backend/migrations: check all workspaces

This commit fixes a bug that (in the case of the `local` backend) would only check if the selected workspace had a state when deciding to preform a migration.

When the selected workspace didn’t have a state (but other existing workspace(s) did), the migration would not be preformed and the other workspaces would be ignored.

command/meta_backend.go

@@ -907,9 +907,14 @@ func (m *Meta) backend_C_r_s(
 		return nil, fmt.Errorf(errBackendLocalRead, err)
 	}
 
-	env := m.Workspace()
+	workspaces, err := localB.States()
+	if err != nil {
+		return nil, fmt.Errorf(errBackendLocalRead, err)
+	}
 
-	localState, err := localB.State(env)
+	var localStates []state.State
+	for _, workspace := range workspaces {
+		localState, err := localB.State(workspace)
 		if err != nil {
 			return nil, fmt.Errorf(errBackendLocalRead, err)
 		}
@@ -917,10 +922,13 @@ func (m *Meta) backend_C_r_s(
 			return nil, fmt.Errorf(errBackendLocalRead, err)
 		}
 
-	// If the local state is not empty, we need to potentially do a
-	// state migration to the new backend (with user permission), unless the
-	// destination is also "local"
+		// We only care about non-empty states.
 		if localS := localState.State(); !localS.Empty() {
+			localStates = append(localStates, localState)
+		}
+	}
+
+	if len(localStates) > 0 {
 		// Perform the migration
 		err = m.backendMigrateState(&backendMigrateOpts{
 			OneType: "local",
@@ -946,6 +954,7 @@ func (m *Meta) backend_C_r_s(
 		}
 
 		if erase {
+			for _, localState := range localStates {
 				// We always delete the local state, unless that was our new state too.
 				if err := localState.WriteState(nil); err != nil {
 					return nil, fmt.Errorf(errBackendMigrateLocalDelete, err)
@@ -955,6 +964,7 @@ func (m *Meta) backend_C_r_s(
 				}
 			}
 		}
+	}
 
 	if m.stateLock {
 		stateLocker := clistate.NewLocker(context.Background(), m.stateLockTimeout, m.Ui, m.Colorize())