pulumi/pkg/resource/deploy/deployment_executor.go

// Copyright 2016-2018, Pulumi Corporation.
//
// 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 deploy

import (
	"context"
	"errors"
	"fmt"
	"strings"

	"github.com/pulumi/pulumi/pkg/v3/resource/deploy/providers"
	"github.com/pulumi/pulumi/pkg/v3/resource/graph"
	"github.com/pulumi/pulumi/sdk/v3/go/common/diag"
	"github.com/pulumi/pulumi/sdk/v3/go/common/display"
	"github.com/pulumi/pulumi/sdk/v3/go/common/resource"
	"github.com/pulumi/pulumi/sdk/v3/go/common/util/contract"
	"github.com/pulumi/pulumi/sdk/v3/go/common/util/logging"
	"github.com/pulumi/pulumi/sdk/v3/go/common/util/result"
)

// deploymentExecutor is responsible for taking a deployment and driving it to completion.
// Its primary responsibility is to own a `stepGenerator` and `stepExecutor`, serving
// as the glue that links the two subsystems together.
type deploymentExecutor struct {
	deployment *Deployment // The deployment that we are executing

	stepGen  *stepGenerator // step generator owned by this deployment
	stepExec *stepExecutor  // step executor owned by this deployment
}

// A set is returned of all the target URNs to facilitate later callers.  The set can be 'nil'
// indicating no targets, or will be non-nil and non-empty if there are targets.  Only URNs in the
// original array are in the set.  i.e. it's only checked for containment.  The value of the map is
// unused.
func createTargetMap(targets []resource.URN) map[resource.URN]bool {
	if len(targets) == 0 {
		return nil
	}

	targetMap := make(map[resource.URN]bool)
	for _, target := range targets {
		targetMap[target] = true
	}

	return targetMap
}

// checkTargets validates that all the targets passed in refer to existing resources.  Diagnostics
// are generated for any target that cannot be found.  The target must either have existed in the stack
// prior to running the operation, or it must be the urn for a resource that was created.
func (ex *deploymentExecutor) checkTargets(targets []resource.URN, op display.StepOp) result.Result {
	if len(targets) == 0 {
		return nil
	}

	olds := ex.deployment.olds
	var news map[resource.URN]bool
	if ex.stepGen != nil {
		news = ex.stepGen.urns
	}

	hasUnknownTarget := false
	for _, target := range targets {
		hasOld := false
		if _, has := olds[target]; has {
			hasOld = true
		}

		hasNew := news != nil && news[target]
		if !hasOld && !hasNew {
			hasUnknownTarget = true

			logging.V(7).Infof("Resource to %v (%v) could not be found in the stack.", op, target)
			if strings.Contains(string(target), "$") {
				ex.deployment.Diag().Errorf(diag.GetTargetCouldNotBeFoundError(), target)
			} else {
				ex.deployment.Diag().Errorf(diag.GetTargetCouldNotBeFoundDidYouForgetError(), target)
			}
		}
	}

	if hasUnknownTarget {
		return result.Bail()
	}

	return nil
}

func (ex *deploymentExecutor) printPendingOperationsWarning() {
	pendingOperations := ""
	for _, op := range ex.deployment.prev.PendingOperations {
		pendingOperations = pendingOperations + fmt.Sprintf("  * %s, interrupted while %s\n", op.Resource.URN, op.Type)
	}

	resolutionMessage := `These resources are in an unknown state because the Pulumi CLI was interrupted while
waiting for changes to these resources to complete. You should confirm whether or not the
operations listed completed successfully by checking the state of the appropriate provider.
For example, if you are using AWS, you can confirm using the AWS Console.
	
Once you have confirmed the status of the interrupted operations, you can repair your stack
using 'pulumi refresh' which will refresh the state from the provider you are using and 
clear the pending operations if there are any.

Note that 'pulumi refresh' will not clear pending CREATE operations since those could have resulted in resources 
which are not tracked by pulumi. To repair the stack and remove pending CREATE operation, 
use 'pulumi stack export' which will  export your stack to a file. For each operation that succeeded,
remove that operation from the "pending_operations" section of the file. Once this is complete,
use 'pulumi stack import' to import the repaired stack.`

	warning := "Attempting to deploy or update resources " +
		fmt.Sprintf("with %d pending operations from previous deployment.\n", len(ex.deployment.prev.PendingOperations)) +
		pendingOperations +
		resolutionMessage

	ex.deployment.Diag().Warningf(diag.RawMessage("" /*urn*/, warning))
}

// reportExecResult issues an appropriate diagnostic depending on went wrong.
func (ex *deploymentExecutor) reportExecResult(message string, preview bool) {
	kind := "update"
	if preview {
		kind = "preview"
	}

	ex.reportError("", errors.New(kind+" "+message))
}

// reportError reports a single error to the executor's diag stream with the indicated URN for context.
func (ex *deploymentExecutor) reportError(urn resource.URN, err error) {
	ex.deployment.Diag().Errorf(diag.RawMessage(urn, err.Error()))
}

// Execute executes a deployment to completion, using the given cancellation context and running a preview
// or update.
func (ex *deploymentExecutor) Execute(callerCtx context.Context, opts Options, preview bool) (*Plan, result.Result) {
	// Set up a goroutine that will signal cancellation to the deployment's plugins if the caller context is cancelled.
	// We do not hang this off of the context we create below because we do not want the failure of a single step to
	// cause other steps to fail.
	done := make(chan bool)
	defer close(done)
	go func() {
		select {
		case <-callerCtx.Done():
			logging.V(4).Infof("deploymentExecutor.Execute(...): signalling cancellation to providers...")
			cancelErr := ex.deployment.ctx.Host.SignalCancellation()
			if cancelErr != nil {
				logging.V(4).Infof("deploymentExecutor.Execute(...): failed to signal cancellation to providers: %v", cancelErr)
			}
		case <-done:
			logging.V(4).Infof("deploymentExecutor.Execute(...): exiting provider canceller")
		}
	}()

	// If this deployment is an import, run the imports and exit.
	if ex.deployment.isImport {
		return ex.importResources(callerCtx, opts, preview)
	}

	// Before doing anything else, optionally refresh each resource in the base checkpoint.
	if opts.Refresh {
		if res := ex.refresh(callerCtx, opts, preview); res != nil {
			return nil, res
		}
		if opts.RefreshOnly {
			return nil, nil
		}
	} else if ex.deployment.prev != nil && len(ex.deployment.prev.PendingOperations) > 0 && !preview {
		// Print a warning for users that there are pending operations.
		// Explain that these operations can be cleared using pulumi refresh (except for CREATE operations)
		// since these require user intevention:
		ex.printPendingOperationsWarning()
	}

	// The set of -t targets provided on the command line.  'nil' means 'update everything'.
	// Non-nil means 'update only in this set'.  We don't error if the user specifies a target
	// during `update` that we don't know about because it might be the urn for a resource they
	// want to create.
	updateTargetsOpt := createTargetMap(opts.UpdateTargets)
	replaceTargetsOpt := createTargetMap(opts.ReplaceTargets)
	destroyTargetsOpt := createTargetMap(opts.DestroyTargets)
	if res := ex.checkTargets(opts.ReplaceTargets, OpReplace); res != nil {
		return nil, res
	}
	if res := ex.checkTargets(opts.DestroyTargets, OpDelete); res != nil {
		return nil, res
	}

	if (updateTargetsOpt != nil || replaceTargetsOpt != nil) && destroyTargetsOpt != nil {
		contract.Failf("Should not be possible to have both .DestroyTargets and .UpdateTargets or .ReplaceTargets")
	}

	// Begin iterating the source.
	src, res := ex.deployment.source.Iterate(callerCtx, opts, ex.deployment)
	if res != nil {
		return nil, res
	}

	// Set up a step generator for this deployment.
	ex.stepGen = newStepGenerator(ex.deployment, opts, updateTargetsOpt, replaceTargetsOpt)

	// Retire any pending deletes that are currently present in this deployment.
	if res := ex.retirePendingDeletes(callerCtx, opts, preview); res != nil {
		return nil, res
	}

	// Derive a cancellable context for this deployment. We will only cancel this context if some piece of the
	// deployment's execution fails.
	ctx, cancel := context.WithCancel(callerCtx)

	// Set up a step generator and executor for this deployment.
	ex.stepExec = newStepExecutor(ctx, cancel, ex.deployment, opts, preview, false)

	// We iterate the source in its own goroutine because iteration is blocking and we want the main loop to be able to
	// respond to cancellation requests promptly.
	type nextEvent struct {
		Event  SourceEvent
		Result result.Result
	}
	incomingEvents := make(chan nextEvent)
	go func() {
		for {
			event, sourceErr := src.Next()
			select {
			case incomingEvents <- nextEvent{event, sourceErr}:
				if event == nil {
					return
				}
			case <-done:
				logging.V(4).Infof("deploymentExecutor.Execute(...): incoming events goroutine exiting")
				return
			}
		}
	}()

	// The main loop. We'll continuously select for incoming events and the cancellation signal. There are
	// a three ways we can exit this loop:
	//  1. The SourceIterator sends us a `nil` event. This means that we're done processing source events and
	//     we should begin processing deletes.
	//  2. The SourceIterator sends us an error. This means some error occurred in the source program and we
	//     should bail.
	//  3. The stepExecCancel cancel context gets canceled. This means some error occurred in the step executor
	//     and we need to bail. This can also happen if the user hits Ctrl-C.
	canceled, res := func() (bool, result.Result) {
		logging.V(4).Infof("deploymentExecutor.Execute(...): waiting for incoming events")
		for {
			select {
			case event := <-incomingEvents:
				logging.V(4).Infof("deploymentExecutor.Execute(...): incoming event (nil? %v, %v)", event.Event == nil,
					event.Result)

				if event.Result != nil {
					if !event.Result.IsBail() {
						logging.V(4).Infof("deploymentExecutor.Execute(...): error running Program: %v", event.Result.Error())
					}
					cancel()

					// We reported any errors above.  So we can just bail now.
					return false, result.Bail()
				}

				if event.Event == nil {
					res := ex.performDeletes(ctx, updateTargetsOpt, destroyTargetsOpt)
					if res != nil {
						if resErr := res.Error(); resErr != nil {
							logging.V(4).Infof("deploymentExecutor.Execute(...): error performing deletes: %v", resErr)
							ex.reportError("", resErr)
							return false, result.Bail()
						}
					}
					return false, res
				}

				if res := ex.handleSingleEvent(event.Event); res != nil {
					if resErr := res.Error(); resErr != nil {
						logging.V(4).Infof("deploymentExecutor.Execute(...): error handling event: %v", resErr)
						ex.reportError(ex.deployment.generateEventURN(event.Event), resErr)
					}
					cancel()
					return false, result.Bail()
				}
			case <-ctx.Done():
				logging.V(4).Infof("deploymentExecutor.Execute(...): context finished: %v", ctx.Err())

				// NOTE: we use the presence of an error in the caller context in order to distinguish caller-initiated
				// cancellation from internally-initiated cancellation.
				return callerCtx.Err() != nil, nil
			}
		}
	}()

	ex.stepExec.WaitForCompletion()
	logging.V(4).Infof("deploymentExecutor.Execute(...): step executor has completed")

	// Now that we've performed all steps in the deployment, ensure that the list of targets to update was
	// valid.  We have to do this *after* performing the steps as the target list may have referred
	// to a resource that was created in one of hte steps.
	if res == nil {
		res = ex.checkTargets(opts.UpdateTargets, OpUpdate)
	}

	// Check that we did operations for everything expected in the plan. We mutate ResourcePlan.Ops as we run
	// so by the time we get here everything in the map should have an empty ops list (except for unneeded
	// deletes). We skip this check if we already have an error, chances are if the deployment failed lots of
	// operations wouldn't have got a chance to run so we'll spam errors about all of those failed operations
	// making it less clear to the user what the root cause error was.
	if res == nil && ex.deployment.plan != nil {
		for urn, resourcePlan := range ex.deployment.plan.ResourcePlans {
			if len(resourcePlan.Ops) != 0 {
				if len(resourcePlan.Ops) == 1 && resourcePlan.Ops[0] == OpDelete {
					// We haven't done a delete for this resource check if it was in the snapshot,
					// if it's already gone this wasn't done because it wasn't needed
					found := false
					for i := range ex.deployment.prev.Resources {
						if ex.deployment.prev.Resources[i].URN == urn {
							found = true
							break
						}
					}

					// Didn't find the resource in the old snapshot so this was just an unneeded delete
					if !found {
						continue
					}
				}

				err := fmt.Errorf("expected resource operations for %v but none were seen", urn)
				logging.V(4).Infof("deploymentExecutor.Execute(...): error handling event: %v", err)
				ex.reportError(urn, err)
				res = result.Bail()
			}
		}
	}

	if res != nil && res.IsBail() {
		return nil, res
	}

	// If the step generator and step executor were both successful, then we send all the resources
	// observed to be analyzed. Otherwise, this step is skipped.
	if res == nil && !ex.stepExec.Errored() {
		res := ex.stepGen.AnalyzeResources()
		if res != nil {
			if resErr := res.Error(); resErr != nil {
				logging.V(4).Infof("deploymentExecutor.Execute(...): error analyzing resources: %v", resErr)
				ex.reportError("", resErr)
			}
			return nil, result.Bail()
		}
	}

	// Figure out if execution failed and why. Step generation and execution errors trump cancellation.
	if res != nil || ex.stepExec.Errored() || ex.stepGen.Errored() {
		// TODO(cyrusn): We seem to be losing any information about the original 'res's errors.  Should
		// we be doing a merge here?
		ex.reportExecResult("failed", preview)
		return nil, result.Bail()
	} else if canceled {
		ex.reportExecResult("canceled", preview)
		return nil, result.Bail()
	}

	return ex.deployment.newPlans.plan(), res
}

func (ex *deploymentExecutor) performDeletes(
	ctx context.Context, updateTargetsOpt, destroyTargetsOpt map[resource.URN]bool) result.Result {

	defer func() {
		// We're done here - signal completion so that the step executor knows to terminate.
		ex.stepExec.SignalCompletion()
	}()

	prev := ex.deployment.prev
	if prev == nil || len(prev.Resources) == 0 {
		return nil
	}

	logging.V(7).Infof("performDeletes(...): beginning")

	// At this point we have generated the set of resources above that we would normally want to
	// delete.  However, if the user provided -target's we will only actually delete the specific
	// resources that are in the set explicitly asked for.
	var targetsOpt map[resource.URN]bool
	if updateTargetsOpt != nil {
		targetsOpt = updateTargetsOpt
	} else if destroyTargetsOpt != nil {
		targetsOpt = destroyTargetsOpt
	}

	deleteSteps, res := ex.stepGen.GenerateDeletes(targetsOpt)
	if res != nil {
		logging.V(7).Infof("performDeletes(...): generating deletes produced error result")
		return res
	}

	deletes := ex.stepGen.ScheduleDeletes(deleteSteps)

	// ScheduleDeletes gives us a list of lists of steps. Each list of steps can safely be executed
	// in parallel, but each list must execute completes before the next list can safely begin
	// executing.
	//
	// This is not "true" delete parallelism, since there may be resources that could safely begin
	// deleting but we won't until the previous set of deletes fully completes. This approximation
	// is conservative, but correct.
	for _, antichain := range deletes {
		logging.V(4).Infof("deploymentExecutor.Execute(...): beginning delete antichain")
		tok := ex.stepExec.ExecuteParallel(antichain)
		tok.Wait(ctx)
		logging.V(4).Infof("deploymentExecutor.Execute(...): antichain complete")
	}

	// After executing targeted deletes, we may now have resources that depend on the resource that
	// were deleted.  Go through and clean things up accordingly for them.
	if targetsOpt != nil {
		resourceToStep := make(map[*resource.State]Step)
		for _, step := range deleteSteps {
			resourceToStep[ex.deployment.olds[step.URN()]] = step
		}

		ex.rebuildBaseState(resourceToStep, false /*refresh*/)
	}

	return nil
}

// handleSingleEvent handles a single source event. For all incoming events, it produces a chain that needs
// to be executed and schedules the chain for execution.
func (ex *deploymentExecutor) handleSingleEvent(event SourceEvent) result.Result {
	contract.Require(event != nil, "event != nil")

	var steps []Step
	var res result.Result
	switch e := event.(type) {
	case RegisterResourceEvent:
		logging.V(4).Infof("deploymentExecutor.handleSingleEvent(...): received RegisterResourceEvent")
		steps, res = ex.stepGen.GenerateSteps(e)
	case ReadResourceEvent:
		logging.V(4).Infof("deploymentExecutor.handleSingleEvent(...): received ReadResourceEvent")
		steps, res = ex.stepGen.GenerateReadSteps(e)
	case RegisterResourceOutputsEvent:
		logging.V(4).Infof("deploymentExecutor.handleSingleEvent(...): received register resource outputs")
		return ex.stepExec.ExecuteRegisterResourceOutputs(e)
	}

	if res != nil {
		return res
	}

	ex.stepExec.ExecuteSerial(steps)
	return nil
}

// retirePendingDeletes deletes all resources that are pending deletion. Run before the start of a deployment, this pass
// ensures that the engine never sees any resources that are pending deletion from a previous deployment.
//
// retirePendingDeletes re-uses the deployment executor's step generator but uses its own step executor.
func (ex *deploymentExecutor) retirePendingDeletes(callerCtx context.Context, opts Options,
	preview bool) result.Result {

	contract.Require(ex.stepGen != nil, "ex.stepGen != nil")
	steps := ex.stepGen.GeneratePendingDeletes()
	if len(steps) == 0 {
		logging.V(4).Infoln("deploymentExecutor.retirePendingDeletes(...): no pending deletions")
		return nil
	}

	logging.V(4).Infof("deploymentExecutor.retirePendingDeletes(...): executing %d steps", len(steps))
	ctx, cancel := context.WithCancel(callerCtx)

	stepExec := newStepExecutor(ctx, cancel, ex.deployment, opts, preview, false)
	antichains := ex.stepGen.ScheduleDeletes(steps)
	// Submit the deletes for execution and wait for them all to retire.
	for _, antichain := range antichains {
		for _, step := range antichain {
			ex.deployment.Ctx().StatusDiag.Infof(diag.RawMessage(step.URN(), "completing deletion from previous update"))
		}

		tok := stepExec.ExecuteParallel(antichain)
		tok.Wait(ctx)
	}

	stepExec.SignalCompletion()
	stepExec.WaitForCompletion()

	// Like Refresh, we use the presence of an error in the caller's context to detect whether or not we have been
	// cancelled.
	canceled := callerCtx.Err() != nil
	if stepExec.Errored() {
		ex.reportExecResult("failed", preview)
		return result.Bail()
	} else if canceled {
		ex.reportExecResult("canceled", preview)
		return result.Bail()
	}
	return nil
}

// import imports a list of resources into a stack.
func (ex *deploymentExecutor) importResources(
	callerCtx context.Context,
	opts Options,
	preview bool) (*Plan, result.Result) {

	if len(ex.deployment.imports) == 0 {
		return nil, nil
	}

	// Create an executor for this import.
	ctx, cancel := context.WithCancel(callerCtx)
	stepExec := newStepExecutor(ctx, cancel, ex.deployment, opts, preview, true)

	importer := &importer{
		deployment: ex.deployment,
		executor:   stepExec,
		preview:    preview,
	}
	res := importer.importResources(ctx)
	stepExec.SignalCompletion()
	stepExec.WaitForCompletion()

	// NOTE: we use the presence of an error in the caller context in order to distinguish caller-initiated
	// cancellation from internally-initiated cancellation.
	canceled := callerCtx.Err() != nil

	if res != nil || stepExec.Errored() {
		if res != nil && res.Error() != nil {
			ex.reportExecResult(fmt.Sprintf("failed: %s", res.Error()), preview)
		} else {
			ex.reportExecResult("failed", preview)
		}
		return nil, result.Bail()
	} else if canceled {
		ex.reportExecResult("canceled", preview)
		return nil, result.Bail()
	}
	return ex.deployment.newPlans.plan(), nil
}

// refresh refreshes the state of the base checkpoint file for the current deployment in memory.
func (ex *deploymentExecutor) refresh(callerCtx context.Context, opts Options, preview bool) result.Result {
	prev := ex.deployment.prev
	if prev == nil || len(prev.Resources) == 0 {
		return nil
	}

	// Make sure if there were any targets specified, that they all refer to existing resources.
	if res := ex.checkTargets(opts.RefreshTargets, OpRefresh); res != nil {
		return res
	}

	// If the user did not provide any --target's, create a refresh step for each resource in the
	// old snapshot.  If they did provider --target's then only create refresh steps for those
	// specific targets.
	steps := []Step{}
	resourceToStep := map[*resource.State]Step{}
	targetMapOpt := createTargetMap(opts.RefreshTargets)
	for _, res := range prev.Resources {
		if targetMapOpt == nil || targetMapOpt[res.URN] {
			step := NewRefreshStep(ex.deployment, res, nil)
			steps = append(steps, step)
			resourceToStep[res] = step
		}
	}

	// Fire up a worker pool and issue each refresh in turn.
	ctx, cancel := context.WithCancel(callerCtx)
	stepExec := newStepExecutor(ctx, cancel, ex.deployment, opts, preview, true)
	stepExec.ExecuteParallel(steps)
	stepExec.SignalCompletion()
	stepExec.WaitForCompletion()

	ex.rebuildBaseState(resourceToStep, true /*refresh*/)

	// NOTE: we use the presence of an error in the caller context in order to distinguish caller-initiated
	// cancellation from internally-initiated cancellation.
	canceled := callerCtx.Err() != nil

	if stepExec.Errored() {
		ex.reportExecResult("failed", preview)
		return result.Bail()
	} else if canceled {
		ex.reportExecResult("canceled", preview)
		return result.Bail()
	}
	return nil
}

func (ex *deploymentExecutor) rebuildBaseState(resourceToStep map[*resource.State]Step, refresh bool) {
	// Rebuild this deployment's map of old resources and dependency graph, stripping out any deleted
	// resources and repairing dependency lists as necessary. Note that this updates the base
	// snapshot _in memory_, so it is critical that any components that use the snapshot refer to
	// the same instance and avoid reading it concurrently with this rebuild.
	//
	// The process of repairing dependency lists is a bit subtle. Because multiple physical
	// resources may share a URN, the ability of a particular URN to be referenced in a dependency
	// list can change based on the dependent resource's position in the resource list. For example,
	// consider the following list of resources, where each resource is a (URN, ID, Dependencies)
	// tuple:
	//
	//     [ (A, 0, []), (B, 0, [A]), (A, 1, []), (A, 2, []), (C, 0, [A]) ]
	//
	// Let `(A, 0, [])` and `(A, 2, [])` be deleted by the refresh. This produces the following
	// intermediate list before dependency lists are repaired:
	//
	//     [ (B, 0, [A]), (A, 1, []), (C, 0, [A]) ]
	//
	// In order to repair the dependency lists, we iterate over the intermediate resource list,
	// keeping track of which URNs refer to at least one physical resource at each point in the
	// list, and remove any dependencies that refer to URNs that do not refer to any physical
	// resources. This process produces the following final list:
	//
	//     [ (B, 0, []), (A, 1, []), (C, 0, [A]) ]
	//
	// Note that the correctness of this process depends on the fact that the list of resources is a
	// topological sort of its corresponding dependency graph, so a resource always appears in the
	// list after any resources on which it may depend.
	resources := []*resource.State{}
	referenceable := make(map[resource.URN]bool)
	olds := make(map[resource.URN]*resource.State)
	for _, s := range ex.deployment.prev.Resources {
		var old, new *resource.State
		if step, has := resourceToStep[s]; has {
			// We produced a refresh step for this specific resource.  Use the new information about
			// its dependencies during the update.
			old = step.Old()
			new = step.New()
		} else {
			// We didn't do anything with this resource.  However, we still may want to update its
			// dependencies.  So use this resource itself as the 'new' one to update.
			old = s
			new = s
		}

		if new == nil {
			if refresh {
				contract.Assertf(old.Custom, "Expected custom resource")
				contract.Assert(!providers.IsProviderType(old.Type))
			}
			continue
		}

		// Remove any deleted resources from this resource's dependency list.
		if len(new.Dependencies) != 0 {
			deps := make([]resource.URN, 0, len(new.Dependencies))
			for _, d := range new.Dependencies {
				if referenceable[d] {
					deps = append(deps, d)
				}
			}
			new.Dependencies = deps
		}

		// Add this resource to the resource list and mark it as referenceable.
		resources = append(resources, new)
		referenceable[new.URN] = true

		// Do not record resources that are pending deletion in the "olds" lookup table.
		if !new.Delete {
			olds[new.URN] = new
		}
	}

	undangleParentResources(olds, resources)

	ex.deployment.prev.Resources = resources
	ex.deployment.olds, ex.deployment.depGraph = olds, graph.NewDependencyGraph(resources)
}

func undangleParentResources(undeleted map[resource.URN]*resource.State, resources []*resource.State) {
	// Since a refresh may delete arbitrary resources, we need to handle the case where
	// the parent of a still existing resource is deleted.
	//
	// Invalid parents need to be fixed since otherwise they leave the state invalid, and
	// the user sees an error:
	// ```
	// snapshot integrity failure; refusing to use it: child resource ${validURN} refers to missing parent ${deletedURN}
	// ```
	// To solve the problem we traverse the topologically sorted list of resources in
	// order, setting newly invalidated parent URNS to the URN of the parent's parent.
	//
	// This can be illustrated by an example. Consider the graph of resource parents:
	//
	//         A            xBx
	//       /   \           |
	//    xCx      D        xEx
	//     |     /   \       |
	//     F    G     xHx    I
	//
	// When a capital letter is marked for deletion, it is bracketed by `x`s.
	// We can obtain a topological sort by reading left to right, top to bottom.
	//
	// A..D -> valid parents, so we do nothing
	// E -> The parent of E is marked for deletion, so set E.Parent to E.Parent.Parent.
	//      Since B (E's parent) has no parent, we set E.Parent to "".
	// F -> The parent of F is marked for deletion, so set F.Parent to F.Parent.Parent.
	//      We set F.Parent to "A"
	// G, H -> valid parents, do nothing
	// I -> The parent of I is marked for deletion, so set I.Parent to I.Parent.Parent.
	//      The parent of I has parent "", (since we addressed the parent of E
	//      previously), so we set I.Parent = "".
	//
	// The new graph looks like this:
	//
	//         A        xBx   xEx   I
	//       / | \
	//     xCx F  D
	//          /   \
	//         G    xHx
	// We observe that it is perfectly valid for deleted nodes to be leaf nodes, but they
	// cannot be intermediary nodes.
	_, hasEmptyValue := undeleted[""]
	contract.Assertf(!hasEmptyValue, "the zero value for an URN is not a valid URN")
	availableParents := map[resource.URN]resource.URN{}
	for _, r := range resources {
		if _, ok := undeleted[r.Parent]; !ok {
			// Since existing must obey a topological sort, we have already addressed
			// p.Parent. Since we know that it doesn't dangle, and that r.Parent no longer
			// exists, we set r.Parent as r.Parent.Parent.
			r.Parent = availableParents[r.Parent]
		}
		availableParents[r.URN] = r.Parent
	}
}