core: close provider/provisioner connections

Currently Terraform is leaking goroutines and with that memory. I know
strictly speaking this maybe isn’t a real concern for Terraform as it’s
mostly used as a short running command line executable.

But there are a few of us out there that are using Terraform in some
long running processes and then this starts to become a problem.

Next to that it’s of course good programming practise to clean up
resources when they're not needed anymore. So even for the standard
command line use case, this seems an improvement in resource management.

Personally I see no downsides as the primary connection to the plugin
is kept alive (the plugin is not killed) and only unused connections
that will never be used again are closed to free up any related
goroutines and memory.

rpc/mux_broker.go

@@ -3,9 +3,9 @@ package rpc
 import (
 	"encoding/binary"
 	"fmt"
+	"math/rand"
 	"net"
 	"sync"
-	"sync/atomic"
 	"time"
 
 	"github.com/hashicorp/yamux"
@@ -17,7 +17,6 @@ import (
 // or accept a connection from, and the broker handles the details of
 // holding these channels open while they're being negotiated.
 type muxBroker struct {
-	nextId  uint32
 	session *yamux.Session
 	streams map[uint32]*muxBrokerPending
 
@@ -95,9 +94,12 @@ func (m *muxBroker) Dial(id uint32) (net.Conn, error) {
 	return stream, nil
 }
 
-// NextId returns a unique ID to use next.
+// NextId returns a unique ID to use next. There is no need for seeding the
+// rand package as the returned ID's aren't stored or used anywhere outside
+// the current runtime. So it's perfectly fine to get the same pseudo-random
+// numbers each time terraform is running.
 func (m *muxBroker) NextId() uint32 {
-	return atomic.AddUint32(&m.nextId, 1)
+	return rand.Uint32()
 }
 
 // Run starts the brokering and should be executed in a goroutine, since it

rpc/resource_provider.go

@@ -174,6 +174,10 @@ func (p *ResourceProvider) Resources() []terraform.ResourceType {
 	return result
 }
 
+func (p *ResourceProvider) Close() error {
+	return p.Client.Close()
+}
+
 // ResourceProviderServer is a net/rpc compatible structure for serving
 // a ResourceProvider. This should not be used directly.
 type ResourceProviderServer struct {

rpc/resource_provider_test.go

@@ -488,3 +488,31 @@ func TestResourceProvider_validateResource_warns(t *testing.T) {
 		t.Fatalf("bad: %#v", w)
 	}
 }
+
+func TestResourceProvider_close(t *testing.T) {
+	client, _ := testNewClientServer(t)
+	defer client.Close()
+
+	provider, err := client.ResourceProvider()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	var p interface{}
+	p = provider
+	pCloser, ok := p.(terraform.ResourceProviderCloser)
+	if !ok {
+		t.Fatal("should be a ResourceProviderCloser")
+	}
+
+	if err := pCloser.Close(); err != nil {
+		t.Fatalf("failed to close provider: %s", err)
+	}
+
+	// The connection should be closed now, so if we to make a
+	// new call we should get an error.
+	err = provider.Configure(&terraform.ResourceConfig{})
+	if err == nil {
+		t.Fatal("should have error")
+	}
+}

rpc/resource_provisioner.go

@@ -63,6 +63,10 @@ func (p *ResourceProvisioner) Apply(
 	return err
 }
 
+func (p *ResourceProvisioner) Close() error {
+	return p.Client.Close()
+}
+
 type ResourceProvisionerValidateArgs struct {
 	Config *terraform.ResourceConfig
 }

rpc/resource_provisioner_test.go

@@ -132,3 +132,34 @@ func TestResourceProvisioner_validate_warns(t *testing.T) {
 		t.Fatalf("bad: %#v", w)
 	}
 }
+
+func TestResourceProvisioner_close(t *testing.T) {
+	client, _ := testNewClientServer(t)
+	defer client.Close()
+
+	provisioner, err := client.ResourceProvisioner()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	var p interface{}
+	p = provisioner
+	pCloser, ok := p.(terraform.ResourceProvisionerCloser)
+	if !ok {
+		t.Fatal("should be a ResourceProvisionerCloser")
+	}
+
+	if err := pCloser.Close(); err != nil {
+		t.Fatalf("failed to close provisioner: %s", err)
+	}
+
+	// The connection should be closed now, so if we to make a
+	// new call we should get an error.
+	o := &terraform.MockUIOutput{}
+	s := &terraform.InstanceState{}
+	c := &terraform.ResourceConfig{}
+	err = provisioner.Apply(o, s, c)
+	if err == nil {
+		t.Fatal("should have error")
+	}
+}

terraform/eval_context.go

@@ -28,6 +28,9 @@ type EvalContext interface {
 	// initialized) or returns nil if the provider isn't initialized.
 	Provider(string) ResourceProvider
 
+	// CloseProvider closes provider connections that aren't needed anymore.
+	CloseProvider(string) error
+
 	// ConfigureProvider configures the provider with the given
 	// configuration. This is a separate context call because this call
 	// is used to store the provider configuration for inheritance lookups
@@ -51,6 +54,10 @@ type EvalContext interface {
 	// initialized) or returns nil if the provisioner isn't initialized.
 	Provisioner(string) ResourceProvisioner
 
+	// CloseProvisioner closes provisioner connections that aren't needed
+	// anymore.
+	CloseProvisioner(string) error
+
 	// Interpolate takes the given raw configuration and completes
 	// the interpolations, returning the processed ResourceConfig.
 	//

terraform/eval_context_builtin.go

@@ -114,6 +114,28 @@ func (ctx *BuiltinEvalContext) Provider(n string) ResourceProvider {
 	return ctx.ProviderCache[PathCacheKey(providerPath)]
 }
 
+func (ctx *BuiltinEvalContext) CloseProvider(n string) error {
+	ctx.once.Do(ctx.init)
+
+	ctx.ProviderLock.Lock()
+	defer ctx.ProviderLock.Unlock()
+
+	providerPath := make([]string, len(ctx.Path())+1)
+	copy(providerPath, ctx.Path())
+	providerPath[len(providerPath)-1] = n
+
+	var provider interface{}
+	provider = ctx.ProviderCache[PathCacheKey(providerPath)]
+	if provider != nil {
+		if p, ok := provider.(ResourceProviderCloser); ok {
+			delete(ctx.ProviderCache, PathCacheKey(providerPath))
+			return p.Close()
+		}
+	}
+
+	return nil
+}
+
 func (ctx *BuiltinEvalContext) ConfigureProvider(
 	n string, cfg *ResourceConfig) error {
 	p := ctx.Provider(n)
@@ -222,6 +244,28 @@ func (ctx *BuiltinEvalContext) Provisioner(n string) ResourceProvisioner {
 	return ctx.ProvisionerCache[PathCacheKey(provPath)]
 }
 
+func (ctx *BuiltinEvalContext) CloseProvisioner(n string) error {
+	ctx.once.Do(ctx.init)
+
+	ctx.ProvisionerLock.Lock()
+	defer ctx.ProvisionerLock.Unlock()
+
+	provPath := make([]string, len(ctx.Path())+1)
+	copy(provPath, ctx.Path())
+	provPath[len(provPath)-1] = n
+
+	var prov interface{}
+	prov = ctx.ProvisionerCache[PathCacheKey(provPath)]
+	if prov != nil {
+		if p, ok := prov.(ResourceProvisionerCloser); ok {
+			delete(ctx.ProvisionerCache, PathCacheKey(provPath))
+			return p.Close()
+		}
+	}
+
+	return nil
+}
+
 func (ctx *BuiltinEvalContext) Interpolate(
 	cfg *config.RawConfig, r *Resource) (*ResourceConfig, error) {
 	if cfg != nil {

terraform/eval_context_mock.go

@@ -25,6 +25,10 @@ type MockEvalContext struct {
 	ProviderName     string
 	ProviderProvider ResourceProvider
 
+	CloseProviderCalled   bool
+	CloseProviderName     string
+	CloseProviderProvider ResourceProvider
+
 	ProviderInputCalled bool
 	ProviderInputName   string
 	ProviderInputConfig map[string]interface{}
@@ -55,6 +59,10 @@ type MockEvalContext struct {
 	ProvisionerName        string
 	ProvisionerProvisioner ResourceProvisioner
 
+	CloseProvisionerCalled      bool
+	CloseProvisionerName        string
+	CloseProvisionerProvisioner ResourceProvisioner
+
 	InterpolateCalled       bool
 	InterpolateConfig       *config.RawConfig
 	InterpolateResource     *Resource
@@ -105,6 +113,12 @@ func (c *MockEvalContext) Provider(n string) ResourceProvider {
 	return c.ProviderProvider
 }
 
+func (c *MockEvalContext) CloseProvider(n string) error {
+	c.CloseProviderCalled = true
+	c.CloseProviderName = n
+	return nil
+}
+
 func (c *MockEvalContext) ConfigureProvider(n string, cfg *ResourceConfig) error {
 	c.ConfigureProviderCalled = true
 	c.ConfigureProviderName = n
@@ -150,6 +164,12 @@ func (c *MockEvalContext) Provisioner(n string) ResourceProvisioner {
 	return c.ProvisionerProvisioner
 }
 
+func (c *MockEvalContext) CloseProvisioner(n string) error {
+	c.CloseProvisionerCalled = true
+	c.CloseProvisionerName = n
+	return nil
+}
+
 func (c *MockEvalContext) Interpolate(
 	config *config.RawConfig, resource *Resource) (*ResourceConfig, error) {
 	c.InterpolateCalled = true

terraform/eval_provider.go

@@ -71,6 +71,17 @@ func (n *EvalInitProvider) Eval(ctx EvalContext) (interface{}, error) {
 	return ctx.InitProvider(n.Name)
 }
 
+// EvalCloseProvider is an EvalNode implementation that closes provider
+// connections that aren't needed anymore.
+type EvalCloseProvider struct {
+	Name string
+}
+
+func (n *EvalCloseProvider) Eval(ctx EvalContext) (interface{}, error) {
+	ctx.CloseProvider(n.Name)
+	return nil, nil
+}
+
 // EvalGetProvider is an EvalNode implementation that retrieves an already
 // initialized provider instance for the given name.
 type EvalGetProvider struct {

terraform/eval_provider_test.go

@@ -112,6 +112,22 @@ func TestEvalInitProvider(t *testing.T) {
 	}
 }
 
+func TestEvalCloseProvider(t *testing.T) {
+	n := &EvalCloseProvider{Name: "foo"}
+	provider := &MockResourceProvider{}
+	ctx := &MockEvalContext{CloseProviderProvider: provider}
+	if _, err := n.Eval(ctx); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if !ctx.CloseProviderCalled {
+		t.Fatal("should be called")
+	}
+	if ctx.CloseProviderName != "foo" {
+		t.Fatalf("bad: %#v", ctx.CloseProviderName)
+	}
+}
+
 func TestEvalGetProvider_impl(t *testing.T) {
 	var _ EvalNode = new(EvalGetProvider)
 }

terraform/eval_provisioner.go

@@ -15,6 +15,17 @@ func (n *EvalInitProvisioner) Eval(ctx EvalContext) (interface{}, error) {
 	return ctx.InitProvisioner(n.Name)
 }
 
+// EvalCloseProvisioner is an EvalNode implementation that closes provisioner
+// connections that aren't needed anymore.
+type EvalCloseProvisioner struct {
+	Name string
+}
+
+func (n *EvalCloseProvisioner) Eval(ctx EvalContext) (interface{}, error) {
+	ctx.CloseProvisioner(n.Name)
+	return nil, nil
+}
+
 // EvalGetProvisioner is an EvalNode implementation that retrieves an already
 // initialized provisioner instance for the given name.
 type EvalGetProvisioner struct {

terraform/eval_provisioner_test.go

@@ -24,6 +24,22 @@ func TestEvalInitProvisioner(t *testing.T) {
 	}
 }
 
+func TestEvalCloseProvisioner(t *testing.T) {
+	n := &EvalCloseProvisioner{Name: "foo"}
+	provisioner := &MockResourceProvisioner{}
+	ctx := &MockEvalContext{CloseProvisionerProvisioner: provisioner}
+	if _, err := n.Eval(ctx); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if !ctx.CloseProvisionerCalled {
+		t.Fatal("should be called")
+	}
+	if ctx.CloseProvisionerName != "foo" {
+		t.Fatalf("bad: %#v", ctx.CloseProvisionerName)
+	}
+}
+
 func TestEvalGetProvisioner_impl(t *testing.T) {
 	var _ EvalNode = new(EvalGetProvisioner)
 }

terraform/evaltree_provider.go

@@ -72,3 +72,9 @@ func ProviderEvalTree(n string, config *config.RawConfig) EvalNode {
 
 	return &EvalSequence{Nodes: seq}
 }
+
+// CloseProviderEvalTree returns the evaluation tree for closing
+// provider connections that aren't needed anymore.
+func CloseProviderEvalTree(n string) EvalNode {
+	return &EvalCloseProvider{Name: n}
+}

terraform/graph_builder.go

@@ -116,12 +116,14 @@ func (b *BuiltinGraphBuilder) Steps(path []string) []GraphTransformer {
 		// Provider-related transformations
 		&MissingProviderTransformer{Providers: b.Providers},
 		&ProviderTransformer{},
+		&CloseProviderTransformer{},
 		&PruneProviderTransformer{},
 		&DisableProviderTransformer{},
 
 		// Provisioner-related transformations
 		&MissingProvisionerTransformer{Provisioners: b.Provisioners},
 		&ProvisionerTransformer{},
+		&CloseProvisionerTransformer{},
 		&PruneProvisionerTransformer{},
 
 		// Run our vertex-level transforms

terraform/graph_builder_test.go

@@ -196,6 +196,8 @@ aws_instance.db
 aws_instance.web
   aws_instance.db
 provider.aws
+provider.aws (close)
+  aws_instance.web
 `
 
 const testBuiltinGraphBuilderVerboseStr = `
@@ -213,8 +215,28 @@ aws_instance.web (destroy tainted)
 aws_instance.web (destroy)
   provider.aws
 provider.aws
+provider.aws (close)
+  aws_instance.web
 `
 
+const testBuiltinGraphBuilderMultiLevelStr = `
+module.foo.module.bar.output.value
+  module.foo.module.bar.var.bar
+module.foo.module.bar.plan-destroy
+module.foo.module.bar.var.bar
+  module.foo.var.foo
+module.foo.plan-destroy
+module.foo.var.foo
+root
+  module.foo.module.bar.output.value
+  module.foo.module.bar.plan-destroy
+  module.foo.plan-destroy
+`
+
+/*
+TODO: Commented out this const as it's likely this needs to
+be updated when the TestBuiltinGraphBuilder_modules test is
+enabled again.
 const testBuiltinGraphBuilderModuleStr = `
 aws_instance.web
   aws_instance.web (destroy)
@@ -231,17 +253,4 @@ module.consul (expanded)
   provider.aws
 provider.aws
 `
-
-const testBuiltinGraphBuilderMultiLevelStr = `
-module.foo.module.bar.output.value
-  module.foo.module.bar.var.bar
-module.foo.module.bar.plan-destroy
-module.foo.module.bar.var.bar
-  module.foo.var.foo
-module.foo.plan-destroy
-module.foo.var.foo
-root
-  module.foo.module.bar.output.value
-  module.foo.module.bar.plan-destroy
-  module.foo.plan-destroy
-`
+*/

terraform/resource_provider.go

@@ -71,6 +71,12 @@ type ResourceProvider interface {
 	Refresh(*InstanceInfo, *InstanceState) (*InstanceState, error)
 }
 
+// ResourceProviderCloser is an interface that providers that can close
+// connections that aren't needed anymore must implement.
+type ResourceProviderCloser interface {
+	Close() error
+}
+
 // ResourceType is a type of resource that a resource provider can manage.
 type ResourceType struct {
 	Name string

terraform/resource_provisioner.go

@@ -23,6 +23,12 @@ type ResourceProvisioner interface {
 	Apply(UIOutput, *InstanceState, *ResourceConfig) error
 }
 
+// ResourceProvisionerCloser is an interface that provisioners that can close
+// connections that aren't needed anymore must implement.
+type ResourceProvisionerCloser interface {
+	Close() error
+}
+
 // ResourceProvisionerFactory is a function type that creates a new instance
 // of a resource provisioner.
 type ResourceProvisionerFactory func() (ResourceProvisioner, error)

terraform/transform_provider.go

@@ -17,6 +17,13 @@ type GraphNodeProvider interface {
 	ProviderConfig() *config.RawConfig
 }
 
+// GraphNodeCloseProvider is an interface that nodes that can be a close
+// provider must implement. The CloseProviderName returned is the name of
+// the provider they satisfy.
+type GraphNodeCloseProvider interface {
+	CloseProviderName() string
+}
+
 // GraphNodeProviderConsumer is an interface that nodes that require
 // a provider must implement. ProvidedBy must return the name of the provider
 // to use.
@@ -98,6 +105,37 @@ func (t *ProviderTransformer) Transform(g *Graph) error {
 	return err
 }
 
+// CloseProviderTransformer is a GraphTransformer that adds nodes to the
+// graph that will close open provider connections that aren't needed anymore.
+// A provider connection is not needed anymore once all depended resources
+// in the graph are evaluated.
+type CloseProviderTransformer struct{}
+
+func (t *CloseProviderTransformer) Transform(g *Graph) error {
+	m := closeProviderVertexMap(g)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeProviderConsumer); ok {
+			for _, p := range pv.ProvidedBy() {
+				source := m[p]
+
+				if source == nil {
+					// Create a new graphNodeCloseProvider and add it to the graph
+					source = &graphNodeCloseProvider{ProviderNameValue: p}
+					g.Add(source)
+
+					// Make sure we also add the new graphNodeCloseProvider to the map
+					// so we don't create and add any duplicate graphNodeCloseProviders.
+					m[p] = source
+				}
+
+				g.Connect(dag.BasicEdge(source, v))
+			}
+		}
+	}
+
+	return nil
+}
+
 // MissingProviderTransformer is a GraphTransformer that adds nodes
 // for missing providers into the graph. Specifically, it creates provider
 // configuration nodes for all the providers that we support. These are
@@ -143,6 +181,28 @@ func (t *PruneProviderTransformer) Transform(g *Graph) error {
 	return nil
 }
 
+func providerVertexMap(g *Graph) map[string]dag.Vertex {
+	m := make(map[string]dag.Vertex)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeProvider); ok {
+			m[pv.ProviderName()] = v
+		}
+	}
+
+	return m
+}
+
+func closeProviderVertexMap(g *Graph) map[string]dag.Vertex {
+	m := make(map[string]dag.Vertex)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeCloseProvider); ok {
+			m[pv.CloseProviderName()] = v
+		}
+	}
+
+	return m
+}
+
 type graphNodeDisabledProvider struct {
 	GraphNodeProvider
 }
@@ -258,6 +318,94 @@ func (n *graphNodeDisabledProviderFlat) DependentOn() []string {
 	return result
 }
 
+type graphNodeCloseProvider struct {
+	ProviderNameValue string
+}
+
+func (n *graphNodeCloseProvider) Name() string {
+	return fmt.Sprintf("provider.%s (close)", n.ProviderNameValue)
+}
+
+// GraphNodeEvalable impl.
+func (n *graphNodeCloseProvider) EvalTree() EvalNode {
+	return CloseProviderEvalTree(n.ProviderNameValue)
+}
+
+// GraphNodeDependable impl.
+func (n *graphNodeCloseProvider) DependableName() []string {
+	return []string{n.Name()}
+}
+
+func (n *graphNodeCloseProvider) CloseProviderName() string {
+	return n.ProviderNameValue
+}
+
+// GraphNodeDotter impl.
+func (n *graphNodeCloseProvider) DotNode(name string, opts *GraphDotOpts) *dot.Node {
+	return dot.NewNode(name, map[string]string{
+		"label": n.Name(),
+		"shape": "diamond",
+	})
+}
+
+// GraphNodeDotterOrigin impl.
+func (n *graphNodeCloseProvider) DotOrigin() bool {
+	return true
+}
+
+// GraphNodeFlattenable impl.
+func (n *graphNodeCloseProvider) Flatten(p []string) (dag.Vertex, error) {
+	return &graphNodeCloseProviderFlat{
+		graphNodeCloseProvider: n,
+		PathValue:              p,
+	}, nil
+}
+
+// Same as graphNodeCloseProvider, but for flattening
+type graphNodeCloseProviderFlat struct {
+	*graphNodeCloseProvider
+
+	PathValue []string
+}
+
+func (n *graphNodeCloseProviderFlat) Name() string {
+	return fmt.Sprintf(
+		"%s.%s", modulePrefixStr(n.PathValue), n.graphNodeCloseProvider.Name())
+}
+
+func (n *graphNodeCloseProviderFlat) Path() []string {
+	return n.PathValue
+}
+
+func (n *graphNodeCloseProviderFlat) ProviderName() string {
+	return fmt.Sprintf(
+		"%s.%s", modulePrefixStr(n.PathValue),
+		n.graphNodeCloseProvider.CloseProviderName())
+}
+
+// GraphNodeDependable impl.
+func (n *graphNodeCloseProviderFlat) DependableName() []string {
+	return []string{n.Name()}
+}
+
+func (n *graphNodeCloseProviderFlat) DependentOn() []string {
+	var result []string
+
+	// If we're in a module, then depend on our parent's provider
+	if len(n.PathValue) > 1 {
+		prefix := modulePrefixStr(n.PathValue[:len(n.PathValue)-1])
+		if prefix != "" {
+			prefix += "."
+		}
+
+		result = append(result, fmt.Sprintf(
+			"%s%s",
+			prefix, n.graphNodeCloseProvider.Name()))
+	}
+
+	return result
+}
+
 type graphNodeMissingProvider struct {
 	ProviderNameValue string
 }
@@ -305,17 +453,6 @@ func (n *graphNodeMissingProvider) Flatten(p []string) (dag.Vertex, error) {
 	}, nil
 }
 
-func providerVertexMap(g *Graph) map[string]dag.Vertex {
-	m := make(map[string]dag.Vertex)
-	for _, v := range g.Vertices() {
-		if pv, ok := v.(GraphNodeProvider); ok {
-			m[pv.ProviderName()] = v
-		}
-	}
-
-	return m
-}
-
 // Same as graphNodeMissingProvider, but for flattening
 type graphNodeMissingProviderFlat struct {
 	*graphNodeMissingProvider

terraform/transform_provider_test.go

@@ -30,6 +30,38 @@ func TestProviderTransformer(t *testing.T) {
 	}
 }
 
+func TestCloseProviderTransformer(t *testing.T) {
+	mod := testModule(t, "transform-provider-basic")
+
+	g := Graph{Path: RootModulePath}
+	{
+		tf := &ConfigTransformer{Module: mod}
+		if err := tf.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &ProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &CloseProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTransformCloseProviderBasicStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s", actual)
+	}
+}
+
 func TestMissingProviderTransformer(t *testing.T) {
 	mod := testModule(t, "transform-provider-basic")
 
@@ -41,10 +73,19 @@ func TestMissingProviderTransformer(t *testing.T) {
 		}
 	}
 
+	{
 		transform := &MissingProviderTransformer{Providers: []string{"foo"}}
 		if err := transform.Transform(&g); err != nil {
 			t.Fatalf("err: %s", err)
 		}
+	}
+
+	{
+		transform := &CloseProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
 
 	actual := strings.TrimSpace(g.String())
 	expected := strings.TrimSpace(testTransformMissingProviderBasicStr)
@@ -78,6 +119,13 @@ func TestPruneProviderTransformer(t *testing.T) {
 		}
 	}
 
+	{
+		transform := &CloseProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
 	{
 		transform := &PruneProviderTransformer{}
 		if err := transform.Transform(&g); err != nil {
@@ -117,6 +165,13 @@ func TestDisableProviderTransformer(t *testing.T) {
 		}
 	}
 
+	{
+		transform := &CloseProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
 	{
 		transform := &PruneProviderTransformer{}
 		if err := transform.Transform(&g); err != nil {
@@ -163,6 +218,13 @@ func TestDisableProviderTransformer_keep(t *testing.T) {
 		}
 	}
 
+	{
+		transform := &CloseProviderTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
 	{
 		transform := &PruneProviderTransformer{}
 		if err := transform.Transform(&g); err != nil {
@@ -203,9 +265,19 @@ aws_instance.web
 provider.aws
 `
 
+const testTransformCloseProviderBasicStr = `
+aws_instance.web
+  provider.aws
+provider.aws
+provider.aws (close)
+  aws_instance.web
+`
+
 const testTransformMissingProviderBasicStr = `
 aws_instance.web
 provider.aws
+provider.aws (close)
+  aws_instance.web
 provider.foo
 `
 
@@ -213,12 +285,16 @@ const testTransformPruneProviderBasicStr = `
 foo_instance.web
   provider.foo
 provider.foo
+provider.foo (close)
+  foo_instance.web
 `
 
 const testTransformDisableProviderBasicStr = `
 module.child
   provider.aws (disabled)
   var.foo
+provider.aws (close)
+  module.child
 provider.aws (disabled)
 var.foo
 `
@@ -230,5 +306,8 @@ module.child
   provider.aws
   var.foo
 provider.aws
+provider.aws (close)
+  aws_instance.foo
+  module.child
 var.foo
 `

terraform/transform_provisioner.go

@@ -14,6 +14,13 @@ type GraphNodeProvisioner interface {
 	ProvisionerName() string
 }
 
+// GraphNodeCloseProvisioner is an interface that nodes that can be a close
+// provisioner must implement. The CloseProvisionerName returned is the name
+// of the provisioner they satisfy.
+type GraphNodeCloseProvisioner interface {
+	CloseProvisionerName() string
+}
+
 // GraphNodeProvisionerConsumer is an interface that nodes that require
 // a provisioner must implement. ProvisionedBy must return the name of the
 // provisioner to use.
@@ -49,6 +56,37 @@ func (t *ProvisionerTransformer) Transform(g *Graph) error {
 	return err
 }
 
+// CloseProvisionerTransformer is a GraphTransformer that adds nodes to the
+// graph that will close open provisioner connections that aren't needed
+// anymore. A provisioner connection is not needed anymore once all depended
+// resources in the graph are evaluated.
+type CloseProvisionerTransformer struct{}
+
+func (t *CloseProvisionerTransformer) Transform(g *Graph) error {
+	m := closeProvisionerVertexMap(g)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeProvisionerConsumer); ok {
+			for _, p := range pv.ProvisionedBy() {
+				source := m[p]
+
+				if source == nil {
+					// Create a new graphNodeCloseProvisioner and add it to the graph
+					source = &graphNodeCloseProvisioner{ProvisionerNameValue: p}
+					g.Add(source)
+
+					// Make sure we also add the new graphNodeCloseProvisioner to the map
+					// so we don't create and add any duplicate graphNodeCloseProvisioners.
+					m[p] = source
+				}
+
+				g.Connect(dag.BasicEdge(source, v))
+			}
+		}
+	}
+
+	return nil
+}
+
 // MissingProvisionerTransformer is a GraphTransformer that adds nodes
 // for missing provisioners into the graph. Specifically, it creates provisioner
 // configuration nodes for all the provisioners that we support. These are
@@ -94,6 +132,75 @@ func (t *PruneProvisionerTransformer) Transform(g *Graph) error {
 	return nil
 }
 
+func provisionerVertexMap(g *Graph) map[string]dag.Vertex {
+	m := make(map[string]dag.Vertex)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeProvisioner); ok {
+			m[pv.ProvisionerName()] = v
+		}
+	}
+
+	return m
+}
+
+func closeProvisionerVertexMap(g *Graph) map[string]dag.Vertex {
+	m := make(map[string]dag.Vertex)
+	for _, v := range g.Vertices() {
+		if pv, ok := v.(GraphNodeCloseProvisioner); ok {
+			m[pv.CloseProvisionerName()] = v
+		}
+	}
+
+	return m
+}
+
+type graphNodeCloseProvisioner struct {
+	ProvisionerNameValue string
+}
+
+func (n *graphNodeCloseProvisioner) Name() string {
+	return fmt.Sprintf("provisioner.%s (close)", n.ProvisionerNameValue)
+}
+
+// GraphNodeEvalable impl.
+func (n *graphNodeCloseProvisioner) EvalTree() EvalNode {
+	return &EvalCloseProvisioner{Name: n.ProvisionerNameValue}
+}
+
+func (n *graphNodeCloseProvisioner) CloseProvisionerName() string {
+	return n.ProvisionerNameValue
+}
+
+// GraphNodeFlattenable impl.
+func (n *graphNodeCloseProvisioner) Flatten(p []string) (dag.Vertex, error) {
+	return &graphNodeCloseProvisionerFlat{
+		graphNodeCloseProvisioner: n,
+		PathValue:                 p,
+	}, nil
+}
+
+// Same as graphNodeCloseProvisioner, but for flattening
+type graphNodeCloseProvisionerFlat struct {
+	*graphNodeCloseProvisioner
+
+	PathValue []string
+}
+
+func (n *graphNodeCloseProvisionerFlat) Name() string {
+	return fmt.Sprintf(
+		"%s.%s", modulePrefixStr(n.PathValue), n.graphNodeCloseProvisioner.Name())
+}
+
+func (n *graphNodeCloseProvisionerFlat) Path() []string {
+	return n.PathValue
+}
+
+func (n *graphNodeCloseProvisionerFlat) ProvisionerName() string {
+	return fmt.Sprintf(
+		"%s.%s", modulePrefixStr(n.PathValue),
+		n.graphNodeCloseProvisioner.CloseProvisionerName())
+}
+
 type graphNodeMissingProvisioner struct {
 	ProvisionerNameValue string
 }
@@ -119,17 +226,6 @@ func (n *graphNodeMissingProvisioner) Flatten(p []string) (dag.Vertex, error) {
 	}, nil
 }
 
-func provisionerVertexMap(g *Graph) map[string]dag.Vertex {
-	m := make(map[string]dag.Vertex)
-	for _, v := range g.Vertices() {
-		if pv, ok := v.(GraphNodeProvisioner); ok {
-			m[pv.ProvisionerName()] = v
-		}
-	}
-
-	return m
-}
-
 // Same as graphNodeMissingProvisioner, but for flattening
 type graphNodeMissingProvisionerFlat struct {
 	*graphNodeMissingProvisioner

terraform/transform_provisioner_test.go

@@ -18,10 +18,19 @@ func TestMissingProvisionerTransformer(t *testing.T) {
 		}
 	}
 
+	{
 		transform := &MissingProvisionerTransformer{Provisioners: []string{"foo"}}
 		if err := transform.Transform(&g); err != nil {
 			t.Fatalf("err: %s", err)
 		}
+	}
+
+	{
+		transform := &CloseProvisionerTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
 
 	actual := strings.TrimSpace(g.String())
 	expected := strings.TrimSpace(testTransformMissingProvisionerBasicStr)
@@ -56,6 +65,13 @@ func TestPruneProvisionerTransformer(t *testing.T) {
 		}
 	}
 
+	{
+		transform := &CloseProvisionerTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
 	{
 		transform := &PruneProvisionerTransformer{}
 		if err := transform.Transform(&g); err != nil {
@@ -86,10 +102,14 @@ func TestGraphNodeMissingProvisioner_ProvisionerName(t *testing.T) {
 const testTransformMissingProvisionerBasicStr = `
 aws_instance.web
 provisioner.foo
+provisioner.shell (close)
+  aws_instance.web
 `
 
 const testTransformPruneProvisionerBasicStr = `
 aws_instance.web
   provisioner.foo
 provisioner.foo
+provisioner.foo (close)
+  aws_instance.web
 `