SA1026, SA5008: reimplement XML checking using encoding/xml rules

Same as previous commit, but for XML instead of JSON.

Because we rely on encoding/xml, we can now flag a plethora of invalid
struct tags.

Closes gh-1088

(cherry picked from commit a62bc8e)

staticcheck/fakexml/marshal.go

@@ -0,0 +1,375 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains a modified copy of the encoding/xml encoder.
+// All dynamic behavior has been removed, and reflecttion has been replaced with go/types.
+// This allows us to statically find unmarshable types
+// with the same rules for tags, shadowing and addressability as encoding/xml.
+// This is used for SA1026 and SA5008.
+
+// NOTE(dh): we do not check CanInterface in various places, which means we'll accept more marshaler implementations than encoding/xml does. This will lead to a small amount of false negatives.
+
+package fakexml
+
+import (
+	"fmt"
+	"go/token"
+	"go/types"
+
+	"honnef.co/go/tools/go/types/typeutil"
+	"honnef.co/go/tools/staticcheck/fakereflect"
+)
+
+func Marshal(v types.Type) error {
+	return NewEncoder().Encode(v)
+}
+
+type Encoder struct {
+	seen map[fakereflect.TypeAndCanAddr]struct{}
+}
+
+func NewEncoder() *Encoder {
+	e := &Encoder{
+		seen: map[fakereflect.TypeAndCanAddr]struct{}{},
+	}
+	return e
+}
+
+func (enc *Encoder) Encode(v types.Type) error {
+	rv := fakereflect.TypeAndCanAddr{Type: v}
+	return enc.marshalValue(rv, nil, nil, "x")
+}
+
+func implementsMarshaler(v fakereflect.TypeAndCanAddr) bool {
+	t := v.Type
+	named, ok := t.(*types.Named)
+	if !ok {
+		return false
+	}
+	obj, _, _ := types.LookupFieldOrMethod(named, false, nil, "MarshalXML")
+	if obj == nil {
+		return false
+	}
+	fn, ok := obj.(*types.Func)
+	if !ok {
+		return false
+	}
+	params := fn.Type().(*types.Signature).Params()
+	if params.Len() != 2 {
+		return false
+	}
+	if !typeutil.IsType(params.At(0).Type(), "*encoding/xml.Encoder") {
+		return false
+	}
+	if !typeutil.IsType(params.At(1).Type(), "encoding/xml.StartElement") {
+		return false
+	}
+	rets := fn.Type().(*types.Signature).Results()
+	if rets.Len() != 1 {
+		return false
+	}
+	if !typeutil.IsType(rets.At(0).Type(), "error") {
+		return false
+	}
+	return true
+}
+
+func implementsMarshalerAttr(v fakereflect.TypeAndCanAddr) bool {
+	t := v.Type
+	named, ok := t.(*types.Named)
+	if !ok {
+		return false
+	}
+	obj, _, _ := types.LookupFieldOrMethod(named, false, nil, "MarshalXMLAttr")
+	if obj == nil {
+		return false
+	}
+	fn, ok := obj.(*types.Func)
+	if !ok {
+		return false
+	}
+	params := fn.Type().(*types.Signature).Params()
+	if params.Len() != 1 {
+		return false
+	}
+	if !typeutil.IsType(params.At(0).Type(), "encoding/xml.Name") {
+		return false
+	}
+	rets := fn.Type().(*types.Signature).Results()
+	if rets.Len() != 2 {
+		return false
+	}
+	if !typeutil.IsType(rets.At(0).Type(), "encoding/xml.Attr") {
+		return false
+	}
+	if !typeutil.IsType(rets.At(1).Type(), "error") {
+		return false
+	}
+	return true
+}
+
+var textMarshalerType = types.NewInterfaceType([]*types.Func{
+	types.NewFunc(token.NoPos, nil, "MarshalText", types.NewSignature(nil,
+		types.NewTuple(),
+		types.NewTuple(
+			types.NewVar(token.NoPos, nil, "", types.NewSlice(types.Typ[types.Byte])),
+			types.NewVar(0, nil, "", types.Universe.Lookup("error").Type())),
+		false,
+	)),
+}, nil).Complete()
+
+var N = 0
+
+// marshalValue writes one or more XML elements representing val.
+// If val was obtained from a struct field, finfo must have its details.
+func (e *Encoder) marshalValue(val fakereflect.TypeAndCanAddr, finfo *fieldInfo, startTemplate *StartElement, stack string) error {
+	if _, ok := e.seen[val]; ok {
+		return nil
+	}
+	e.seen[val] = struct{}{}
+
+	// Drill into interfaces and pointers.
+	for val.IsInterface() || val.IsPtr() {
+		if val.IsInterface() {
+			return nil
+		}
+		val = val.Elem()
+	}
+
+	// Check for marshaler.
+	if implementsMarshaler(val) {
+		return nil
+	}
+	if val.CanAddr() {
+		pv := fakereflect.PtrTo(val)
+		if implementsMarshaler(pv) {
+			return nil
+		}
+	}
+
+	// Check for text marshaler.
+	if val.Implements(textMarshalerType) {
+		return nil
+	}
+	if val.CanAddr() {
+		pv := fakereflect.PtrTo(val)
+		if pv.Implements(textMarshalerType) {
+			return nil
+		}
+	}
+
+	// Slices and arrays iterate over the elements. They do not have an enclosing tag.
+	if (val.IsSlice() || val.IsArray()) && !isByteArray(val) && !isByteSlice(val) {
+		if err := e.marshalValue(val.Elem(), finfo, startTemplate, stack+"[0]"); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	tinfo, err := getTypeInfo(val)
+	if err != nil {
+		return err
+	}
+
+	// Create start element.
+	// Precedence for the XML element name is:
+	// 0. startTemplate
+	// 1. XMLName field in underlying struct;
+	// 2. field name/tag in the struct field; and
+	// 3. type name
+	var start StartElement
+
+	if startTemplate != nil {
+		start.Name = startTemplate.Name
+		start.Attr = append(start.Attr, startTemplate.Attr...)
+	} else if tinfo.xmlname != nil {
+		xmlname := tinfo.xmlname
+		if xmlname.name != "" {
+			start.Name.Space, start.Name.Local = xmlname.xmlns, xmlname.name
+		}
+	}
+
+	// Attributes
+	for i := range tinfo.fields {
+		finfo := &tinfo.fields[i]
+		if finfo.flags&fAttr == 0 {
+			continue
+		}
+		fv := finfo.value(val)
+
+		name := Name{Space: finfo.xmlns, Local: finfo.name}
+		if err := e.marshalAttr(&start, name, fv, stack+pathByIndex(val, finfo.idx)); err != nil {
+			return err
+		}
+	}
+
+	if val.IsStruct() {
+		return e.marshalStruct(tinfo, val, stack)
+	} else {
+		return e.marshalSimple(val, stack)
+	}
+}
+
+func isSlice(v fakereflect.TypeAndCanAddr) bool {
+	_, ok := v.Type.Underlying().(*types.Slice)
+	return ok
+}
+
+func isByteSlice(v fakereflect.TypeAndCanAddr) bool {
+	slice, ok := v.Type.Underlying().(*types.Slice)
+	if !ok {
+		return false
+	}
+	basic, ok := slice.Elem().Underlying().(*types.Basic)
+	if !ok {
+		return false
+	}
+	return basic.Kind() == types.Uint8
+}
+
+func isByteArray(v fakereflect.TypeAndCanAddr) bool {
+	slice, ok := v.Type.Underlying().(*types.Array)
+	if !ok {
+		return false
+	}
+	basic, ok := slice.Elem().Underlying().(*types.Basic)
+	if !ok {
+		return false
+	}
+	return basic.Kind() == types.Uint8
+}
+
+// marshalAttr marshals an attribute with the given name and value, adding to start.Attr.
+func (e *Encoder) marshalAttr(start *StartElement, name Name, val fakereflect.TypeAndCanAddr, stack string) error {
+	if implementsMarshalerAttr(val) {
+		return nil
+	}
+
+	if val.CanAddr() {
+		pv := fakereflect.PtrTo(val)
+		if implementsMarshalerAttr(pv) {
+			return nil
+		}
+	}
+
+	if val.Implements(textMarshalerType) {
+		return nil
+	}
+
+	if val.CanAddr() {
+		pv := fakereflect.PtrTo(val)
+		if pv.Implements(textMarshalerType) {
+			return nil
+		}
+	}
+
+	// Dereference or skip nil pointer
+	if val.IsPtr() {
+		val = val.Elem()
+	}
+
+	// Walk slices.
+	if isSlice(val) && !isByteSlice(val) {
+		if err := e.marshalAttr(start, name, val.Elem(), stack+"[0]"); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	if typeutil.IsType(val.Type, "encoding/xml.Attr") {
+		return nil
+	}
+
+	return e.marshalSimple(val, stack)
+}
+
+func (e *Encoder) marshalSimple(val fakereflect.TypeAndCanAddr, stack string) error {
+	switch val.Type.Underlying().(type) {
+	case *types.Basic, *types.Interface:
+		return nil
+	case *types.Slice, *types.Array:
+		basic, ok := val.Elem().Type.Underlying().(*types.Basic)
+		if !ok || basic.Kind() != types.Uint8 {
+			return &UnsupportedTypeError{val.Type, stack}
+		}
+		return nil
+	default:
+		return &UnsupportedTypeError{val.Type, stack}
+	}
+}
+
+func indirect(vf fakereflect.TypeAndCanAddr) fakereflect.TypeAndCanAddr {
+	for vf.IsPtr() {
+		vf = vf.Elem()
+	}
+	return vf
+}
+
+func pathByIndex(t fakereflect.TypeAndCanAddr, index []int) string {
+	path := ""
+	for _, i := range index {
+		if t.IsPtr() {
+			t = t.Elem()
+		}
+		path += "." + t.Field(i).Name
+		t = t.Field(i).Type
+	}
+	return path
+}
+
+func (e *Encoder) marshalStruct(tinfo *typeInfo, val fakereflect.TypeAndCanAddr, stack string) error {
+	for i := range tinfo.fields {
+		finfo := &tinfo.fields[i]
+		if finfo.flags&fAttr != 0 {
+			continue
+		}
+		vf := finfo.value(val)
+
+		switch finfo.flags & fMode {
+		case fCDATA, fCharData:
+			if vf.Implements(textMarshalerType) {
+				continue
+			}
+			if vf.CanAddr() {
+				pv := fakereflect.PtrTo(vf)
+				if pv.Implements(textMarshalerType) {
+					continue
+				}
+			}
+
+			vf = indirect(vf)
+			continue
+
+		case fComment:
+			vf = indirect(vf)
+			if !(isByteSlice(vf) || isByteArray(vf)) {
+				return fmt.Errorf("xml: bad type for comment field of %s", val)
+			}
+			continue
+
+		case fInnerXML:
+			vf = indirect(vf)
+			if typeutil.IsType(vf.Type, "[]byte") || typeutil.IsType(vf.Type, "string") {
+				continue
+			}
+
+		case fElement, fElement | fAny:
+		}
+		if err := e.marshalValue(vf, finfo, nil, stack+pathByIndex(val, finfo.idx)); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// UnsupportedTypeError is returned when Marshal encounters a type
+// that cannot be converted into XML.
+type UnsupportedTypeError struct {
+	Type types.Type
+	Path string
+}
+
+func (e *UnsupportedTypeError) Error() string {
+	return fmt.Sprintf("xml: unsupported type %s, via %s ", e.Type, e.Path)
+}