override.go

package override

import (
	"encoding"
	"fmt"
	"reflect"
	"sort"
	"strconv"
	"strings"

	"github.com/mitchellh/copystructure"
	"github.com/mitchellh/mapstructure"
	"github.com/mitchellh/reflectwalk"
	"github.com/pkg/errors"
)

const (
	structTagKey   = "override"
	redactKeyword  = "redact"
	elementKeyword = "element-key="
	omitField      = "-"
)

// Validator is a type that can validate itself.
// If an element is a Validator, then Validate() is called
// whenever it is modified.
type Validator interface {
	Validate() error
}

// Override specifies what configuration values should be overridden and how.
// Configuration options can be overridden as well as elements of a section
// can be deleted or created.
type Override struct {
	// Section is the name of the section to override.
	Section string
	// Element is the name of the element within a section to override.
	// If the section is not a slice of structs then this can remain empty.
	Element string
	// Options is a set of option name to value to override existing values.
	Options map[string]interface{}
	// Delete indicates whether the specified element should be deleted.
	Delete bool
	// Create indicates whether to create a new element in the specified section.
	// To create a new element leave the element name empty in this Override object
	// and provide the value in the Options map under the element key.
	Create bool
}

// Validate that the values set on this Override are self-consistent.
func (o Override) Validate() error {
	if o.Section == "" {
		return errors.New("section cannot be empty")
	}
	if o.Delete && o.Element == "" {
		return errors.New("element cannot be empty if deleting an element")
	}
	if o.Create && o.Element != "" {
		return errors.New("element must be empty if creating an element, set the element key value via the options")
	}
	if o.Delete && len(o.Options) > 0 {
		return errors.New("cannot delete an element and provide options in the same override")
	}
	if o.Delete && o.Create {
		return errors.New("cannot create and delete an element in the same override")
	}
	return nil
}

// ElementKeys returns a map of section name to element key for each section.
func ElementKeys(config interface{}) (map[string]string, error) {
	// walk the config and read all sections
	walker := newSectionWalker()
	if err := reflectwalk.Walk(config, walker); err != nil {
		return nil, errors.Wrap(err, "failed to read sections from configuration object")
	}

	return walker.elementKeysMap(), nil
}

// OverrideConfig applies all given overrides and returns a map of all configuration sections, even if they were not overridden.
// The overrides are all applied to the same object and the original configuration object remains unmodified.
//
// Values must be of the same type as the named option field, or have another means of converting the value.
//
// Numeric types will be converted to the absolute type using Go's default conversion mechanisms.
// Strings and Stringer types will be parsed for numeric values if possible.
// TextUnmarshaler types will attempt to unmarshal string values.
//
// Mismatched types or failure to convert the value will result in an error.
//
// An element value that is a Validator will be validated and any encounted error returned.
//
// When a new element is being created if the element type is a Initer, then the zero value of the
// element will first have defaults set before the overrides are applied.
//
// The underlying configuration object is not modified, but rather a copy is returned via the Element type.
func OverrideConfig(config interface{}, os []Override) (map[string]Section, error) {
	// First make a copy into which we can apply the updates.
	copy, err := copystructure.Copy(config)
	if err != nil {
		return nil, errors.Wrap(err, "failed to copy configuration object")
	}

	// Apply all overrides to the same copy
	for _, o := range os {
		// We do not need to keep a reference to the section since we are going to walk the entire copy next
		_, err := applyOverride(copy, o)
		if err != nil {
			return nil, errors.Wrapf(err, "failed to override configuration %s/%s", o.Section, o.Element)
		}
	}

	// Walk the copy to return all sections
	walker := newSectionWalker()
	if err := reflectwalk.Walk(copy, walker); err != nil {
		return nil, errors.Wrap(err, "failed to read sections from configuration object")
	}

	return walker.sectionsMap(), nil
}

// applyOverride applies the given override to the specified object.
func applyOverride(object interface{}, o Override) (Element, error) {
	if err := o.Validate(); err != nil {
		return Element{}, errors.Wrap(err, "invalid override")
	}
	walker := newOverrideWalker(o)

	// walk the copy and apply the updates
	if err := reflectwalk.Walk(object, walker); err != nil {
		return Element{}, err
	}
	unused := walker.unused()
	if len(unused) > 0 {
		return Element{}, fmt.Errorf("unknown options %v in section %s", unused, o.Section)
	}
	// Return the modified copy
	element := walker.elementObject()
	if element.value == nil && !o.Delete {
		return Element{}, fmt.Errorf("unknown section %s", o.Section)
	}
	// Validate new value
	if v, ok := element.value.(Validator); ok {
		if err := v.Validate(); err != nil {
			return Element{}, errors.Wrap(err, "failed validation")
		}
	}
	return element, nil
}

// overrideWalker applies the changes onto the walked value.
type overrideWalker struct {
	depthWalker

	o Override

	used               map[string]bool
	elementValue       reflect.Value
	currentSectionName string
	currentElementName string
	currentSlice       reflect.Value
	elementKey         string
}

func newOverrideWalker(o Override) *overrideWalker {
	return &overrideWalker{
		o:    o,
		used: make(map[string]bool, len(o.Options)),
	}
}

func (w *overrideWalker) unused() []string {
	unused := make([]string, 0, len(w.o.Options))
	for name := range w.o.Options {
		if !w.used[name] {
			unused = append(unused, name)
		}
	}
	return unused
}

func (w *overrideWalker) elementObject() Element {
	if w.elementValue.IsValid() {
		return Element{
			value:   w.elementValue.Interface(),
			element: w.o.Element,
		}
	}
	return Element{}
}

func (w *overrideWalker) Struct(reflect.Value) error {
	return nil
}

func (w *overrideWalker) StructField(f reflect.StructField, v reflect.Value) error {
	switch w.depth {
	// Section level
	case 0:
		name, ok := getSectionName(f)
		if ok {
			// Only override the section if a struct tag was present
			w.currentSectionName = name
			if w.o.Section == w.currentSectionName {
				w.elementValue = v
				w.elementKey = getElementKey(f)
			}
		} else {
			w.currentSectionName = ""
		}
	// Option level
	case 1:
		// Skip this field if its not for the section/element we care about
		if w.currentSectionName != w.o.Section || w.currentElementName != w.o.Element {
			break
		}

		name := fieldName(f)
		if name == omitField {
			break
		}
		setValue, ok := w.o.Options[name]
		if !ok {
			name = strings.ToLower(name)
			setValue, ok = w.o.Options[name]
		}
		if ok {
			if !w.o.Create && name == w.elementKey {
				return fmt.Errorf("cannot override element key %s", name)
			}
			if err := weakCopyValue(v, reflect.ValueOf(setValue)); err != nil {
				return errors.Wrapf(err, "cannot set option %s", name)
			}
			w.used[name] = true
		}
	}
	return nil
}

// Initer set defaults on the receiving object.
// If a type is a Initer and a new value needs to be created of that type,
// then Init() is called on a new instance of that type.
type Initer interface {
	Init()
}

var initerType = reflect.TypeOf((*Initer)(nil)).Elem()

func (w *overrideWalker) Slice(v reflect.Value) error {
	if w.o.Section != w.currentSectionName || w.depth != 1 {
		return nil
	}
	w.currentSlice = v
	switch {
	case w.o.Delete:
		// Explictly set the section value to the zero value
		w.elementValue = reflect.Value{}
	case w.o.Create:
		// Create a new element in the slice
		var n reflect.Value
		et := v.Type().Elem()
		if et.Kind() == reflect.Ptr {
			n = reflect.New(et.Elem())
		} else {
			n = reflect.New(et)
		}
		// If the type is a initer, call Default
		if n.Type().Implements(initerType) {
			n.Interface().(Initer).Init()
		}
		// Indirect the value if we didn't want a pointer
		if et.Kind() != reflect.Ptr {
			n = reflect.Indirect(n)
		}
		v.Set(reflect.Append(v, n))
		// Set element key
		if w.elementKey == "" {
			return fmt.Errorf("cannot create new element, no element key found. An element key must be specified via the `%s:\",%s<field name>\"` struct tag", structTagKey, elementKeyword)
		}
		// Get the value that is now part of the slice.
		n = v.Index(v.Len() - 1)
		elementField := findFieldByElementKey(n, w.elementKey)
		if !elementField.IsValid() {
			return fmt.Errorf("could not find field with the name of the element key %q", w.elementKey)
		}
		if elementField.Kind() != reflect.String {
			return fmt.Errorf("element key field must be of type string, got %s", elementField.Type())
		}
		setValue, ok := w.o.Options[w.elementKey]
		if !ok {
			return fmt.Errorf("element key %q not present in options", w.elementKey)
		}
		str, ok := setValue.(string)
		if !ok {
			return fmt.Errorf("type of element key must be a string, got %T ", setValue)
		}
		w.o.Element = str
		w.used[w.elementKey] = true
		if err := weakCopyValue(elementField, reflect.ValueOf(setValue)); err != nil {
			return errors.Wrapf(err, "cannot set element key %q on new element", w.elementKey)
		}
	default:
		// We are modifying an existing slice element.
		// Nothing to do here.
	}
	return nil
}

func (w *overrideWalker) SliceElem(idx int, v reflect.Value) error {
	if w.depth == 1 && w.currentSectionName == w.o.Section && w.o.Element != "" {
		w.currentElementName = ""
		if w.elementKey == "" {
			return fmt.Errorf("an element key must be specified via the `%s:\",%s<field name>\"` struct tag", structTagKey, elementKeyword)
		}
		// Get current element name via field on current value
		elementField := findFieldByElementKey(v, w.elementKey)
		if !elementField.IsValid() {
			return fmt.Errorf("could not find field with name %q on value of type %s", w.elementKey, v.Type())
		}
		if elementField.Kind() != reflect.String {
			return fmt.Errorf("element key field must be of type string, got %s", elementField.Type())
		}
		w.currentElementName = elementField.String()
		if w.o.Element == w.currentElementName {
			if w.o.Delete {
				// Delete the element from the slice by re-slicing the element out
				w.currentSlice.Set(
					reflect.AppendSlice(
						w.currentSlice.Slice(0, idx),
						w.currentSlice.Slice(idx+1, w.currentSlice.Len()),
					),
				)
			} else {
				w.elementValue = v
			}
		}
	}
	return nil
}

var textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()

// weakCopyValue copies the value of dst into src, where numeric and interface types are copied weakly.
func weakCopyValue(dst, src reflect.Value) (err error) {
	defer func() {
		// This shouldn't be necessary but it better to catch a panic here,
		// where we can provide context instead of crashing the server or letting it bouble up.
		if r := recover(); r != nil {
			if e, ok := r.(error); ok {
				err = e
			} else {
				err = fmt.Errorf("%v", r)
			}
		}
	}()
	if !dst.CanSet() {
		return errors.New("not settable")
	}
	if src.Kind() == reflect.Interface {
		src = src.Elem()
	}
	srcK := src.Kind()
	dstK := dst.Kind()

	// Get addressable value since it may implement
	// the TextUnmarshaler interface
	var addrDst reflect.Value
	if d := reflect.Indirect(dst); d.CanAddr() {
		addrDst = d.Addr()
	}

	if srcK == dstK {
		if dst.Type() == src.Type() {
			// Perform normal copy
			dst.Set(src)
		} else {
			// Perform recursive copy into elements
			switch dstK {
			case reflect.Map:
				// Make new map
				dst.Set(reflect.MakeMap(dst.Type()))
				for _, key := range src.MapKeys() {
					value := reflect.Indirect(reflect.New(dst.Type().Elem()))
					if err := weakCopyValue(value, src.MapIndex(key)); err != nil {
						return errors.Wrap(err, "failed to copy map value")
					}
					dst.SetMapIndex(key, value)
				}
			case reflect.Slice:
				// Make new slice
				dst.Set(reflect.MakeSlice(dst.Type(), src.Len(), src.Len()))
				for i := 0; i < src.Len(); i++ {
					value := reflect.Indirect(reflect.New(dst.Type().Elem()))
					if err := weakCopyValue(value, src.Index(i)); err != nil {
						return errors.Wrap(err, "failed to copy slice value")
					}
					dst.Index(i).Set(value)
				}
			default:
				return fmt.Errorf("cannot copy mismatched types got %s exp %s", src.Type().String(), dst.Type().String())
			}
		}
	} else if addrDst.Type().Implements(textUnmarshalerType) {
		um := addrDst.Interface().(encoding.TextUnmarshaler)
		var text []byte
		if src.Type().Implements(stringerType) || srcK == reflect.String {
			text = []byte(src.String())
		} else {
			return fmt.Errorf("cannot unmarshal %s into %s", srcK, dstK)
		}
		if err := um.UnmarshalText(text); err != nil {
			errors.Wrap(err, "failed to unmarshal text")
		}
	} else if isNumericKind(dstK) {
		// Perform weak numeric copy
		if isNumericKind(srcK) {
			dst.Set(src.Convert(dst.Type()))
			return nil
		} else {
			var str string
			if src.Type().Implements(stringerType) || srcK == reflect.String {
				str = src.String()
			} else {
				return fmt.Errorf("cannot convert %s into %s", srcK, dstK)
			}
			switch {
			case isIntKind(dstK):
				if i, err := strconv.ParseInt(str, 10, 64); err == nil {
					dst.SetInt(i)
					return nil
				}
			case isUintKind(dstK):
				if i, err := strconv.ParseUint(str, 10, 64); err == nil {
					dst.SetUint(i)
					return nil
				}
			case isFloatKind(dstK):
				if f, err := strconv.ParseFloat(str, 64); err == nil {
					dst.SetFloat(f)
					return nil
				}
			}
			return fmt.Errorf("cannot convert string %q into %s", str, dstK)
		}
	} else if dstK == reflect.Struct && srcK == reflect.Map {
		dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
			ErrorUnused: true,
			Result:      addrDst.Interface(),
			DecodeHook:  mapstructure.StringToTimeDurationHookFunc(),
		})
		if err != nil {
			return errors.Wrap(err, "failed to initialize mapstructure decoder")
		}
		if err := dec.Decode(src.Interface()); err != nil {
			return errors.Wrapf(err, "failed to decode options into %s", addrDst.Type())
		}
		return nil
	} else {
		return fmt.Errorf("wrong kind %s, expected value of kind %s: %t", srcK, dstK, srcK == dstK)
	}
	return nil
}

// Stringer is a type that can provide a string value of itself.
// If a value is a Stringer and needs to be copied into a numeric value,
// then String() is called and parsed as a numeric value if possible.
type Stringer interface {
	String() string
}

var stringerType = reflect.TypeOf((*Stringer)(nil)).Elem()

func isNumericKind(k reflect.Kind) bool {
	// Ignoring complex kinds since we cannot convert them
	return k >= reflect.Int && k <= reflect.Float64
}
func isIntKind(k reflect.Kind) bool {
	return k >= reflect.Int && k <= reflect.Int64
}
func isUintKind(k reflect.Kind) bool {
	return k >= reflect.Uint && k <= reflect.Uint64
}
func isFloatKind(k reflect.Kind) bool {
	return k == reflect.Float32 || k == reflect.Float64
}

// Element provides access to the underlying value or a map of redacted values.
type Element struct {
	value   interface{}
	element string
}

// ElementID returns the value of the field specified by the element key.
// It is unique for all elements within a Section.
func (e Element) ElementID() string {
	return e.element
}

// Value returns the underlying value of the configuration element.
func (e Element) Value() interface{} {
	return e.value
}

// Redacted returns the options for the element in a map.
// Any fields with the `override:",redact"` tag set will be replaced
// with a boolean value indicating whether a non-zero value was set.
func (e Element) Redacted() (map[string]interface{}, []string, error) {
	walker := newRedactWalker()
	// walk the section and collect redacted options
	if err := reflectwalk.Walk(e.value, walker); err != nil {
		return nil, nil, errors.Wrap(err, "failed to redact section")
	}
	return walker.optionsMap(), walker.redactedList(), nil
}

// getElementKey returns the name of the field that is used to uniquely identify elements of a list.
func getElementKey(f reflect.StructField) string {
	parts := strings.Split(f.Tag.Get(structTagKey), ",")
	if len(parts) > 1 {
		for _, p := range parts[1:] {
			if strings.HasPrefix(p, elementKeyword) {
				return strings.TrimPrefix(p, elementKeyword)
			}
		}
	}
	return ""
}

func findFieldByElementKey(v reflect.Value, elementKey string) reflect.Value {
	v = reflect.Indirect(v)
	if v.Kind() != reflect.Struct {
		return reflect.Value{}
	}
	field := v.FieldByName(elementKey)
	if field.IsValid() {
		return field
	}

	t := v.Type()
	for i := 0; i < t.NumField(); i++ {
		field := v.Field(i)
		// Skip any unexported fields
		if !field.CanSet() {
			continue
		}
		name := fieldName(t.Field(i))
		if name == elementKey {
			return field
		}
	}
	return reflect.Value{}
}

// redactWalker reads the the sections from the walked values and redacts and sensitive fields.
type redactWalker struct {
	depthWalker
	options  map[string]interface{}
	redacted []string
}

func newRedactWalker() *redactWalker {
	return &redactWalker{
		options: make(map[string]interface{}),
	}
}

func (w *redactWalker) optionsMap() map[string]interface{} {
	return w.options
}

func (w *redactWalker) redactedList() []string {
	return w.redacted
}

func (w *redactWalker) Struct(reflect.Value) error {
	return nil
}

func (w *redactWalker) StructField(f reflect.StructField, v reflect.Value) error {
	switch w.depth {
	// Top level
	case 0:
		name := fieldName(f)
		if name == omitField {
			break
		}
		var value interface{}
		if isRedacted(f) {
			value = !isZero(v)
			// Add field to redacted list
			w.redacted = append(w.redacted, name)
		} else {
			value = v.Interface()
		}
		w.options[name] = value
	// Ignore all other levels
	default:
	}
	return nil
}

func isRedacted(f reflect.StructField) bool {
	parts := strings.Split(f.Tag.Get(structTagKey), ",")
	if len(parts) > 1 {
		for _, p := range parts[1:] {
			if p == redactKeyword {
				return true
			}
		}
	}
	return false
}

// isZero returns whether if v is equal to the zero value of its type.
func isZero(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.Func, reflect.Map, reflect.Slice:
		return v.IsNil()
	case reflect.Array:
		// Check arrays linearly since its element type may not be comparable.
		z := true
		for i := 0; i < v.Len() && z; i++ {
			z = z && isZero(v.Index(i))
		}
		return z
	case reflect.Struct:
		// Check structs recusively since not all of its field may be comparable
		z := true
		for i := 0; i < v.NumField() && z; i++ {
			f := v.Field(i)
			z = z && isZero(f)
		}
		return z
	default:
		// Compare other types directly:
		z := reflect.Zero(v.Type())
		return v.Interface() == z.Interface()
	}
}

// Section is a list of Elements.
// Elements are sorted by their element ID.
type Section []Element

func (s Section) Len() int           { return len(s) }
func (s Section) Less(i, j int) bool { return s[i].ElementID() < s[j].ElementID() }
func (s Section) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

// sectionWalker reads the sections from the walked values and redacts any sensitive fields.
type sectionWalker struct {
	depthWalker
	sections           map[string]Section
	currentSectionName string
	elementKeys        map[string]string
	uniqueElements     map[string]bool
}

func newSectionWalker() *sectionWalker {
	return &sectionWalker{
		sections:       make(map[string]Section),
		elementKeys:    make(map[string]string),
		uniqueElements: make(map[string]bool),
	}
}

func (w *sectionWalker) sectionsMap() map[string]Section {
	for _, sectionList := range w.sections {
		sort.Sort(sectionList)
	}
	return w.sections
}

func (w *sectionWalker) elementKeysMap() map[string]string {
	return w.elementKeys
}

func (w *sectionWalker) Struct(reflect.Value) error {
	return nil
}

func (w *sectionWalker) StructField(f reflect.StructField, v reflect.Value) error {
	switch w.depth {
	// Section level
	case 0:
		name, ok := getSectionName(f)
		if ok {
			w.currentSectionName = name
			w.uniqueElements = make(map[string]bool)
			elementKey := getElementKey(f)
			w.elementKeys[name] = elementKey
			if k := reflect.Indirect(v).Kind(); k == reflect.Struct {
				w.sections[name] = Section{{
					value: v.Interface(),
				}}
			}
		} else {
			w.currentSectionName = ""
		}
	// Skip all other levels
	default:
	}
	return nil
}

func (w *sectionWalker) Slice(reflect.Value) error {
	return nil
}

func (w *sectionWalker) SliceElem(idx int, v reflect.Value) error {
	// Skip sections that we are not interested in
	if w.currentSectionName == "" {
		return nil
	}
	switch w.depth {
	//Option level
	case 1:
		// Get element value from object
		var element string
		elementKey, ok := w.elementKeys[w.currentSectionName]
		if !ok {
			return fmt.Errorf("no element key found for section %q, %v", w.currentSectionName, v)
		}
		elementField := findFieldByElementKey(v, elementKey)
		if elementField.IsValid() {
			if elementField.Kind() != reflect.String {
				return fmt.Errorf("element key field must be of type string, got %s", elementField.Type())
			}
			element = elementField.String()
		} else {
			return fmt.Errorf("could not find field with the name of the element key %q on element object", elementKey)
		}

		if w.uniqueElements[element] {
			return fmt.Errorf("section %q has duplicate elements: %q", w.currentSectionName, element)
		}
		w.uniqueElements[element] = true
		w.sections[w.currentSectionName] = append(w.sections[w.currentSectionName], Element{
			value:   v.Interface(),
			element: element,
		})
		// Skip all other levels
	default:
	}
	return nil
}

// getSectionName returns the name of the section based off its `override` struct tag.
// If no tag is present the Go field name is returned and the second return value is false.
func getSectionName(f reflect.StructField) (string, bool) {
	parts := strings.Split(f.Tag.Get(structTagKey), ",")
	if parts[0] != "" {
		return parts[0], true
	}
	return f.Name, false
}

// depthWalker keeps track of the current depth count into nested structs.
type depthWalker struct {
	depth int
}

func (w *depthWalker) Enter(l reflectwalk.Location) error {
	if l == reflectwalk.StructField {
		w.depth++
	}
	return nil
}

func (w *depthWalker) Exit(l reflectwalk.Location) error {
	if l == reflectwalk.StructField {
		w.depth--
	}
	return nil
}

// fieldName returns the name of a field based on the value of the `override` struct tag.
// If no override struct tag is found the field name is returned.
func fieldName(f reflect.StructField) (name string) {
	parts := strings.Split(f.Tag.Get(structTagKey), ",")
	name = parts[0]
	if name == "" {
		name = f.Name
	}
	return
}