core: Make an instances.Expander available to every graph walk

This is not used yet, but in future commits will be used as a
"blackboard" to centrally aggregate the information pertaining to
expansion of resources and modules (using "count" or "for_each") to help
ensure consistent treatment of the expansion process during a graph walk.

In practice this only really makes sense for the plan walk, because the
apply walk doesn't do any dynamic expansion.

terraform/context.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
+	"github.com/hashicorp/terraform/instances"
 	"github.com/hashicorp/terraform/lang"
 	"github.com/hashicorp/terraform/plans"
 	"github.com/hashicorp/terraform/providers"
@@ -788,6 +789,7 @@ func (c *Context) graphWalker(operation walkOperation) *ContextGraphWalker {
 		Context:            c,
 		State:              c.state.SyncWrapper(),
 		Changes:            c.changes.SyncWrapper(),
+		InstanceExpander:   instances.NewExpander(),
 		Operation:          operation,
 		StopContext:        c.runContext,
 		RootVariableValues: c.variables,

terraform/eval_context_builtin.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"sync"
 
+	"github.com/hashicorp/terraform/instances"
 	"github.com/hashicorp/terraform/plans"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/provisioners"
@@ -53,16 +54,17 @@ type BuiltinEvalContext struct {
 	VariableValues     map[string]map[string]cty.Value
 	VariableValuesLock *sync.Mutex
 
-	Components          contextComponentFactory
-	Hooks               []Hook
-	InputValue          UIInput
-	ProviderCache       map[string]providers.Interface
-	ProviderInputConfig map[string]map[string]cty.Value
-	ProviderLock        *sync.Mutex
-	ProvisionerCache    map[string]provisioners.Interface
-	ProvisionerLock     *sync.Mutex
-	ChangesValue        *plans.ChangesSync
-	StateValue          *states.SyncState
+	Components            contextComponentFactory
+	Hooks                 []Hook
+	InputValue            UIInput
+	ProviderCache         map[string]providers.Interface
+	ProviderInputConfig   map[string]map[string]cty.Value
+	ProviderLock          *sync.Mutex
+	ProvisionerCache      map[string]provisioners.Interface
+	ProvisionerLock       *sync.Mutex
+	ChangesValue          *plans.ChangesSync
+	StateValue            *states.SyncState
+	InstanceExpanderValue *instances.Expander
 
 	once sync.Once
 }
@@ -359,5 +361,9 @@ func (ctx *BuiltinEvalContext) State() *states.SyncState {
 	return ctx.StateValue
 }
 
+func (ctx *BuiltinEvalContext) InstanceExpander() *instances.Expander {
+	return ctx.InstanceExpanderValue
+}
+
 func (ctx *BuiltinEvalContext) init() {
 }

terraform/eval_context_mock.go

@@ -5,6 +5,7 @@ import (
 	"github.com/hashicorp/hcl/v2/hcldec"
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs/configschema"
+	"github.com/hashicorp/terraform/instances"
 	"github.com/hashicorp/terraform/lang"
 	"github.com/hashicorp/terraform/plans"
 	"github.com/hashicorp/terraform/providers"
@@ -124,6 +125,9 @@ type MockEvalContext struct {
 
 	StateCalled bool
 	StateState  *states.SyncState
+
+	InstanceExpanderCalled   bool
+	InstanceExpanderExpander *instances.Expander
 }
 
 // MockEvalContext implements EvalContext
@@ -327,3 +331,8 @@ func (c *MockEvalContext) State() *states.SyncState {
 	c.StateCalled = true
 	return c.StateState
 }
+
+func (c *MockEvalContext) InstanceExpander() *instances.Expander {
+	c.InstanceExpanderCalled = true
+	return c.InstanceExpanderExpander
+}

terraform/graph_walk_context.go

@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs/configschema"
 	"github.com/hashicorp/terraform/dag"
+	"github.com/hashicorp/terraform/instances"
 	"github.com/hashicorp/terraform/plans"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/provisioners"
@@ -24,8 +25,9 @@ type ContextGraphWalker struct {
 
 	// Configurable values
 	Context            *Context
-	State              *states.SyncState  // Used for safe concurrent access to state
-	Changes            *plans.ChangesSync // Used for safe concurrent writes to changes
+	State              *states.SyncState   // Used for safe concurrent access to state
+	Changes            *plans.ChangesSync  // Used for safe concurrent writes to changes
+	InstanceExpander   *instances.Expander // Tracks our gradual expansion of module and resource instances
 	Operation          walkOperation
 	StopContext        context.Context
 	RootVariableValues InputValues
@@ -75,22 +77,23 @@ func (w *ContextGraphWalker) EnterPath(path addrs.ModuleInstance) EvalContext {
 	}
 
 	ctx := &BuiltinEvalContext{
-		StopContext:         w.StopContext,
-		PathValue:           path,
-		Hooks:               w.Context.hooks,
-		InputValue:          w.Context.uiInput,
-		Components:          w.Context.components,
-		Schemas:             w.Context.schemas,
-		ProviderCache:       w.providerCache,
-		ProviderInputConfig: w.Context.providerInputConfig,
-		ProviderLock:        &w.providerLock,
-		ProvisionerCache:    w.provisionerCache,
-		ProvisionerLock:     &w.provisionerLock,
-		ChangesValue:        w.Changes,
-		StateValue:          w.State,
-		Evaluator:           evaluator,
-		VariableValues:      w.variableValues,
-		VariableValuesLock:  &w.variableValuesLock,
+		StopContext:           w.StopContext,
+		PathValue:             path,
+		Hooks:                 w.Context.hooks,
+		InputValue:            w.Context.uiInput,
+		InstanceExpanderValue: w.InstanceExpander,
+		Components:            w.Context.components,
+		Schemas:               w.Context.schemas,
+		ProviderCache:         w.providerCache,
+		ProviderInputConfig:   w.Context.providerInputConfig,
+		ProviderLock:          &w.providerLock,
+		ProvisionerCache:      w.provisionerCache,
+		ProvisionerLock:       &w.provisionerLock,
+		ChangesValue:          w.Changes,
+		StateValue:            w.State,
+		Evaluator:             evaluator,
+		VariableValues:        w.variableValues,
+		VariableValuesLock:    &w.variableValuesLock,
 	}
 
 	w.contexts[key] = ctx