jsonpath.go

/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package printers

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"reflect"

	"k8s.io/apimachinery/pkg/runtime"
	"k8s.io/client-go/util/jsonpath"
)

// exists returns true if it would be possible to call the index function
// with these arguments.
//
// TODO: how to document this for users?
//
// index returns the result of indexing its first argument by the following
// 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 exists(item interface{}, indices ...interface{}) bool {
	v := reflect.ValueOf(item)
	for _, i := range indices {
		index := reflect.ValueOf(i)
		var isNil bool
		if v, isNil = indirect(v); isNil {
			return false
		}
		switch v.Kind() {
		case reflect.Array, reflect.Slice, reflect.String:
			var x int64
			switch index.Kind() {
			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
				x = index.Int()
			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
				x = int64(index.Uint())
			default:
				return false
			}
			if x < 0 || x >= int64(v.Len()) {
				return false
			}
			v = v.Index(int(x))
		case reflect.Map:
			if !index.IsValid() {
				index = reflect.Zero(v.Type().Key())
			}
			if !index.Type().AssignableTo(v.Type().Key()) {
				return false
			}
			if x := v.MapIndex(index); x.IsValid() {
				v = x
			} else {
				v = reflect.Zero(v.Type().Elem())
			}
		default:
			return false
		}
	}
	if _, isNil := indirect(v); isNil {
		return false
	}
	return true
}

// stolen from text/template
// indirect returns the item at the end of indirection, and a bool to indicate if it's nil.
// We indirect through pointers and empty interfaces (only) because
// non-empty interfaces have methods we might need.
func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
	for ; v.Kind() == reflect.Pointer || v.Kind() == reflect.Interface; v = v.Elem() {
		if v.IsNil() {
			return v, true
		}
		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
			break
		}
	}
	return v, false
}

// JSONPathPrinter is an implementation of ResourcePrinter which formats data with jsonpath expression.
type JSONPathPrinter struct {
	rawTemplate string
	*jsonpath.JSONPath
}

func NewJSONPathPrinter(tmpl string) (*JSONPathPrinter, error) {
	j := jsonpath.New("out")
	if err := j.Parse(tmpl); err != nil {
		return nil, err
	}
	return &JSONPathPrinter{
		rawTemplate: tmpl,
		JSONPath:    j,
	}, nil
}

// PrintObj formats the obj with the JSONPath Template.
func (j *JSONPathPrinter) PrintObj(obj runtime.Object, w io.Writer) error {
	// we use reflect.Indirect here in order to obtain the actual value from a pointer.
	// we need an actual value in order to retrieve the package path for an object.
	// using reflect.Indirect indiscriminately is valid here, as all runtime.Objects are supposed to be pointers.
	if InternalObjectPreventer.IsForbidden(reflect.Indirect(reflect.ValueOf(obj)).Type().PkgPath()) {
		return fmt.Errorf(InternalObjectPrinterErr)
	}

	var queryObj interface{} = obj
	if unstructured, ok := obj.(runtime.Unstructured); ok {
		queryObj = unstructured.UnstructuredContent()
	} else {
		data, err := json.Marshal(obj)
		if err != nil {
			return err
		}
		queryObj = map[string]interface{}{}
		if err := json.Unmarshal(data, &queryObj); err != nil {
			return err
		}
	}

	if err := j.JSONPath.Execute(w, queryObj); err != nil {
		buf := bytes.NewBuffer(nil)
		fmt.Fprintf(buf, "Error executing template: %v. Printing more information for debugging the template:\n", err)
		fmt.Fprintf(buf, "\ttemplate was:\n\t\t%v\n", j.rawTemplate)
		fmt.Fprintf(buf, "\tobject given to jsonpath engine was:\n\t\t%#v\n\n", queryObj)
		return fmt.Errorf("error executing jsonpath %q: %v\n", j.rawTemplate, buf.String())
	}
	return nil
}