command: remove legacy remote state on migration

Fixes #12871

We were forgetting to remove the legacy remote state from the actual
state value when migrating. This only causes an issue when saving a plan
since the plan contains the state itself and causes an error where both
a backend + legacy state exist.

If saved plans aren't used this causes no noticable issue.

Due to buggy upgrades already existing in the wild, I also added code to
clear the remote section if it exists in a standard unchanged backend

backend/local/backend_plan.go

@@ -110,6 +110,12 @@ func (b *Local) opPlan(
 		// Write the backend if we have one
 		plan.Backend = op.PlanOutBackend
 
+		// This works around a bug (#12871) which is no longer possible to
+		// trigger but will exist for already corrupted upgrades.
+		if plan.Backend != nil && plan.State != nil {
+			plan.State.Remote = nil
+		}
+
 		log.Printf("[INFO] backend/local: writing plan output to: %s", path)
 		f, err := os.Create(path)
 		if err == nil {

command/meta_backend_migrate.go

@@ -235,6 +235,15 @@ func (m *Meta) backendMigrateState_s_s(opts *backendMigrateOpts) error {
 	one := stateOne.State()
 	two := stateTwo.State()
 
+	// Clear the legacy remote state in both cases. If we're at the migration
+	// step then this won't be used anymore.
+	if one != nil {
+		one.Remote = nil
+	}
+	if two != nil {
+		two.Remote = nil
+	}
+
 	var confirmFunc func(state.State, state.State, *backendMigrateOpts) (bool, error)
 	switch {
 	// No migration necessary

command/meta_backend_test.go

@@ -771,6 +771,13 @@ func TestMetaBackend_configureNewLegacyCopy(t *testing.T) {
 		}
 	}
 
+	// Verify we have no configured legacy in the state itself
+	{
+		if !state.Remote.Empty() {
+			t.Fatalf("legacy has remote state: %#v", state.Remote)
+		}
+	}
+
 	// Write some state
 	state = terraform.NewState()
 	state.Lineage = "changing"

command/test-fixtures/backend-new-legacy/local-state-old.tfstate

@@ -2,5 +2,11 @@
     "version": 3,
     "terraform_version": "0.8.2",
     "serial": 7,
-    "lineage": "backend-new-legacy"
+    "lineage": "backend-new-legacy",
+    "remote": {
+        "type": "local",
+        "config": {
+            "path": "local-state-old.tfstate"
+        }
+    }
 }