pulumi/pkg/resource/deploy/step_generator.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 (
	"github.com/pulumi/pulumi/pkg/diag"
	"github.com/pulumi/pulumi/pkg/resource"
	"github.com/pulumi/pulumi/pkg/resource/deploy/providers"
	"github.com/pulumi/pulumi/pkg/resource/graph"
	"github.com/pulumi/pulumi/pkg/resource/plugin"
	"github.com/pulumi/pulumi/pkg/tokens"
	"github.com/pulumi/pulumi/pkg/util/contract"
	"github.com/pulumi/pulumi/pkg/util/logging"
	"github.com/pulumi/pulumi/pkg/util/result"
)

// stepGenerator is responsible for turning resource events into steps that
// can be fed to the plan executor. It does this by consulting the plan
// and calculating the appropriate step action based on the requested goal
// state and the existing state of the world.
type stepGenerator struct {
	plan *Plan   // the plan to which this step generator belongs
	opts Options // options for this step generator

	urns           map[resource.URN]bool    // set of URNs discovered for this plan
	reads          map[resource.URN]bool    // set of URNs read for this plan
	deletes        map[resource.URN]bool    // set of URNs deleted in this plan
	replaces       map[resource.URN]bool    // set of URNs replaced in this plan
	updates        map[resource.URN]bool    // set of URNs updated in this plan
	creates        map[resource.URN]bool    // set of URNs created in this plan
	sames          map[resource.URN]bool    // set of URNs that were not changed in this plan
	pendingDeletes map[*resource.State]bool // set of resources (not URNs!) that are pending deletion

	// a map from URN to a list of property keys that caused the replacement of a dependent resource during a
	// delete-before-replace.
	dependentReplaceKeys map[resource.URN][]resource.PropertyKey
}

// GenerateReadSteps is responsible for producing one or more steps required to service
// a ReadResourceEvent coming from the language host.
func (sg *stepGenerator) GenerateReadSteps(event ReadResourceEvent) ([]Step, result.Result) {
	urn := sg.plan.generateURN(event.Parent(), event.Type(), event.Name())
	newState := resource.NewState(event.Type(),
		urn,
		true,  /*custom*/
		false, /*delete*/
		event.ID(),
		event.Properties(),
		make(resource.PropertyMap), /* outputs */
		event.Parent(),
		false, /*protect*/
		true,  /*external*/
		event.Dependencies(),
		nil, /* initErrors */
		event.Provider(),
		nil,   /* propertyDependencies */
		false, /* deleteBeforeCreate */
		event.AdditionalSecretOutputs())
	old, hasOld := sg.plan.Olds()[urn]

	// If the snapshot has an old resource for this URN and it's not external, we're going
	// to have to delete the old resource and conceptually replace it with the resource we
	// are about to read.
	//
	// We accomplish this through the "read-replacement" step, which atomically reads a resource
	// and marks the resource it is replacing as pending deletion.
	//
	// In the event that the new "read" resource's ID matches the existing resource,
	// we do not need to delete the resource - we know exactly what resource we are going
	// to get from the read.
	//
	// This operation is tenatively called "relinquish" - it semantically represents the
	// release of a resource from the management of Pulumi.
	if hasOld && !old.External && old.ID != event.ID() {
		logging.V(7).Infof(
			"stepGenerator.GenerateReadSteps(...): replacing existing resource %s, ids don't match", urn)
		sg.replaces[urn] = true
		return []Step{
			NewReadReplacementStep(sg.plan, event, old, newState),
			NewReplaceStep(sg.plan, old, newState, nil, nil, true),
		}, nil
	}

	if bool(logging.V(7)) && hasOld && old.ID == event.ID() {
		logging.V(7).Infof("stepGenerator.GenerateReadSteps(...): recognized relinquish of resource %s", urn)
	}

	sg.reads[urn] = true
	return []Step{
		NewReadStep(sg.plan, event, old, newState),
	}, nil
}

// GenerateSteps produces one or more steps required to achieve the goal state
// specified by the incoming RegisterResourceEvent.
//
// If the given resource is a custom resource, the step generator will invoke Diff
// and Check on the provider associated with that resource. If those fail, an error
// is returned.
func (sg *stepGenerator) GenerateSteps(event RegisterResourceEvent) ([]Step, result.Result) {
	var invalid bool // will be set to true if this object fails validation.

	goal := event.Goal()
	// generate an URN for this new resource.
	urn := sg.plan.generateURN(goal.Parent, goal.Type, goal.Name)
	if sg.urns[urn] {
		invalid = true
		// TODO[pulumi/pulumi-framework#19]: improve this error message!
		sg.plan.Diag().Errorf(diag.GetDuplicateResourceURNError(urn), urn)
	}
	sg.urns[urn] = true

	// Check for an old resource so that we can figure out if this is a create, delete, etc., and/or to diff.
	old, hasOld := sg.plan.Olds()[urn]
	var oldInputs resource.PropertyMap
	var oldOutputs resource.PropertyMap
	if hasOld {
		oldInputs = old.Inputs
		oldOutputs = old.Outputs
	}

	// Create the desired inputs from the goal state
	inputs := goal.Properties
	if hasOld {
		// Set inputs back to their old values (if any) for any "ignored" properties
		inputs = sg.processIgnoreChanges(inputs, oldInputs, goal.IgnoreChanges)
	}

	// Produce a new state object that we'll build up as operations are performed.  Ultimately, this is what will
	// get serialized into the checkpoint file.
	new := resource.NewState(goal.Type, urn, goal.Custom, false, "", inputs, nil, goal.Parent, goal.Protect, false,
		goal.Dependencies, goal.InitErrors, goal.Provider, goal.PropertyDependencies, false, goal.AdditionalSecretOutputs)

	// Fetch the provider for this resource.
	prov, res := sg.loadResourceProvider(urn, goal.Custom, goal.Provider, goal.Type)
	if res != nil {
		return nil, res
	}

	// We only allow unknown property values to be exposed to the provider if we are performing an update preview.
	allowUnknowns := sg.plan.preview

	// We may be re-creating this resource if it got deleted earlier in the execution of this plan.
	_, recreating := sg.deletes[urn]

	// We may be creating this resource if it previously existed in the snapshot as an External resource
	wasExternal := hasOld && old.External

	// Ensure the provider is okay with this resource and fetch the inputs to pass to subsequent methods.
	var err error
	if prov != nil {
		var failures []plugin.CheckFailure

		// If we are re-creating this resource because it was deleted earlier, the old inputs are now
		// invalid (they got deleted) so don't consider them. Similarly, if the old resource was External,
		// don't consider those inputs since Pulumi does not own them.
		if recreating || wasExternal {
			inputs, failures, err = prov.Check(urn, nil, goal.Properties, allowUnknowns)
		} else {
			inputs, failures, err = prov.Check(urn, oldInputs, inputs, allowUnknowns)
		}

		if err != nil {
			return nil, result.FromError(err)
		} else if sg.issueCheckErrors(new, urn, failures) {
			invalid = true
		}
		new.Inputs = inputs
	}

	// Next, give each analyzer -- if any -- a chance to inspect the resource too.
	for _, a := range sg.plan.analyzers {
		var analyzer plugin.Analyzer
		analyzer, err = sg.plan.ctx.Host.Analyzer(a)
		if err != nil {
			return nil, result.FromError(err)
		} else if analyzer == nil {
			return nil, result.Errorf("analyzer '%v' could not be loaded from your $PATH", a)
		}
		var failures []plugin.AnalyzeFailure
		failures, err = analyzer.Analyze(new.Type, inputs)
		if err != nil {
			return nil, result.FromError(err)
		}
		for _, failure := range failures {
			invalid = true
			sg.plan.Diag().Errorf(
				diag.GetAnalyzeResourceFailureError(urn), a, urn, failure.Property, failure.Reason)
		}
	}

	// If the resource isn't valid, don't proceed any further.
	if invalid {
		return nil, result.Bail()
	}

	// There are four cases we need to consider when figuring out what to do with this resource.
	//
	// Case 1: recreating
	//  In this case, we have seen a resource with this URN before and we have already issued a
	//  delete step for it. This happens when the engine has to delete a resource before it has
	//  enough information about whether that resource still exists. A concrete example is
	//  when a resource depends on a resource that is delete-before-replace: the engine must first
	//  delete the dependent resource before depending the DBR resource, but the engine can't know
	//  yet whether the dependent resource is being replaced or deleted.
	//
	//  In this case, we are seeing the resource again after deleting it, so it must be a replacement.
	//
	//  Logically, recreating implies hasOld, since in order to delete something it must have
	//  already existed.
	contract.Assert(!recreating || hasOld)
	if recreating {
		logging.V(7).Infof("Planner decided to re-create replaced resource '%v' deleted due to dependent DBR", urn)

		// Unmark this resource as deleted, we now know it's being replaced instead.
		delete(sg.deletes, urn)
		sg.replaces[urn] = true
		keys := sg.dependentReplaceKeys[urn]
		return []Step{
			NewReplaceStep(sg.plan, old, new, nil, nil, false),
			NewCreateReplacementStep(sg.plan, event, old, new, keys, nil, false),
		}, nil
	}

	// Case 2: wasExternal
	//  In this case, the resource we are operating upon exists in the old snapshot, but it
	//  was "external" - Pulumi does not own its lifecycle. Conceptually, this operation is
	//  akin to "taking ownership" of a resource that we did not previously control.
	//
	//  Since we are not allowed to manipulate the existing resource, we must create a resource
	//  to take its place. Since this is technically a replacement operation, we pend deletion of
	//  read until the end of the plan.
	if wasExternal {
		logging.V(7).Infof("Planner recognized '%s' as old external resource, creating instead", urn)
		sg.creates[urn] = true
		if err != nil {
			return nil, result.FromError(err)
		}

		return []Step{
			NewCreateReplacementStep(sg.plan, event, old, new, nil, nil, true),
			NewReplaceStep(sg.plan, old, new, nil, nil, true),
		}, nil
	}

	// Case 3: hasOld
	//  In this case, the resource we are operating upon now exists in the old snapshot.
	//  It must be an update or a replace. Which operation we do depends on the the specific change made to the
	//  resource's properties:
	//  - If the resource's provider reference changed, the resource must be replaced. This behavior is founded upon
	//    the assumption that providers are recreated iff their configuration changed in such a way that they are no
	//    longer able to manage existing resources.
	//  - Otherwise, we invoke the resource's provider's `Diff` method. If this method indicates that the resource must
	//    be replaced, we do so. If it does not, we update the resource in place.
	if hasOld {
		contract.Assert(old != nil && old.Type == new.Type)

		var diff plugin.DiffResult
		if old.Provider != new.Provider {
			diff = plugin.DiffResult{Changes: plugin.DiffSome, ReplaceKeys: []resource.PropertyKey{"provider"}}
		} else {
			// Determine whether the change resulted in a diff.
			d, diffErr := sg.diff(urn, old.ID, oldInputs, oldOutputs, inputs, prov, allowUnknowns)
			if diffErr != nil {
				// If the plugin indicated that the diff is unavailable, assume that the resource will be updated and
				// report the message contained in the error.
				//nolint
				if _, ok := diffErr.(plugin.DiffUnavailableError); ok {
					d = plugin.DiffResult{Changes: plugin.DiffSome}
					sg.plan.ctx.Diag.Warningf(diag.RawMessage(urn, diffErr.Error()))
				} else {
					return nil, result.FromError(diffErr)
				}
			}
			diff = d
		}

		// Ensure that we received a sensible response.
		if diff.Changes != plugin.DiffNone && diff.Changes != plugin.DiffSome {
			return nil, result.Errorf(
				"unrecognized diff state for %s: %d", urn, diff.Changes)
		}

		// If there were changes, check for a replacement vs. an in-place update.
		if diff.Changes == plugin.DiffSome {
			if diff.Replace() {
				sg.replaces[urn] = true

				// If we are going to perform a replacement, we need to recompute the default values.  The above logic
				// had assumed that we were going to carry them over from the old resource, which is no longer true.
				if prov != nil {
					var failures []plugin.CheckFailure
					inputs, failures, err = prov.Check(urn, nil, goal.Properties, allowUnknowns)
					if err != nil {
						return nil, result.FromError(err)
					} else if sg.issueCheckErrors(new, urn, failures) {
						return nil, result.Bail()
					}
					new.Inputs = inputs
				}

				if logging.V(7) {
					logging.V(7).Infof("Planner decided to replace '%v' (oldprops=%v inputs=%v)",
						urn, oldInputs, new.Inputs)
				}

				// We have two approaches to performing replacements:
				//
				//     * CreateBeforeDelete: the default mode first creates a new instance of the resource, then
				//       updates all dependent resources to point to the new one, and finally after all of that,
				//       deletes the old resource.  This ensures minimal downtime.
				//
				//     * DeleteBeforeCreate: this mode can be used for resources that cannot be tolerate having
				//       side-by-side old and new instances alive at once.  This first deletes the resource and
				//       then creates the new one.  This may result in downtime, so is less preferred.  Note that
				//       until pulumi/pulumi#624 is resolved, we cannot safely perform this operation on resources
				//       that have dependent resources (we try to delete the resource while they refer to it).
				//
				// The provider is responsible for requesting which of these two modes to use.

				if diff.DeleteBeforeReplace || goal.DeleteBeforeReplace {
					logging.V(7).Infof("Planner decided to delete-before-replacement for resource '%v'", urn)
					contract.Assert(sg.plan.depGraph != nil)

					// DeleteBeforeCreate implies that we must immediately delete the resource. For correctness,
					// we must also eagerly delete all resources that depend directly or indirectly on the resource
					// being replaced and would be replaced by a change to the relevant dependency.
					//
					// To do this, we'll utilize the dependency information contained in the snapshot if it is
					// trustworthy, which is interpreted by the DependencyGraph type.
					var steps []Step
					if sg.opts.TrustDependencies {
						toReplace, res := sg.calculateDependentReplacements(old)
						if res != nil {
							return nil, res
						}

						// Deletions must occur in reverse dependency order, and `deps` is returned in dependency
						// order, so we iterate in reverse.
						for i := len(toReplace) - 1; i >= 0; i-- {
							dependentResource := toReplace[i].res

							// If we already deleted this resource due to some other DBR, don't do it again.
							if sg.deletes[dependentResource.URN] {
								continue
							}

							sg.dependentReplaceKeys[dependentResource.URN] = toReplace[i].keys

							logging.V(7).Infof("Planner decided to delete '%v' due to dependence on condemned resource '%v'",
								dependentResource.URN, urn)

							steps = append(steps, NewDeleteReplacementStep(sg.plan, dependentResource, true))
							// Mark the condemned resource as deleted. We won't know until later in the plan whether
							// or not we're going to be replacing this resource.
							sg.deletes[dependentResource.URN] = true
						}
					}

					return append(steps,
						NewDeleteReplacementStep(sg.plan, old, true),
						NewReplaceStep(sg.plan, old, new, diff.ReplaceKeys, diff.ChangedKeys, false),
						NewCreateReplacementStep(sg.plan, event, old, new, diff.ReplaceKeys, diff.ChangedKeys, false),
					), nil
				}

				return []Step{
					NewCreateReplacementStep(sg.plan, event, old, new, diff.ReplaceKeys, diff.ChangedKeys, true),
					NewReplaceStep(sg.plan, old, new, diff.ReplaceKeys, diff.ChangedKeys, true),
					// note that the delete step is generated "later" on, after all creates/updates finish.
				}, nil
			}

			// If we fell through, it's an update.
			sg.updates[urn] = true
			if logging.V(7) {
				logging.V(7).Infof("Planner decided to update '%v' (oldprops=%v inputs=%v", urn, oldInputs, new.Inputs)
			}
			return []Step{NewUpdateStep(sg.plan, event, old, new, diff.StableKeys, diff.ChangedKeys)}, nil
		}

		// If resource was unchanged, but there were initialization errors, generate an empty update
		// step to attempt to "continue" awaiting initialization.
		if len(old.InitErrors) > 0 {
			sg.updates[urn] = true
			return []Step{NewUpdateStep(sg.plan, event, old, new, diff.StableKeys, nil)}, nil
		}

		// No need to update anything, the properties didn't change.
		sg.sames[urn] = true
		if logging.V(7) {
			logging.V(7).Infof("Planner decided not to update '%v' (same) (inputs=%v)", urn, new.Inputs)
		}
		return []Step{NewSameStep(sg.plan, event, old, new)}, nil
	}

	// Case 4: Not Case 1, 2, or 3
	//  If a resource isn't being recreated and it's not being updated or replaced,
	//  it's just being created.
	sg.creates[urn] = true
	logging.V(7).Infof("Planner decided to create '%v' (inputs=%v)", urn, new.Inputs)
	return []Step{NewCreateStep(sg.plan, event, new)}, nil
}

func (sg *stepGenerator) GenerateDeletes() []Step {
	// To compute the deletion list, we must walk the list of old resources *backwards*.  This is because the list is
	// stored in dependency order, and earlier elements are possibly leaf nodes for later elements.  We must not delete
	// dependencies prior to their dependent nodes.
	var dels []Step
	if prev := sg.plan.prev; prev != nil {
		for i := len(prev.Resources) - 1; i >= 0; i-- {
			// If this resource is explicitly marked for deletion or wasn't seen at all, delete it.
			res := prev.Resources[i]
			if res.Delete {
				// The below assert is commented-out because it's believed to be wrong.
				//
				// The original justification for this assert is that the author (swgillespie) believed that
				// it was impossible for a single URN to be deleted multiple times in the same program.
				// This has empirically been proven to be false - it is possible using today engine to construct
				// a series of actions that puts arbitrarily many pending delete resources with the same URN in
				// the snapshot.
				//
				// It is not clear whether or not this is OK. I (swgillespie), the author of this comment, have
				// seen no evidence that it is *not* OK. However, concerns were raised about what this means for
				// structural resources, and so until that question is answered, I am leaving this comment and
				// assert in the code.
				//
				// Regardless, it is better to admit strange behavior in corner cases than it is to crash the CLI
				// whenever we see multiple deletes for the same URN.
				// contract.Assert(!sg.deletes[res.URN])
				if sg.pendingDeletes[res] {
					logging.V(7).Infof(
						"Planner ignoring pending-delete resource (%v, %v) that was already deleted", res.URN, res.ID)
					continue
				}

				if sg.deletes[res.URN] {
					logging.V(7).Infof(
						"Planner is deleting pending-delete urn '%v' that has already been deleted", res.URN)
				}

				logging.V(7).Infof("Planner decided to delete '%v' due to replacement", res.URN)
				sg.deletes[res.URN] = true
				dels = append(dels, NewDeleteReplacementStep(sg.plan, res, false))
			} else if !sg.sames[res.URN] && !sg.updates[res.URN] && !sg.replaces[res.URN] && !sg.reads[res.URN] {
				// NOTE: we deliberately do not check sg.deletes here, as it is possible for us to issue multiple
				// delete steps for the same URN if the old checkpoint contained pending deletes.
				logging.V(7).Infof("Planner decided to delete '%v'", res.URN)
				sg.deletes[res.URN] = true
				if !res.PendingReplacement {
					dels = append(dels, NewDeleteStep(sg.plan, res))
				} else {
					dels = append(dels, NewRemovePendingReplaceStep(sg.plan, res))
				}
			}
		}
	}
	return dels
}

// GeneratePendingDeletes generates delete steps for all resources that are pending deletion. This function should be
// called at the start of a plan in order to find all resources that are pending deletion from the prevous plan.
func (sg *stepGenerator) GeneratePendingDeletes() []Step {
	var dels []Step
	if prev := sg.plan.prev; prev != nil {
		logging.V(7).Infof("stepGenerator.GeneratePendingDeletes(): scanning previous snapshot for pending deletes")
		for i := len(prev.Resources) - 1; i >= 0; i-- {
			res := prev.Resources[i]
			if res.Delete {
				logging.V(7).Infof(
					"stepGenerator.GeneratePendingDeletes(): resource (%v, %v) is pending deletion", res.URN, res.ID)
				sg.pendingDeletes[res] = true
				dels = append(dels, NewDeleteStep(sg.plan, res))
			}
		}
	}
	return dels
}

// scheduleDeletes takes a list of steps that will delete resources and "schedules" them by producing a list of list of
// steps, where each list can be executed in parallel but a previous list must be executed to completion before advacing
// to the next list.
//
// In lieu of tracking per-step dependencies and orienting the step executor around these dependencies, this function
// provides a conservative approximation of what deletions can safely occur in parallel. The insight here is that the
// resource dependency graph is a partially-ordered set and all partially-ordered sets can be easily decomposed into
// antichains - subsets of the set that are all not comparable to one another. (In this definition, "not comparable"
// means "do not depend on one another").
//
// The algorithm for decomposing a poset into antichains is:
//  1. While there exist elements in the poset,
//    1a. There must exist at least one "maximal" element of the poset. Let E_max be those elements.
//    2a. Remove all elements E_max from the poset. E_max is an antichain.
//    3a. Goto 1.
//
// Translated to our dependency graph:
//  1. While the set of condemned resources is not empty:
//    1a. Remove all resources with no outgoing edges from the graph and add them to the current antichain.
//    2a. Goto 1.
//
// The resulting list of antichains is a list of list of steps that can be safely executed in parallel. Since we must
// process deletes in reverse (so we don't delete resources upon which other resources depend), we reverse the list and
// hand it back to the plan executor for safe execution.
func (sg *stepGenerator) ScheduleDeletes(deleteSteps []Step) []antichain {
	var antichains []antichain                // the list of parallelizable steps we intend to return.
	dg := sg.plan.depGraph                    // the current plan's dependency graph.
	condemned := make(graph.ResourceSet)      // the set of condemned resources.
	stepMap := make(map[*resource.State]Step) // a map from resource states to the steps that delete them.

	// If we don't trust the dependency graph we've been given, we must be conservative and delete everything serially.
	if !sg.opts.TrustDependencies {
		logging.V(7).Infof("Planner does not trust dependency graph, scheduling deletions serially")
		for _, step := range deleteSteps {
			antichains = append(antichains, antichain{step})
		}

		return antichains
	}

	logging.V(7).Infof("Planner trusts dependency graph, scheduling deletions in parallel")

	// For every step we've been given, record it as condemned and save the step that will be used to delete it. We'll
	// iteratively place these steps into antichains as we remove elements from the condemned set.
	for _, step := range deleteSteps {
		condemned[step.Res()] = true
		stepMap[step.Res()] = step
	}

	for len(condemned) > 0 {
		var steps antichain
		logging.V(7).Infof("Planner beginning schedule of new deletion antichain")
		for res := range condemned {
			// Does res have any outgoing edges to resources that haven't already been removed from the graph?
			condemnedDependencies := dg.DependenciesOf(res).Intersect(condemned)
			if len(condemnedDependencies) == 0 {
				// If not, it's safe to delete res at this stage.
				logging.V(7).Infof("Planner scheduling deletion of '%v'", res.URN)
				steps = append(steps, stepMap[res])
			}

			// If one of this resource's dependencies or this resource's parent hasn't been removed from the graph yet,
			// it can't be deleted this round.
		}

		// For all reosurces that are to be deleted in this round, remove them from the graph.
		for _, step := range steps {
			delete(condemned, step.Res())
		}

		antichains = append(antichains, steps)
	}

	// Up until this point, all logic has been "backwards" - we're scheduling resources for deletion when all of their
	// dependencies finish deletion, but that's exactly the opposite of what we need to do. We can only delete a
	// resource when all *resources that depend on it* complete deletion. Our solution is still correct, though, it's
	// just backwards.
	//
	// All we have to do here is reverse the list and then our solution is correct.
	for i := len(antichains)/2 - 1; i >= 0; i-- {
		opp := len(antichains) - 1 - i
		antichains[i], antichains[opp] = antichains[opp], antichains[i]
	}

	return antichains
}

// diff returns a DiffResult for the given resource.
func (sg *stepGenerator) diff(urn resource.URN, id resource.ID, oldInputs, oldOutputs, newInputs resource.PropertyMap,
	prov plugin.Provider, allowUnknowns bool) (plugin.DiffResult, error) {

	// Workaround #1251: unexpected replaces.
	//
	// The legacy/desired behavior here is that if the provider-calculated inputs for a resource did not change,
	// then the resource itself should not change. Unfortunately, we (correctly?) pass the entire current state
	// of the resource to Diff, which includes calculated/output properties that may differ from those present
	// in the input properties. This can cause unexpected diffs.
	//
	// For now, simply apply the legacy diffing behavior before deferring to the provider.
	if oldInputs.DeepEquals(newInputs) {
		return plugin.DiffResult{Changes: plugin.DiffNone}, nil
	}

	// If there is no provider for this resource, simply return a "diffs exist" result.
	if prov == nil {
		return plugin.DiffResult{Changes: plugin.DiffSome}, nil
	}

	// Grab the diff from the provider. At this point we know that there were changes to the Pulumi inputs, so if the
	// provider returns an "unknown" diff result, pretend it returned "diffs exist".
	diff, err := prov.Diff(urn, id, oldOutputs, newInputs, allowUnknowns)
	if err != nil {
		return diff, err
	}
	if diff.Changes == plugin.DiffUnknown {
		diff.Changes = plugin.DiffSome
	}
	return diff, nil
}

// issueCheckErrors prints any check errors to the diagnostics sink.
func (sg *stepGenerator) issueCheckErrors(new *resource.State, urn resource.URN,
	failures []plugin.CheckFailure) bool {
	if len(failures) == 0 {
		return false
	}
	inputs := new.Inputs
	for _, failure := range failures {
		if failure.Property != "" {
			sg.plan.Diag().Errorf(diag.GetResourcePropertyInvalidValueError(urn),
				new.Type, urn.Name(), failure.Property, inputs[failure.Property], failure.Reason)
		} else {
			sg.plan.Diag().Errorf(
				diag.GetResourceInvalidError(urn), new.Type, urn.Name(), failure.Reason)
		}
	}
	return true
}

// processIgnoreChanges sets the value for each ignoreChanges property in inputs to the value from oldInputs.  This has
// the effect of ensuring that no changes will be made for the corresponding property.
func (sg *stepGenerator) processIgnoreChanges(inputs, oldInputs resource.PropertyMap,
	ignoreChanges []string) resource.PropertyMap {

	ignoredInputs := inputs.Copy()
	for _, ignoreChange := range ignoreChanges {
		ignoreChangePropertyKey := resource.PropertyKey(ignoreChange)
		if oldValue, has := oldInputs[ignoreChangePropertyKey]; has {
			ignoredInputs[ignoreChangePropertyKey] = oldValue
		} else {
			delete(ignoredInputs, ignoreChangePropertyKey)
		}
	}
	return ignoredInputs
}

func (sg *stepGenerator) loadResourceProvider(
	urn resource.URN, custom bool, provider string, typ tokens.Type) (plugin.Provider, result.Result) {

	// If this is not a custom resource, then it has no provider by definition.
	if !custom {
		return nil, nil
	}

	// If this resource is a provider resource, use the plan's provider registry for its CRUD operations.
	// Otherwise, resolve the the resource's provider reference.
	if providers.IsProviderType(typ) {
		return sg.plan.providers, nil
	}

	contract.Assert(provider != "")
	ref, refErr := providers.ParseReference(provider)
	if refErr != nil {
		sg.plan.Diag().Errorf(diag.GetBadProviderError(urn), provider, urn, refErr)
		return nil, result.Bail()
	}
	p, ok := sg.plan.GetProvider(ref)
	if !ok {
		sg.plan.Diag().Errorf(diag.GetUnknownProviderError(urn), provider, urn, refErr)
		return nil, result.Bail()
	}
	return p, nil
}

type dependentReplace struct {
	res  *resource.State
	keys []resource.PropertyKey
}

func (sg *stepGenerator) calculateDependentReplacements(root *resource.State) ([]dependentReplace, result.Result) {
	// We need to compute the set of resources that may be replaced by a change to the resource under consideration.
	// We do this by taking the complete set of transitive dependents on the resource under consideration and
	// removing any resources that would not be replaced by changes to their dependencies. We determine whether or not
	// a resource may be replaced by substituting unknowns for input properties that may change due to deletion of the
	// resources their value depends on and calling the resource provider's `Diff` method.
	//
	// This is perhaps clearer when described by example. Consider the following dependency graph:
	//
	//       A
	//     __|__
	//     B   C
	//     |  _|_
	//     D  E F
	//
	// In this graph, all of B, C, D, E, and F transitively depend on A. It may be the case, however, that changes to
	// the specific properties of any of those resources R that would occur if a resource on the path to A were deleted
	// and recreated may not cause R to be replaced. For example, the edge from B to A may be a simple `dependsOn` edge
	// such that a change to B does not actually influence any of B's input properties.  More commonly, the edge from B
	// to A may be due to a property from A being used as the input to a property of B that does not require B to be
	// replaced upon a change. In these cases, neither B nor D would need to be deleted before A could be deleted.
	var toReplace []dependentReplace
	replaceSet := map[resource.URN]bool{root.URN: true}

	requiresReplacement := func(r *resource.State) (bool, []resource.PropertyKey, result.Result) {
		// Neither component nor external resources require replacement.
		if !r.Custom || r.External {
			return false, nil, nil
		}

		// If the resource's provider is in the replace set, we mustreplace this resource.
		if r.Provider != "" {
			ref, err := providers.ParseReference(r.Provider)
			if err != nil {
				return false, nil, result.FromError(err)
			}
			if replaceSet[ref.URN()] {
				return true, nil, nil
			}
		}

		// Scan the properties of this resource in order to determine whether or not any of them depend on a resource
		// that requires replacement and build a set of input properties for the provider diff.
		hasDependencyInReplaceSet, inputsForDiff := false, resource.PropertyMap{}
		for pk, pv := range r.Inputs {
			for _, propertyDep := range r.PropertyDependencies[pk] {
				if replaceSet[propertyDep] {
					hasDependencyInReplaceSet = true
					pv = resource.MakeComputed(resource.NewStringProperty("<unknown>"))
				}
			}
			inputsForDiff[pk] = pv
		}

		// If none of this resource's properties depend on a resource in the replace set, then none of the properties
		// may change and this resource does not need to be replaced.
		if !hasDependencyInReplaceSet {
			return false, nil, nil
		}

		// Otherwise, fetch the resource's provider. Since we have filtered out component resources, this resource must
		// have a provider.
		prov, res := sg.loadResourceProvider(r.URN, r.Custom, r.Provider, r.Type)
		if res != nil {
			return false, nil, res
		}
		contract.Assert(prov != nil)

		// Call the provider's `Diff` method and return.
		diff, err := prov.Diff(r.URN, r.ID, r.Outputs, inputsForDiff, true)
		if err != nil {
			return false, nil, result.FromError(err)
		}
		return diff.Replace(), diff.ReplaceKeys, nil
	}

	// Walk the root resource's dependents in order and build up the set of resources that require replacement.
	for _, d := range sg.plan.depGraph.DependingOn(root) {
		replace, keys, res := requiresReplacement(d)
		if res != nil {
			return nil, res
		}
		if replace {
			toReplace, replaceSet[d.URN] = append(toReplace, dependentReplace{res: d, keys: keys}), true
		}
	}

	// Return the list of resources to replace.
	return toReplace, nil
}

// newStepGenerator creates a new step generator that operates on the given plan.
func newStepGenerator(plan *Plan, opts Options) *stepGenerator {
	return &stepGenerator{
		plan:                 plan,
		opts:                 opts,
		urns:                 make(map[resource.URN]bool),
		reads:                make(map[resource.URN]bool),
		creates:              make(map[resource.URN]bool),
		sames:                make(map[resource.URN]bool),
		replaces:             make(map[resource.URN]bool),
		updates:              make(map[resource.URN]bool),
		deletes:              make(map[resource.URN]bool),
		pendingDeletes:       make(map[*resource.State]bool),
		dependentReplaceKeys: make(map[resource.URN][]resource.PropertyKey),
	}
}