fix: Fail apply command if the plan file was generated for a workspac…

…e that isn't the selected workspace.

internal/command/meta_backend.go

@@ -333,6 +333,21 @@ func (m *Meta) selectWorkspace(b backend.Backend) error {
 func (m *Meta) BackendForLocalPlan(settings plans.Backend) (backendrun.OperationsBackend, tfdiags.Diagnostics) {
 	var diags tfdiags.Diagnostics
 
+	// Check the workspace name in the plan matches the current workspace
+	w, err := m.Workspace()
+	if err != nil {
+		diags = diags.Append(fmt.Errorf("error determining current workspace when initializing a backend from the plan file: %w", err))
+		return nil, diags
+	}
+	if w != settings.Workspace {
+		diags = diags.Append(&errWrongWorkspaceForPlan{
+			currentWorkspace: w,
+			plannedWorkspace: settings.Workspace,
+		})
+		return nil, diags
+	}
+
+	// Proceed with initializing the backend from the configuration in the plan file
 	f := backendInit.Backend(settings.Type)
 	if f == nil {
 		diags = diags.Append(errBackendSavedUnknown{settings.Type})

internal/command/meta_backend_errors.go

@@ -9,6 +9,27 @@ import (
 	"github.com/hashicorp/terraform/internal/tfdiags"
 )
 
+// errWrongWorkspaceForPlan is a custom error used to alert users that the plan file they are applying
+// describes a workspace that doesn't match the currently selected workspace.
+type errWrongWorkspaceForPlan struct {
+	plannedWorkspace string
+	currentWorkspace string
+}
+
+func (e *errWrongWorkspaceForPlan) Error() string {
+	return fmt.Sprintf(`The plan file describes changes to the %q workspace, but the %q workspace is currently selected in the working directory.
+
+Applying this plan with the incorrect workspace selected could result in state being stored in an unexpected location, or a downstream error
+when Terraform attempts apply a plan using the other workspace's state.
+
+If you'd like to continue to use the plan file, you must run "terraform workspace select %s" to select the correct workspace.
+In future make sure the selected workspace is not changed between creating and applying a plan file.`,
+		e.plannedWorkspace,
+		e.currentWorkspace,
+		e.plannedWorkspace,
+	)
+}
+
 // errBackendLocalRead is a custom error used to alert users that state
 // files on their local filesystem were not erased successfully after
 // migrating that state to a remote-state backend.

internal/command/meta_backend_test.go

@@ -1837,6 +1837,52 @@ func TestMetaBackend_planLocalMatch(t *testing.T) {
 	}
 }
 
+// A plan that contains a workspace that isn't the currently selected workspace
+func TestMetaBackend_planLocal_mismatchedWorkspace(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := t.TempDir()
+	testCopyDir(t, testFixturePath("backend-plan-local"), td)
+	t.Chdir(td)
+
+	backendConfigBlock := cty.ObjectVal(map[string]cty.Value{
+		"path":          cty.NullVal(cty.String),
+		"workspace_dir": cty.NullVal(cty.String),
+	})
+	backendConfigRaw, err := plans.NewDynamicValue(backendConfigBlock, backendConfigBlock.Type())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defaultWorkspace := "default"
+	backendConfig := plans.Backend{
+		Type:      "local",
+		Config:    backendConfigRaw,
+		Workspace: defaultWorkspace,
+	}
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+	selectedWorkspace := "foobar"
+	err = m.SetWorkspace(selectedWorkspace)
+	if err != nil {
+		t.Fatalf("error in test setup: %s", err)
+	}
+
+	// Get the backend
+	_, diags := m.BackendForLocalPlan(backendConfig)
+	if !diags.HasErrors() {
+		t.Fatalf("expected an error but got none: %s", diags.ErrWithWarnings())
+	}
+	expectedMsg := fmt.Sprintf("The plan file describes changes to the %q workspace, but the %q workspace is currently selected in the working directory",
+		defaultWorkspace,
+		selectedWorkspace,
+	)
+	if !strings.Contains(diags.Err().Error(), expectedMsg) {
+		t.Fatalf("expected error to include %q, but got:\n%s",
+			expectedMsg,
+			diags.Err())
+	}
+}
+
 // init a backend using -backend-config options multiple times
 func TestMetaBackend_configureBackendWithExtra(t *testing.T) {
 	// Create a temporary working directory that is empty