context.go

package terraform

import (
	"bytes"
	"context"
	"fmt"
	"log"
	"sync"

	"github.com/hashicorp/terraform-plugin-sdk/internal/addrs"
	"github.com/hashicorp/terraform-plugin-sdk/internal/configs"
	"github.com/hashicorp/terraform-plugin-sdk/internal/lang"
	"github.com/hashicorp/terraform-plugin-sdk/internal/plans"
	"github.com/hashicorp/terraform-plugin-sdk/internal/providers"
	"github.com/hashicorp/terraform-plugin-sdk/internal/provisioners"
	"github.com/hashicorp/terraform-plugin-sdk/internal/states"
	"github.com/hashicorp/terraform-plugin-sdk/internal/states/statefile"
	"github.com/hashicorp/terraform-plugin-sdk/internal/tfdiags"
	"github.com/zclconf/go-cty/cty"
)

// InputMode defines what sort of input will be asked for when Input
// is called on Context.
type InputMode byte

const (
	// InputModeVar asks for all variables
	InputModeVar InputMode = 1 << iota

	// InputModeVarUnset asks for variables which are not set yet.
	// InputModeVar must be set for this to have an effect.
	InputModeVarUnset

	// InputModeProvider asks for provider variables
	InputModeProvider

	// InputModeStd is the standard operating mode and asks for both variables
	// and providers.
	InputModeStd = InputModeVar | InputModeProvider
)

// ContextOpts are the user-configurable options to create a context with
// NewContext.
type ContextOpts struct {
	Config    *configs.Config
	Changes   *plans.Changes
	State     *states.State
	Targets   []addrs.Targetable
	Variables InputValues
	Meta      *ContextMeta
	Destroy   bool

	Hooks            []Hook
	Parallelism      int
	ProviderResolver providers.Resolver
	Provisioners     map[string]ProvisionerFactory

	// If non-nil, will apply as additional constraints on the provider
	// plugins that will be requested from the provider resolver.
	ProviderSHA256s    map[string][]byte
	SkipProviderVerify bool

	UIInput UIInput
}

// ContextMeta is metadata about the running context. This is information
// that this package or structure cannot determine on its own but exposes
// into Terraform in various ways. This must be provided by the Context
// initializer.
type ContextMeta struct {
	Env string // Env is the state environment
}

// Context represents all the context that Terraform needs in order to
// perform operations on infrastructure. This structure is built using
// NewContext.
type Context struct {
	config    *configs.Config
	changes   *plans.Changes
	state     *states.State
	targets   []addrs.Targetable
	variables InputValues
	meta      *ContextMeta
	destroy   bool

	hooks      []Hook
	components contextComponentFactory
	schemas    *Schemas
	sh         *stopHook
	uiInput    UIInput

	l                   sync.Mutex // Lock acquired during any task
	parallelSem         Semaphore
	providerInputConfig map[string]map[string]cty.Value
	providerSHA256s     map[string][]byte
	runCond             *sync.Cond
	runContext          context.Context
	runContextCancel    context.CancelFunc
	shadowErr           error
}

// (additional methods on Context can be found in context_*.go files.)

// NewContext creates a new Context structure.
//
// Once a Context is created, the caller must not access or mutate any of
// the objects referenced (directly or indirectly) by the ContextOpts fields.
//
// If the returned diagnostics contains errors then the resulting context is
// invalid and must not be used.
func NewContext(opts *ContextOpts) (*Context, tfdiags.Diagnostics) {
	log.Printf("[TRACE] terraform.NewContext: starting")
	diags := CheckCoreVersionRequirements(opts.Config)
	// If version constraints are not met then we'll bail early since otherwise
	// we're likely to just see a bunch of other errors related to
	// incompatibilities, which could be overwhelming for the user.
	if diags.HasErrors() {
		return nil, diags
	}

	// Copy all the hooks and add our stop hook. We don't append directly
	// to the Config so that we're not modifying that in-place.
	sh := new(stopHook)
	hooks := make([]Hook, len(opts.Hooks)+1)
	copy(hooks, opts.Hooks)
	hooks[len(opts.Hooks)] = sh

	state := opts.State
	if state == nil {
		state = states.NewState()
	}

	// Determine parallelism, default to 10. We do this both to limit
	// CPU pressure but also to have an extra guard against rate throttling
	// from providers.
	par := opts.Parallelism
	if par == 0 {
		par = 10
	}

	// Set up the variables in the following sequence:
	//    0 - Take default values from the configuration
	//    1 - Take values from TF_VAR_x environment variables
	//    2 - Take values specified in -var flags, overriding values
	//        set by environment variables if necessary. This includes
	//        values taken from -var-file in addition.
	var variables InputValues
	if opts.Config != nil {
		// Default variables from the configuration seed our map.
		variables = DefaultVariableValues(opts.Config.Module.Variables)
	}
	// Variables provided by the caller (from CLI, environment, etc) can
	// override the defaults.
	variables = variables.Override(opts.Variables)

	// Bind available provider plugins to the constraints in config
	var providerFactories map[string]providers.Factory
	if opts.ProviderResolver != nil {
		deps := ConfigTreeDependencies(opts.Config, state)
		reqd := deps.AllPluginRequirements()
		if opts.ProviderSHA256s != nil && !opts.SkipProviderVerify {
			reqd.LockExecutables(opts.ProviderSHA256s)
		}
		log.Printf("[TRACE] terraform.NewContext: resolving provider version selections")

		var providerDiags tfdiags.Diagnostics
		providerFactories, providerDiags = resourceProviderFactories(opts.ProviderResolver, reqd)
		diags = diags.Append(providerDiags)

		if diags.HasErrors() {
			return nil, diags
		}
	} else {
		providerFactories = make(map[string]providers.Factory)
	}

	components := &basicComponentFactory{
		providers:    providerFactories,
		provisioners: opts.Provisioners,
	}

	log.Printf("[TRACE] terraform.NewContext: loading provider schemas")
	schemas, err := LoadSchemas(opts.Config, opts.State, components)
	if err != nil {
		diags = diags.Append(err)
		return nil, diags
	}

	changes := opts.Changes
	if changes == nil {
		changes = plans.NewChanges()
	}

	config := opts.Config
	if config == nil {
		config = configs.NewEmptyConfig()
	}

	log.Printf("[TRACE] terraform.NewContext: complete")

	return &Context{
		components: components,
		schemas:    schemas,
		destroy:    opts.Destroy,
		changes:    changes,
		hooks:      hooks,
		meta:       opts.Meta,
		config:     config,
		state:      state,
		targets:    opts.Targets,
		uiInput:    opts.UIInput,
		variables:  variables,

		parallelSem:         NewSemaphore(par),
		providerInputConfig: make(map[string]map[string]cty.Value),
		providerSHA256s:     opts.ProviderSHA256s,
		sh:                  sh,
	}, nil
}

func (c *Context) Schemas() *Schemas {
	return c.schemas
}

type ContextGraphOpts struct {
	// If true, validates the graph structure (checks for cycles).
	Validate bool

	// Legacy graphs only: won't prune the graph
	Verbose bool
}

// Graph returns the graph used for the given operation type.
//
// The most extensive or complex graph type is GraphTypePlan.
func (c *Context) Graph(typ GraphType, opts *ContextGraphOpts) (*Graph, tfdiags.Diagnostics) {
	if opts == nil {
		opts = &ContextGraphOpts{Validate: true}
	}

	log.Printf("[INFO] terraform: building graph: %s", typ)
	switch typ {
	case GraphTypeApply:
		return (&ApplyGraphBuilder{
			Config:     c.config,
			Changes:    c.changes,
			State:      c.state,
			Components: c.components,
			Schemas:    c.schemas,
			Targets:    c.targets,
			Destroy:    c.destroy,
			Validate:   opts.Validate,
		}).Build(addrs.RootModuleInstance)

	case GraphTypeValidate:
		// The validate graph is just a slightly modified plan graph
		fallthrough
	case GraphTypePlan:
		// Create the plan graph builder
		p := &PlanGraphBuilder{
			Config:     c.config,
			State:      c.state,
			Components: c.components,
			Schemas:    c.schemas,
			Targets:    c.targets,
			Validate:   opts.Validate,
		}

		// Some special cases for other graph types shared with plan currently
		var b GraphBuilder = p
		switch typ {
		case GraphTypeValidate:
			b = ValidateGraphBuilder(p)
		}

		return b.Build(addrs.RootModuleInstance)

	case GraphTypePlanDestroy:
		return (&DestroyPlanGraphBuilder{
			Config:     c.config,
			State:      c.state,
			Components: c.components,
			Schemas:    c.schemas,
			Targets:    c.targets,
			Validate:   opts.Validate,
		}).Build(addrs.RootModuleInstance)

	case GraphTypeRefresh:
		return (&RefreshGraphBuilder{
			Config:     c.config,
			State:      c.state,
			Components: c.components,
			Schemas:    c.schemas,
			Targets:    c.targets,
			Validate:   opts.Validate,
		}).Build(addrs.RootModuleInstance)

	case GraphTypeEval:
		return (&EvalGraphBuilder{
			Config:     c.config,
			State:      c.state,
			Components: c.components,
			Schemas:    c.schemas,
		}).Build(addrs.RootModuleInstance)

	default:
		// Should never happen, because the above is exhaustive for all graph types.
		panic(fmt.Errorf("unsupported graph type %s", typ))
	}
}

// ShadowError returns any errors caught during a shadow operation.
//
// A shadow operation is an operation run in parallel to a real operation
// that performs the same tasks using new logic on copied state. The results
// are compared to ensure that the new logic works the same as the old logic.
// The shadow never affects the real operation or return values.
//
// The result of the shadow operation are only available through this function
// call after a real operation is complete.
//
// For API consumers of Context, you can safely ignore this function
// completely if you have no interest in helping report experimental feature
// errors to Terraform maintainers. Otherwise, please call this function
// after every operation and report this to the user.
//
// IMPORTANT: Shadow errors are _never_ critical: they _never_ affect
// the real state or result of a real operation. They are purely informational
// to assist in future Terraform versions being more stable. Please message
// this effectively to the end user.
//
// This must be called only when no other operation is running (refresh,
// plan, etc.). The result can be used in parallel to any other operation
// running.
func (c *Context) ShadowError() error {
	return c.shadowErr
}

// State returns a copy of the current state associated with this context.
//
// This cannot safely be called in parallel with any other Context function.
func (c *Context) State() *states.State {
	return c.state.DeepCopy()
}

// Eval produces a scope in which expressions can be evaluated for
// the given module path.
//
// This method must first evaluate any ephemeral values (input variables, local
// values, and output values) in the configuration. These ephemeral values are
// not included in the persisted state, so they must be re-computed using other
// values in the state before they can be properly evaluated. The updated
// values are retained in the main state associated with the receiving context.
//
// This function takes no action against remote APIs but it does need access
// to all provider and provisioner instances in order to obtain their schemas
// for type checking.
//
// The result is an evaluation scope that can be used to resolve references
// against the root module. If the returned diagnostics contains errors then
// the returned scope may be nil. If it is not nil then it may still be used
// to attempt expression evaluation or other analysis, but some expressions
// may not behave as expected.
func (c *Context) Eval(path addrs.ModuleInstance) (*lang.Scope, tfdiags.Diagnostics) {
	// This is intended for external callers such as the "terraform console"
	// command. Internally, we create an evaluator in c.walk before walking
	// the graph, and create scopes in ContextGraphWalker.

	var diags tfdiags.Diagnostics
	defer c.acquireRun("eval")()

	// Start with a copy of state so that we don't affect any instances
	// that other methods may have already returned.
	c.state = c.state.DeepCopy()
	var walker *ContextGraphWalker

	graph, graphDiags := c.Graph(GraphTypeEval, nil)
	diags = diags.Append(graphDiags)
	if !diags.HasErrors() {
		var walkDiags tfdiags.Diagnostics
		walker, walkDiags = c.walk(graph, walkEval)
		diags = diags.Append(walker.NonFatalDiagnostics)
		diags = diags.Append(walkDiags)
	}

	if walker == nil {
		// If we skipped walking the graph (due to errors) then we'll just
		// use a placeholder graph walker here, which'll refer to the
		// unmodified state.
		walker = c.graphWalker(walkEval)
	}

	// This is a bit weird since we don't normally evaluate outside of
	// the context of a walk, but we'll "re-enter" our desired path here
	// just to get hold of an EvalContext for it. GraphContextBuiltin
	// caches its contexts, so we should get hold of the context that was
	// previously used for evaluation here, unless we skipped walking.
	evalCtx := walker.EnterPath(path)
	return evalCtx.EvaluationScope(nil, EvalDataForNoInstanceKey), diags
}

// Apply applies the changes represented by this context and returns
// the resulting state.
//
// Even in the case an error is returned, the state may be returned and will
// potentially be partially updated.  In addition to returning the resulting
// state, this context is updated with the latest state.
//
// If the state is required after an error, the caller should call
// Context.State, rather than rely on the return value.
//
// TODO: Apply and Refresh should either always return a state, or rely on the
//       State() method. Currently the helper/resource testing framework relies
//       on the absence of a returned state to determine if Destroy can be
//       called, so that will need to be refactored before this can be changed.
func (c *Context) Apply() (*states.State, tfdiags.Diagnostics) {
	defer c.acquireRun("apply")()

	// Copy our own state
	c.state = c.state.DeepCopy()

	// Build the graph.
	graph, diags := c.Graph(GraphTypeApply, nil)
	if diags.HasErrors() {
		return nil, diags
	}

	// Determine the operation
	operation := walkApply
	if c.destroy {
		operation = walkDestroy
	}

	// Walk the graph
	walker, walkDiags := c.walk(graph, operation)
	diags = diags.Append(walker.NonFatalDiagnostics)
	diags = diags.Append(walkDiags)

	if c.destroy && !diags.HasErrors() {
		// If we know we were trying to destroy objects anyway, and we
		// completed without any errors, then we'll also prune out any
		// leftover empty resource husks (left after all of the instances
		// of a resource with "count" or "for_each" are destroyed) to
		// help ensure we end up with an _actually_ empty state, assuming
		// we weren't destroying with -target here.
		//
		// (This doesn't actually take into account -target, but that should
		// be okay because it doesn't throw away anything we can't recompute
		// on a subsequent "terraform plan" run, if the resources are still
		// present in the configuration. However, this _will_ cause "count = 0"
		// resources to read as unknown during the next refresh walk, which
		// may cause some additional churn if used in a data resource or
		// provider block, until we remove refreshing as a separate walk and
		// just do it as part of the plan walk.)
		c.state.PruneResourceHusks()
	}

	if len(c.targets) > 0 {
		diags = diags.Append(tfdiags.Sourceless(
			tfdiags.Warning,
			"Applied changes may be incomplete",
			`The plan was created with the -target option in effect, so some changes requested in the configuration may have been ignored and the output values may not be fully updated. Run the following command to verify that no other changes are pending:
    terraform plan
	
Note that the -target option is not suitable for routine use, and is provided only for exceptional situations such as recovering from errors or mistakes, or when Terraform specifically suggests to use it as part of an error message.`,
		))
	}

	return c.state, diags
}

// Plan generates an execution plan for the given context.
//
// The execution plan encapsulates the context and can be stored
// in order to reinstantiate a context later for Apply.
//
// Plan also updates the diff of this context to be the diff generated
// by the plan, so Apply can be called after.
func (c *Context) Plan() (*plans.Plan, tfdiags.Diagnostics) {
	defer c.acquireRun("plan")()
	c.changes = plans.NewChanges()

	var diags tfdiags.Diagnostics

	if len(c.targets) > 0 {
		diags = diags.Append(tfdiags.Sourceless(
			tfdiags.Warning,
			"Resource targeting is in effect",
			`You are creating a plan with the -target option, which means that the result of this plan may not represent all of the changes requested by the current configuration.
		
The -target option is not for routine use, and is provided only for exceptional situations such as recovering from errors or mistakes, or when Terraform specifically suggests to use it as part of an error message.`,
		))
	}

	varVals := make(map[string]plans.DynamicValue, len(c.variables))
	for k, iv := range c.variables {
		// We use cty.DynamicPseudoType here so that we'll save both the
		// value _and_ its dynamic type in the plan, so we can recover
		// exactly the same value later.
		dv, err := plans.NewDynamicValue(iv.Value, cty.DynamicPseudoType)
		if err != nil {
			diags = diags.Append(tfdiags.Sourceless(
				tfdiags.Error,
				"Failed to prepare variable value for plan",
				fmt.Sprintf("The value for variable %q could not be serialized to store in the plan: %s.", k, err),
			))
			continue
		}
		varVals[k] = dv
	}

	p := &plans.Plan{
		VariableValues:  varVals,
		TargetAddrs:     c.targets,
		ProviderSHA256s: c.providerSHA256s,
	}

	var operation walkOperation
	if c.destroy {
		operation = walkPlanDestroy
	} else {
		// Set our state to be something temporary. We do this so that
		// the plan can update a fake state so that variables work, then
		// we replace it back with our old state.
		old := c.state
		if old == nil {
			c.state = states.NewState()
		} else {
			c.state = old.DeepCopy()
		}
		defer func() {
			c.state = old
		}()

		operation = walkPlan
	}

	// Build the graph.
	graphType := GraphTypePlan
	if c.destroy {
		graphType = GraphTypePlanDestroy
	}
	graph, graphDiags := c.Graph(graphType, nil)
	diags = diags.Append(graphDiags)
	if graphDiags.HasErrors() {
		return nil, diags
	}

	// Do the walk
	walker, walkDiags := c.walk(graph, operation)
	diags = diags.Append(walker.NonFatalDiagnostics)
	diags = diags.Append(walkDiags)
	if walkDiags.HasErrors() {
		return nil, diags
	}
	p.Changes = c.changes

	return p, diags
}

// Refresh goes through all the resources in the state and refreshes them
// to their latest state. This will update the state that this context
// works with, along with returning it.
//
// Even in the case an error is returned, the state may be returned and
// will potentially be partially updated.
func (c *Context) Refresh() (*states.State, tfdiags.Diagnostics) {
	defer c.acquireRun("refresh")()

	// Copy our own state
	c.state = c.state.DeepCopy()

	// Refresh builds a partial changeset as part of its work because it must
	// create placeholder stubs for any resource instances that'll be created
	// in subsequent plan so that provider configurations and data resources
	// can interpolate from them. This plan is always thrown away after
	// the operation completes, restoring any existing changeset.
	oldChanges := c.changes
	defer func() { c.changes = oldChanges }()
	c.changes = plans.NewChanges()

	// Build the graph.
	graph, diags := c.Graph(GraphTypeRefresh, nil)
	if diags.HasErrors() {
		return nil, diags
	}

	// Do the walk
	_, walkDiags := c.walk(graph, walkRefresh)
	diags = diags.Append(walkDiags)
	if walkDiags.HasErrors() {
		return nil, diags
	}

	// During our walk we will have created planned object placeholders in
	// state for resource instances that are in configuration but not yet
	// created. These were created only to allow expression evaluation to
	// work properly in provider and data blocks during the walk and must
	// now be discarded, since a subsequent plan walk is responsible for
	// creating these "for real".
	// TODO: Consolidate refresh and plan into a single walk, so that the
	// refresh walk doesn't need to emulate various aspects of the plan
	// walk in order to properly evaluate provider and data blocks.
	c.state.SyncWrapper().RemovePlannedResourceInstanceObjects()

	return c.state, diags
}

// Stop stops the running task.
//
// Stop will block until the task completes.
func (c *Context) Stop() {
	log.Printf("[WARN] terraform: Stop called, initiating interrupt sequence")

	c.l.Lock()
	defer c.l.Unlock()

	// If we're running, then stop
	if c.runContextCancel != nil {
		log.Printf("[WARN] terraform: run context exists, stopping")

		// Tell the hook we want to stop
		c.sh.Stop()

		// Stop the context
		c.runContextCancel()
		c.runContextCancel = nil
	}

	// Grab the condition var before we exit
	if cond := c.runCond; cond != nil {
		log.Printf("[INFO] terraform: waiting for graceful stop to complete")
		cond.Wait()
	}

	log.Printf("[WARN] terraform: stop complete")
}

// Validate performs semantic validation of the configuration, and returning
// any warnings or errors.
//
// Syntax and structural checks are performed by the configuration loader,
// and so are not repeated here.
func (c *Context) Validate() tfdiags.Diagnostics {
	defer c.acquireRun("validate")()

	var diags tfdiags.Diagnostics

	// Validate input variables. We do this only for the values supplied
	// by the root module, since child module calls are validated when we
	// visit their graph nodes.
	if c.config != nil {
		varDiags := checkInputVariables(c.config.Module.Variables, c.variables)
		diags = diags.Append(varDiags)
	}

	// If we have errors at this point then we probably won't be able to
	// construct a graph without producing redundant errors, so we'll halt early.
	if diags.HasErrors() {
		return diags
	}

	// Build the graph so we can walk it and run Validate on nodes.
	// We also validate the graph generated here, but this graph doesn't
	// necessarily match the graph that Plan will generate, so we'll validate the
	// graph again later after Planning.
	graph, graphDiags := c.Graph(GraphTypeValidate, nil)
	diags = diags.Append(graphDiags)
	if graphDiags.HasErrors() {
		return diags
	}

	// Walk
	walker, walkDiags := c.walk(graph, walkValidate)
	diags = diags.Append(walker.NonFatalDiagnostics)
	diags = diags.Append(walkDiags)
	if walkDiags.HasErrors() {
		return diags
	}

	return diags
}

// Config returns the configuration tree associated with this context.
func (c *Context) Config() *configs.Config {
	return c.config
}

// Variables will return the mapping of variables that were defined
// for this Context. If Input was called, this mapping may be different
// than what was given.
func (c *Context) Variables() InputValues {
	return c.variables
}

// SetVariable sets a variable after a context has already been built.
func (c *Context) SetVariable(k string, v cty.Value) {
	c.variables[k] = &InputValue{
		Value:      v,
		SourceType: ValueFromCaller,
	}
}

func (c *Context) acquireRun(phase string) func() {
	// With the run lock held, grab the context lock to make changes
	// to the run context.
	c.l.Lock()
	defer c.l.Unlock()

	// Wait until we're no longer running
	for c.runCond != nil {
		c.runCond.Wait()
	}

	// Build our lock
	c.runCond = sync.NewCond(&c.l)

	// Create a new run context
	c.runContext, c.runContextCancel = context.WithCancel(context.Background())

	// Reset the stop hook so we're not stopped
	c.sh.Reset()

	// Reset the shadow errors
	c.shadowErr = nil

	return c.releaseRun
}

func (c *Context) releaseRun() {
	// Grab the context lock so that we can make modifications to fields
	c.l.Lock()
	defer c.l.Unlock()

	// End our run. We check if runContext is non-nil because it can be
	// set to nil if it was cancelled via Stop()
	if c.runContextCancel != nil {
		c.runContextCancel()
	}

	// Unlock all waiting our condition
	cond := c.runCond
	c.runCond = nil
	cond.Broadcast()

	// Unset the context
	c.runContext = nil
}

func (c *Context) walk(graph *Graph, operation walkOperation) (*ContextGraphWalker, tfdiags.Diagnostics) {
	log.Printf("[DEBUG] Starting graph walk: %s", operation.String())

	walker := c.graphWalker(operation)

	// Watch for a stop so we can call the provider Stop() API.
	watchStop, watchWait := c.watchStop(walker)

	// Walk the real graph, this will block until it completes
	diags := graph.Walk(walker)

	// Close the channel so the watcher stops, and wait for it to return.
	close(watchStop)
	<-watchWait

	return walker, diags
}

func (c *Context) graphWalker(operation walkOperation) *ContextGraphWalker {
	return &ContextGraphWalker{
		Context:            c,
		State:              c.state.SyncWrapper(),
		Changes:            c.changes.SyncWrapper(),
		Operation:          operation,
		StopContext:        c.runContext,
		RootVariableValues: c.variables,
	}
}

// watchStop immediately returns a `stop` and a `wait` chan after dispatching
// the watchStop goroutine. This will watch the runContext for cancellation and
// stop the providers accordingly.  When the watch is no longer needed, the
// `stop` chan should be closed before waiting on the `wait` chan.
// The `wait` chan is important, because without synchronizing with the end of
// the watchStop goroutine, the runContext may also be closed during the select
// incorrectly causing providers to be stopped. Even if the graph walk is done
// at that point, stopping a provider permanently cancels its StopContext which
// can cause later actions to fail.
func (c *Context) watchStop(walker *ContextGraphWalker) (chan struct{}, <-chan struct{}) {
	stop := make(chan struct{})
	wait := make(chan struct{})

	// get the runContext cancellation channel now, because releaseRun will
	// write to the runContext field.
	done := c.runContext.Done()

	go func() {
		defer close(wait)
		// Wait for a stop or completion
		select {
		case <-done:
			// done means the context was canceled, so we need to try and stop
			// providers.
		case <-stop:
			// our own stop channel was closed.
			return
		}

		// If we're here, we're stopped, trigger the call.
		log.Printf("[TRACE] Context: requesting providers and provisioners to gracefully stop")

		{
			// Copy the providers so that a misbehaved blocking Stop doesn't
			// completely hang Terraform.
			walker.providerLock.Lock()
			ps := make([]providers.Interface, 0, len(walker.providerCache))
			for _, p := range walker.providerCache {
				ps = append(ps, p)
			}
			defer walker.providerLock.Unlock()

			for _, p := range ps {
				// We ignore the error for now since there isn't any reasonable
				// action to take if there is an error here, since the stop is still
				// advisory: Terraform will exit once the graph node completes.
				p.Stop()
			}
		}

		{
			// Call stop on all the provisioners
			walker.provisionerLock.Lock()
			ps := make([]provisioners.Interface, 0, len(walker.provisionerCache))
			for _, p := range walker.provisionerCache {
				ps = append(ps, p)
			}
			defer walker.provisionerLock.Unlock()

			for _, p := range ps {
				// We ignore the error for now since there isn't any reasonable
				// action to take if there is an error here, since the stop is still
				// advisory: Terraform will exit once the graph node completes.
				p.Stop()
			}
		}
	}()

	return stop, wait
}

// ShimLegacyState is a helper that takes the legacy state type and
// converts it to the new state type.
//
// This is implemented as a state file upgrade, so it will not preserve
// parts of the state structure that are not included in a serialized state,
// such as the resolved results of any local values, outputs in non-root
// modules, etc.
func ShimLegacyState(legacy *State) (*states.State, error) {
	if legacy == nil {
		return nil, nil
	}
	var buf bytes.Buffer
	err := WriteState(legacy, &buf)
	if err != nil {
		return nil, err
	}
	f, err := statefile.Read(&buf)
	if err != nil {
		return nil, err
	}
	return f.State, err
}

// MustShimLegacyState is a wrapper around ShimLegacyState that panics if
// the conversion does not succeed. This is primarily intended for tests where
// the given legacy state is an object constructed within the test.
func MustShimLegacyState(legacy *State) *states.State {
	ret, err := ShimLegacyState(legacy)
	if err != nil {
		panic(err)
	}
	return ret
}