datastore/prop.go
func getStructCodecLocked(t reflect.Type) (ret *structCodec, retErr error) {
	c, ok := structCodecs[t]
	if ok {
		return c, nil
	}
	c = &structCodec{
		fields: make(map[string]fieldCodec),
		// We initialize keyField to -1 so that the zero-value is not
		// misinterpreted as index 0.
		keyField: -1,
	}

	// Add c to the structCodecs map before we are sure it is good. If t is
	// a recursive type, it needs to find the incomplete entry for itself in
	// the map.
	structCodecs[t] = c
	defer func() {
		if retErr != nil {
			delete(structCodecs, t)
		}
	}()

	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		// Skip unexported fields.
		// Note that if f is an anonymous, unexported struct field,
		// we will promote its fields.
		if f.PkgPath != "" && !f.Anonymous {
			continue
		}

		tags := strings.Split(f.Tag.Get("datastore"), ",")
		name := tags[0]
		opts := make(map[string]bool)
		for _, t := range tags[1:] {
			opts[t] = true
		}
		switch {
		case name == "":
			if !f.Anonymous {
				name = f.Name
			}
		case name == "-":
			continue
		case name == "__key__":
			if f.Type != typeOfKeyPtr {
				return nil, fmt.Errorf("datastore: __key__ field on struct %v is not a *datastore.Key", t)
			}
			c.keyField = i
		case !validPropertyName(name):
			return nil, fmt.Errorf("datastore: struct tag has invalid property name: %q", name)
		}

		substructType, fIsSlice := reflect.Type(nil), false
		switch f.Type.Kind() {
		case reflect.Struct:
			substructType = f.Type
		case reflect.Slice:
			if f.Type.Elem().Kind() == reflect.Struct {
				substructType = f.Type.Elem()
			}
			fIsSlice = f.Type != typeOfByteSlice
			c.hasSlice = c.hasSlice || fIsSlice
		}

		var sub *structCodec
		if substructType != nil && substructType != typeOfTime && substructType != typeOfGeoPoint {
			var err error
			sub, err = getStructCodecLocked(substructType)
			if err != nil {
				return nil, err
			}
			if !sub.complete {
				return nil, fmt.Errorf("datastore: recursive struct: field %q", f.Name)
			}
			if fIsSlice && sub.hasSlice {
				return nil, fmt.Errorf(
					"datastore: flattening nested structs leads to a slice of slices: field %q", f.Name)
			}
			c.hasSlice = c.hasSlice || sub.hasSlice
			// If f is an anonymous struct field, we promote the substruct's fields up to this level
			// in the linked list of struct codecs.
			if f.Anonymous {
				for subname, subfield := range sub.fields {
					if name != "" {
						subname = name + "." + subname
					}
					if _, ok := c.fields[subname]; ok {
						return nil, fmt.Errorf("datastore: struct tag has repeated property name: %q", subname)
					}
					c.fields[subname] = fieldCodec{
						path:        append([]int{i}, subfield.path...),
						noIndex:     subfield.noIndex || opts["noindex"],
						omitEmpty:   subfield.omitEmpty,
						structCodec: subfield.structCodec,
					}
				}
				continue
			}
		}

		if _, ok := c.fields[name]; ok {
			return nil, fmt.Errorf("datastore: struct tag has repeated property name: %q", name)
		}
		c.fields[name] = fieldCodec{
			path:        []int{i},
			noIndex:     opts["noindex"],
			omitEmpty:   opts["omitempty"],
			structCodec: sub,
		}
	}
	c.complete = true
	return c, nil
}
func getStructCodecLocked(t reflect.Type) (ret *structCodec, retErr error) {
	c, ok := structCodecs[t]
	if ok {
		return c, nil
	}
	c = &structCodec{
		fields: make(map[string]fieldCodec),
		// We initialize keyField to -1 so that the zero-value is not
		// misinterpreted as index 0.
		keyField: -1,
	}

	// Add c to the structCodecs map before we are sure it is good. If t is
	// a recursive type, it needs to find the incomplete entry for itself in
	// the map.
	structCodecs[t] = c
	defer func() {
		if retErr != nil {
			delete(structCodecs, t)
		}
	}()

	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		// Skip unexported fields.
		// Note that if f is an anonymous, unexported struct field,
		// we will promote its fields.
		if f.PkgPath != "" && !f.Anonymous {
			continue
		}

		tags := strings.Split(f.Tag.Get("datastore"), ",")
		name := tags[0]
		opts := make(map[string]bool)
		for _, t := range tags[1:] {
			opts[t] = true
		}
		switch {
		case name == "":
			if !f.Anonymous {
				name = f.Name
			}
		case name == "-":
			continue
		case name == "__key__":
			if f.Type != typeOfKeyPtr {
				return nil, fmt.Errorf("datastore: __key__ field on struct %v is not a *datastore.Key", t)
			}
			c.keyField = i
		case !validPropertyName(name):
			return nil, fmt.Errorf("datastore: struct tag has invalid property name: %q", name)
		}

		substructType, fIsSlice := reflect.Type(nil), false
		switch f.Type.Kind() {
		case reflect.Struct:
			substructType = f.Type
		case reflect.Slice:
			if f.Type.Elem().Kind() == reflect.Struct {
				substructType = f.Type.Elem()
			}
			fIsSlice = f.Type != typeOfByteSlice
			c.hasSlice = c.hasSlice || fIsSlice
		}

		var sub *structCodec
		if substructType != nil && substructType != typeOfTime && substructType != typeOfGeoPoint {
			var err error
			sub, err = getStructCodecLocked(substructType)
			if err != nil {
				return nil, err
			}
			if !sub.complete {
				return nil, fmt.Errorf("datastore: recursive struct: field %q", f.Name)
			}
			if fIsSlice && sub.hasSlice {
				return nil, fmt.Errorf(
					"datastore: flattening nested structs leads to a slice of slices: field %q", f.Name)
			}
			c.hasSlice = c.hasSlice || sub.hasSlice
			// If f is an anonymous struct field, we promote the substruct's fields up to this level
			// in the linked list of struct codecs.
			if f.Anonymous {
				for subname, subfield := range sub.fields {
					if name != "" {
						subname = name + "." + subname
					}
					if _, ok := c.fields[subname]; ok {
						return nil, fmt.Errorf("datastore: struct tag has repeated property name: %q", subname)
					}
					c.fields[subname] = fieldCodec{
						path:        append([]int{i}, subfield.path...),
						noIndex:     subfield.noIndex || opts["noindex"],
						omitEmpty:   subfield.omitEmpty,
						structCodec: subfield.structCodec,
					}
				}
				continue
			}
		}

		if _, ok := c.fields[name]; ok {
			return nil, fmt.Errorf("datastore: struct tag has repeated property name: %q", name)
		}
		c.fields[name] = fieldCodec{
			path:        []int{i},
			noIndex:     opts["noindex"],
			omitEmpty:   opts["omitempty"],
			structCodec: sub,
		}
	}
	c.complete = true
	return c, nil
}
// getStructCodecLocked implements getStructCodec. The structCodecsMutex must
// be held when calling this function.