jsonmerge.go

/*
Copyright 2015 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 jsonmerge

import (
	"encoding/json"
	"fmt"

	"github.com/evanphx/json-patch"
	"github.com/golang/glog"

	"k8s.io/apimachinery/pkg/util/mergepatch"
	"k8s.io/apimachinery/pkg/util/yaml"
)

// Delta represents a change between two JSON documents.
type Delta struct {
	original []byte
	edit     []byte

	preconditions []PreconditionFunc
}

// PreconditionFunc is a test to verify that an incompatible change
// has occurred before an Apply can be successful.
type PreconditionFunc func(interface{}) (hold bool, message string)

// AddPreconditions adds precondition checks to a change which must
// be satisfied before an Apply is considered successful. If a
// precondition returns false, the Apply is failed with
// ErrPreconditionFailed.
func (d *Delta) AddPreconditions(fns ...PreconditionFunc) {
	d.preconditions = append(d.preconditions, fns...)
}

// RequireKeyUnchanged creates a precondition function that fails
// if the provided key is present in the diff (indicating its value
// has changed).
func RequireKeyUnchanged(key string) PreconditionFunc {
	return func(diff interface{}) (bool, string) {
		m, ok := diff.(map[string]interface{})
		if !ok {
			return true, ""
		}
		// the presence of key in a diff means that its value has been changed, therefore
		// we should fail the precondition.
		_, ok = m[key]
		if ok {
			return false, key + " should not be changed\n"
		} else {
			return true, ""
		}
	}
}

// RequireMetadataKeyUnchanged creates a precondition function that fails
// if the metadata.key is present in the diff (indicating its value
// has changed).
func RequireMetadataKeyUnchanged(key string) PreconditionFunc {
	return func(diff interface{}) (bool, string) {
		m, ok := diff.(map[string]interface{})
		if !ok {
			return true, ""
		}
		m1, ok := m["metadata"]
		if !ok {
			return true, ""
		}
		m2, ok := m1.(map[string]interface{})
		if !ok {
			return true, ""
		}
		_, ok = m2[key]
		if ok {
			return false, "metadata." + key + " should not be changed\n"
		} else {
			return true, ""
		}
	}
}

// TestPreconditions test if preconditions hold given the edit
func TestPreconditionsHold(edit []byte, preconditions []PreconditionFunc) (bool, string) {
	diff := make(map[string]interface{})
	if err := json.Unmarshal(edit, &diff); err != nil {
		return false, err.Error()
	}
	for _, fn := range preconditions {
		if hold, msg := fn(diff); !hold {
			return false, msg
		}
	}
	return true, ""
}

// NewDelta accepts two JSON or YAML documents and calculates the difference
// between them.  It returns a Delta object which can be used to resolve
// conflicts against a third version with a common parent, or an error
// if either document is in error.
func NewDelta(from, to []byte) (*Delta, error) {
	d := &Delta{}
	before, err := yaml.ToJSON(from)
	if err != nil {
		return nil, err
	}
	after, err := yaml.ToJSON(to)
	if err != nil {
		return nil, err
	}
	diff, err := jsonpatch.CreateMergePatch(before, after)
	if err != nil {
		return nil, err
	}
	glog.V(6).Infof("Patch created from:\n%s\n%s\n%s", string(before), string(after), string(diff))
	d.original = before
	d.edit = diff
	return d, nil
}

// Apply attempts to apply the changes described by Delta onto latest,
// returning an error if the changes cannot be applied cleanly.
// IsConflicting will be true if the changes overlap, otherwise a
// generic error will be returned.
func (d *Delta) Apply(latest []byte) ([]byte, error) {
	base, err := yaml.ToJSON(latest)
	if err != nil {
		return nil, err
	}
	changes, err := jsonpatch.CreateMergePatch(d.original, base)
	if err != nil {
		return nil, err
	}
	diff1 := make(map[string]interface{})
	if err := json.Unmarshal(d.edit, &diff1); err != nil {
		return nil, err
	}
	diff2 := make(map[string]interface{})
	if err := json.Unmarshal(changes, &diff2); err != nil {
		return nil, err
	}
	for _, fn := range d.preconditions {
		hold1, _ := fn(diff1)
		hold2, _ := fn(diff2)
		if !hold1 || !hold2 {
			return nil, ErrPreconditionFailed
		}
	}

	glog.V(6).Infof("Testing for conflict between:\n%s\n%s", string(d.edit), string(changes))
	hasConflicts, err := mergepatch.HasConflicts(diff1, diff2)
	if err != nil {
		return nil, err
	}
	if hasConflicts {
		return nil, ErrConflict
	}

	return jsonpatch.MergePatch(base, d.edit)
}

// IsConflicting returns true if the provided error indicates a
// conflict exists between the original changes and the applied
// changes.
func IsConflicting(err error) bool {
	return err == ErrConflict
}

// IsPreconditionFailed returns true if the provided error indicates
// a Delta precondition did not succeed.
func IsPreconditionFailed(err error) bool {
	return err == ErrPreconditionFailed
}

var ErrPreconditionFailed = fmt.Errorf("a precondition failed")
var ErrConflict = fmt.Errorf("changes are in conflict")

func (d *Delta) Edit() []byte {
	return d.edit
}