1]), nil
 	}
 	// given item[i:j:k], make sure i <= j <= k.
 	if idx[1] > idx[2] {
 		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[1], idx[2])
 	}
-	return v.Slice3(idx[0], idx[1], idx[2]), nil
+	return item.Slice3(idx[0], idx[1], idx[2]), nil
 }
 
 // Length
 
 // length returns the length of the item, with an error if it has no defined length.
-func length(item interface{}) (int, error) {
-	v := reflect.ValueOf(item)
-	if !v.IsValid() {
-		return 0, fmt.Errorf("len of untyped nil")
-	}
-	v, isNil := indirect(v)
+func length(item reflect.Value) (int, error) {
+	item, isNil := indirect(item)
 	if isNil {
 		return 0, fmt.Errorf("len of nil pointer")
 	}
-	switch v.Kind() {
+	switch item.Kind() {
 	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
-		return v.Len(), nil
+		return item.Len(), nil
 	}
-	return 0, fmt.Errorf("len of type %s", v.Type())
+	return 0, fmt.Errorf("len of type %s", item.Type())
 }
 
 // Function invocation
 
 // call returns the result of evaluating the first argument as a function.
 // The function must return 1 result, or 2 results, the second of which is an error.
 func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
-	v := indirectInterface(fn)
-	if !v.IsValid() {
+	fn = indirectInterface(fn)
+	if !fn.IsValid() {
 		return reflect.Value{}, fmt.Errorf("call of nil")
 	}
-	typ := v.Type()
+	typ := fn.Type()
 	if typ.Kind() != reflect.Func {
 		return reflect.Value{}, fmt.Errorf("non-function of type %s", typ)
 	}
@@ -322,19 +316,19 @@ func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
 	}
 	argv := make([]reflect.Value, len(args))
 	for i, arg := range args {
-		value := indirectInterface(arg)
+		arg = indirectInterface(arg)
 		// Compute the expected type. Clumsy because of variadics.
 		argType := dddType
 		if !typ.IsVariadic() || i < numIn-1 {
 			argType = typ.In(i)
 		}
 
 		var err error
-		if argv[i], err = prepareArg(value, argType); err != nil {
+		if argv[i], err = prepareArg(arg, argType); err != nil {
 			return reflect.Value{}, fmt.Errorf("arg %d: %s", i, err)
 		}
 	}
-	return safeCall(v, argv)
+	return safeCall(fn, argv)
 }
 
 // safeCall runs fun.Call(args), and returns the resulting value and error, if
@@ -440,52 +434,52 @@ func basicKind(v reflect.Value) (kind, error) {
 
 // eq evaluates the comparison a == b || a == c || ...
 func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
-	v1 := indirectInterface(arg1)
-	if v1 != zero {
-		if t1 := v1.Type(); !t1.Comparable() {
-			return false, fmt.Errorf("uncomparable type %s: %v", t1, v1)
+	arg1 = indirectInterface(arg1)
+	if arg1 != zero {
+		if t1 := arg1.Type(); !t1.Comparable() {
+			return false, fmt.Errorf("uncomparable type %s: %v", t1, arg1)
 		}
 	}
 	if len(arg2) == 0 {
 		return false, errNoComparison
 	}
-	k1, _ := basicKind(v1)
+	k1, _ := basicKind(arg1)
 	for _, arg := range arg2 {
-		v2 := indirectInterface(arg)
-		k2, _ := basicKind(v2)
+		arg = indirectInterface(arg)
+		k2, _ := basicKind(arg)
 		truth := false
 		if k1 != k2 {
 			// Special case: Can compare integer values regardless of type's sign.
 			switch {
 			case k1 == intKind && k2 == uintKind:
-				truth = v1.Int() >= 0 && uint64(v1.Int()) == v2.Uint()
+				truth = arg1.Int() >= 0 && uint64(arg1.Int()) == arg.Uint()
 			case k1 == uintKind && k2 == intKind:
-				truth = v2.Int() >= 0 && v1.Uint() == uint64(v2.Int())
+				truth = arg.Int() >= 0 && arg1.Uint() == uint64(arg.Int())
 			default:
 				return false, errBadComparison
 			}
 		} else {
 			switch k1 {
 			case boolKind:
-				truth = v1.Bool() == v2.Bool()
+				truth = arg1.Bool() == arg.Bool()
 			case complexKind:
-				truth = v1.Complex() == v2.Complex()
+				truth = arg1.Complex() == arg.Complex()
 			case floatKind:
-				truth = v1.Float() == v2.Float()
+				truth = arg1.Float() == arg.Float()
 			case intKind:
-				truth = v1.Int() == v2.Int()
+				truth = arg1.Int() == arg.Int()
 			case stringKind:
-				truth = v1.String() == v2.String()
+				truth = arg1.String() == arg.String()
 			case uintKind:
-				truth = v1.Uint() == v2.Uint()
+				truth = arg1.Uint() == arg.Uint()
 			default:
-				if v2 == zero {
-					truth = v1 == v2
+				if arg == zero {
+					truth = arg1 == arg
 				} else {
-					if t2 := v2.Type(); !t2.Comparable() {
-						return false, fmt.Errorf("uncomparable type %s: %v", t2, v2)
+					if t2 := arg.Type(); !t2.Comparable() {
+						return false, fmt.Errorf("uncomparable type %s: %v", t2, arg)
 					}
-					truth = v1.Interface() == v2.Interface()
+					truth = arg1.Interface() == arg.Interface()
 				}
 			}
 		}
@@ -505,13 +499,13 @@ func ne(arg1, arg2 reflect.Value) (bool, error) {
 
 // lt evaluates the comparison a < b.
 func lt(arg1, arg2 reflect.Value) (bool, error) {
-	v1 := indirectInterface(arg1)
-	k1, err := basicKind(v1)
+	arg1 = indirectInterface(arg1)
+	k1, err := basicKind(arg1)
 	if err != nil {
 		return false, err
 	}
-	v2 := indirectInterface(arg2)
-	k2, err := basicKind(v2)
+	arg2 = indirectInterface(arg2)
+	k2, err := basicKind(arg2)
 	if err != nil {
 		return false, err
 	}
@@ -520,9 +514,9 @@ func lt(arg1, arg2 reflect.Value) (bool, error) {
 		// Special case: Can compare integer values regardless of type's sign.
 		switch {
 		case k1 == intKind && k2 == uintKind:
-			truth = v1.Int() < 0 || uint64(v1.Int()) < v2.Uint()
+			truth = arg1.Int() < 0 || uint64(arg1.Int()) < arg2.Uint()
 		case k1 == uintKind && k2 == intKind:
-			truth = v2.Int() >= 0 && v1.Uint() < uint64(v2.Int())
+			truth = arg2.Int() >= 0 && arg1.Uint() < uint64(arg2.Int())
 		default:
 			return false, errBadComparison
 		}
@@ -531,13 +525,13 @@ func lt(arg1, arg2 reflect.Value) (bool, error) {
 		case boolKind, complexKind:
 			return false, errBadComparisonType
 		case floatKind:
-			truth = v1.Float() < v2.Float()
+			truth = arg1.Float() < arg2.Float()
 		case intKind:
-			truth = v1.Int() < v2.Int()
+			truth = arg1.Int() < arg2.Int()
 		case stringKind:
-			truth = v1.String() < v2.String()
+			truth = arg1.String() < arg2.String()
 		case uintKind:
-			truth = v1.Uint() < v2.Uint()
+			truth = arg1.Uint() < arg2.Uint()
 		default:
 			panic("invalid kind")
 		}