expose StopContext in EvalContext

The existing Stopped method predates the use of a context, returning a
simple channel for signaling cancellation. The method was not used
however, so we can just update it to return a context to give us access
to cancellation for ephemeral values.

internal/terraform/eval_context.go

@@ -4,6 +4,8 @@
 package terraform
 
 import (
+	"context"
+
 	"github.com/hashicorp/hcl/v2"
 	"github.com/zclconf/go-cty/cty"
 
@@ -28,9 +30,9 @@ import (
 
 // EvalContext is the interface that is given to eval nodes to execute.
 type EvalContext interface {
-	// Stopped returns a channel that is closed when evaluation is stopped
-	// via Terraform.Context.Stop()
-	Stopped() <-chan struct{}
+	// Stopped returns a context that is canceled when evaluation is stopped via
+	// Terraform.Context.Stop()
+	StopCtx() context.Context
 
 	// Path is the current module path.
 	Path() addrs.ModuleInstance

internal/terraform/eval_context_builtin.go

@@ -102,13 +102,13 @@ func (ctx *BuiltinEvalContext) withScope(scope evalContextScope) EvalContext {
 	return &newCtx
 }
 
-func (ctx *BuiltinEvalContext) Stopped() <-chan struct{} {
+func (ctx *BuiltinEvalContext) StopCtx() context.Context {
 	// This can happen during tests. During tests, we just block forever.
 	if ctx.StopContext == nil {
-		return nil
+		return context.TODO()
 	}
 
-	return ctx.StopContext.Done()
+	return ctx.StopContext
 }
 
 func (ctx *BuiltinEvalContext) Hook(fn func(Hook) (HookAction, error)) error {

internal/terraform/eval_context_mock.go

@@ -4,6 +4,8 @@
 package terraform
 
 import (
+	"context"
+
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/hcldec"
 	"github.com/zclconf/go-cty/cty"
@@ -31,8 +33,8 @@ import (
 // MockEvalContext is a mock version of EvalContext that can be used
 // for tests.
 type MockEvalContext struct {
-	StoppedCalled bool
-	StoppedValue  <-chan struct{}
+	StopCtxCalled bool
+	StopCtxValue  context.Context
 
 	HookCalled bool
 	HookHook   Hook
@@ -164,9 +166,9 @@ type MockEvalContext struct {
 // MockEvalContext implements EvalContext
 var _ EvalContext = (*MockEvalContext)(nil)
 
-func (c *MockEvalContext) Stopped() <-chan struct{} {
-	c.StoppedCalled = true
-	return c.StoppedValue
+func (c *MockEvalContext) StopCtx() context.Context {
+	c.StopCtxCalled = true
+	return c.StopCtxValue
 }
 
 func (c *MockEvalContext) Hook(fn func(Hook) (HookAction, error)) error {