core: Do not assume HCL parser has touched vars

This PR fixes #7824, which crashed when applying a plan file. The bug is
that while a map which has come from the HCL parser reifies as a
[]map[string]interface{}, the variable saved in the plan file was not.
We now cover both cases.

Fixes #7824.

terraform/context.go

@@ -181,11 +181,16 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 							if existingMap, ok := existing.(map[string]interface{}); !ok {
 								panic(fmt.Sprintf("%s is not a map, this is a bug in Terraform.", k))
 							} else {
-								if newMap, ok := varVal.(map[string]interface{}); ok {
-									for newKey, newVal := range newMap {
+								switch typedV := varVal.(type) {
+								case []map[string]interface{}:
+									for newKey, newVal := range typedV[0] {
 										existingMap[newKey] = newVal
 									}
-								} else {
+								case map[string]interface{}:
+									for newKey, newVal := range typedV {
+										existingMap[newKey] = newVal
+									}
+								default:
 									panic(fmt.Sprintf("%s is not a map, this is a bug in Terraform.", k))
 								}
 							}
@@ -208,11 +213,16 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 							if existingMap, ok := existing.(map[string]interface{}); !ok {
 								panic(fmt.Sprintf("%s is not a map, this is a bug in Terraform.", k))
 							} else {
-								if newMap, ok := v.([]map[string]interface{}); ok {
-									for newKey, newVal := range newMap[0] {
+								switch typedV := v.(type) {
+								case []map[string]interface{}:
+									for newKey, newVal := range typedV[0] {
 										existingMap[newKey] = newVal
 									}
-								} else {
+								case map[string]interface{}:
+									for newKey, newVal := range typedV {
+										existingMap[newKey] = newVal
+									}
+								default:
 									panic(fmt.Sprintf("%s is not a map, this is a bug in Terraform.", k))
 								}
 							}

terraform/context_apply_test.go

@@ -4523,6 +4523,55 @@ func TestContext2Apply_singleDestroy(t *testing.T) {
 	}
 }
 
+// GH-7824
+func TestContext2Apply_issue7824(t *testing.T) {
+	p := testProvider("template")
+	p.ResourcesReturn = append(p.ResourcesReturn, ResourceType{
+		Name: "template_file",
+	})
+
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	// Apply cleanly step 0
+	ctx := testContext2(t, &ContextOpts{
+		Module: testModule(t, "issue-7824"),
+		Providers: map[string]ResourceProviderFactory{
+			"template": testProviderFuncFixed(p),
+		},
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Write / Read plan to simulate running it through a Plan file
+	var buf bytes.Buffer
+	if err := WritePlan(plan, &buf); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	planFromFile, err := ReadPlan(&buf)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	ctx, err = planFromFile.Context(&ContextOpts{
+		Providers: map[string]ResourceProviderFactory{
+			"template": testProviderFuncFixed(p),
+		},
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	_, err = ctx.Apply()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}
+
 // GH-5254
 func TestContext2Apply_issue5254(t *testing.T) {
 	// Create a provider. We use "template" here just to match the repro

terraform/test-fixtures/issue-7824/main.tf

@@ -0,0 +1,6 @@
+variable "test" {
+  type = "map"
+  default = {
+    "test" = "1"
+  }
+}