changes.go

package fi

import (
	"github.com/golang/glog"
	"reflect"
)

// An important part of our state synchronization is to compare two tasks, to see what has changed
// Doing so means that the tasks don't have to have this logic, and we can reuse this for dry-run.
// We do so using reflection.  We have a custom notion of equality for Resources and for Tasks that implement HasID.
// A task that implements CompareWithID is compared by ID alone.

// BuildChanges compares the values of a & e, and populates differences into changes,
// except that if a value is nil in e, the corresponding value in a is ignored.
// a, e and changes must all be of the same type
// a is the actual object found, e is the expected value
// Note that the ignore-nil-in-e logic therefore implements the idea that nil value in e means "don't care"
// If a is nil, all the non-nil values in e will be copied over to changes, because every field in e must be applied
func BuildChanges(a, e, changes interface{}) bool {
	changed := false

	vc := reflect.ValueOf(changes)
	vc = vc.Elem()
	t := vc.Type()

	ve := reflect.ValueOf(e)
	ve = ve.Elem()
	if t != ve.Type() {
		panic("mismatched types in BuildChanges")
	}

	va := reflect.ValueOf(a)
	aIsNil := false
	if va.IsNil() {
		aIsNil = true
	}
	if !aIsNil {
		va = va.Elem()

		if t != va.Type() {
			panic("mismatched types in BuildChanges")
		}
	}

	for i := 0; i < ve.NumField(); i++ {
		if t.Field(i).PkgPath != "" {
			// unexported: ignore
			continue
		}

		fve := ve.Field(i)
		if fve.Kind() == reflect.Ptr && fve.IsNil() {
			// Nil expected value means 'don't care'
			continue
		}

		if !aIsNil {
			fva := va.Field(i)

			if equalFieldValues(fva, fve) {
				continue
			}

			glog.V(8).Infof("Field changed %q actual=%q expected=%q", t.Field(i).Name, DebugPrint(fva.Interface()), DebugPrint(fve.Interface()))
		}
		changed = true
		vc.Field(i).Set(fve)
	}

	return changed
}

// equalFieldValues implements our equality checking, with special cases for resources and tasks
func equalFieldValues(a, e reflect.Value) bool {
	if !a.IsValid() || !e.IsValid() {
		return a.IsValid() == e.IsValid()
	}

	if a.Kind() == reflect.Map {
		return equalMapValues(a, e)
	}
	if a.Kind() == reflect.Slice {
		return equalSlice(a, e)
	}
	if (a.Kind() == reflect.Ptr || a.Kind() == reflect.Interface) && !a.IsNil() {
		aHasID, ok := a.Interface().(CompareWithID)
		if ok && (e.Kind() == reflect.Ptr || e.Kind() == reflect.Interface) && !e.IsNil() {
			eHasID, ok := e.Interface().(CompareWithID)
			if ok {
				aID := aHasID.CompareWithID()
				eID := eHasID.CompareWithID()
				if aID != nil && eID != nil && *aID == *eID {
					return true
				}
			}
		}

		aResource, ok := a.Interface().(Resource)
		if ok && (e.Kind() == reflect.Ptr || e.Kind() == reflect.Interface) && !e.IsNil() {
			eResource, ok := e.Interface().(Resource)
			if ok {
				same, err := ResourcesMatch(aResource, eResource)
				if err != nil {
					glog.Fatalf("error while comparing resources: %v", err)
				} else {
					return same
				}
			}
		}
	}
	if reflect.DeepEqual(a.Interface(), e.Interface()) {
		return true
	}
	return false
}

// equalMapValues performs a deep-equality check on a map, but using our custom comparison logic (equalFieldValues)
func equalMapValues(a, e reflect.Value) bool {
	if a.IsNil() != e.IsNil() {
		return false
	}
	if a.IsNil() && e.IsNil() {
		return true
	}
	if a.Len() != e.Len() {
		return false
	}
	for _, k := range a.MapKeys() {
		valA := a.MapIndex(k)
		valE := e.MapIndex(k)

		glog.V(10).Infof("comparing maps: %v %v %v", k, valA, valE)

		if !equalFieldValues(valA, valE) {
			glog.V(4).Infof("unequal map value: %v %v %v", k, valA, valE)
			return false
		}
	}
	return true
}

// equalSlice performs a deep-equality check on a slice, but using our custom comparison logic (equalFieldValues)
func equalSlice(a, e reflect.Value) bool {
	if a.IsNil() != e.IsNil() {
		return false
	}
	if a.IsNil() && e.IsNil() {
		return true
	}
	if a.Len() != e.Len() {
		return false
	}
	for i := 0; i < a.Len(); i++ {
		valA := a.Index(i)
		valE := e.Index(i)

		glog.V(10).Infof("comparing slices: %d %v %v", i, valA, valE)

		if !equalFieldValues(valA, valE) {
			glog.V(4).Infof("unequal slice value: %d %v %v", i, valA, valE)
			return false
		}
	}
	return true
}