return errors if selectors reduce all entries (#427)

* return errors if selectors reduce all entries

During a `backup details` or `restore` command, if the
selectors produce 0 results, return an error rather than
running a no-op process.

src/cli/backup/exchange.go

@@ -329,8 +329,11 @@ func detailsExchangeCmd(cmd *cobra.Command, args []string) error {
 	}
 
 	ds := sel.Reduce(d)
-	print.Entries(ds.Entries)
+	if len(ds.Entries) == 0 {
+		return errors.New("nothing to display: no items in the backup match the provided selectors")
+	}
 
+	print.Entries(ds.Entries)
 	return nil
 }
 

src/internal/operations/restore.go

@@ -98,6 +98,10 @@ func (op *RestoreOperation) Run(ctx context.Context) error {
 
 	// format the details and retrieve the items from kopia
 	fds := er.Reduce(d)
+	if len(fds.Entries) == 0 {
+		return errors.New("nothing to restore: no items in the backup match the provided selectors")
+	}
+
 	// todo: use path pkg for this
 	fdsPaths := fds.Paths()
 	paths := make([][]string, len(fdsPaths))

src/internal/operations/restore_test.go

@@ -186,3 +186,58 @@ func (suite *RestoreOpIntegrationSuite) TestRestore_Run() {
 	assert.Zero(t, ro.Results.WriteErrors, "errors while writing restore data")
 	assert.Equal(t, bo.Results.ItemsWritten, ro.Results.ItemsWritten, "backup and restore wrote the same num of items")
 }
+
+func (suite *RestoreOpIntegrationSuite) TestRestore_Run_ErrorNoResults() {
+	t := suite.T()
+	ctx := context.Background()
+
+	m365User := "lidiah@8qzvrj.onmicrosoft.com"
+	acct, err := ctesting.NewM365Account()
+	require.NoError(t, err)
+
+	// need to initialize the repository before we can test connecting to it.
+	st, err := ctesting.NewPrefixedS3Storage(t)
+	require.NoError(t, err)
+
+	k := kopia.NewConn(st)
+	require.NoError(t, k.Initialize(ctx))
+	defer k.Close(ctx)
+
+	w, err := kopia.NewWrapper(k)
+	require.NoError(t, err)
+	defer w.Close(ctx)
+
+	ms, err := kopia.NewModelStore(k)
+	require.NoError(t, err)
+	defer ms.Close(ctx)
+
+	sw := store.NewKopiaStore(ms)
+
+	bsel := selectors.NewExchangeBackup()
+	bsel.Include(bsel.Users([]string{m365User}))
+
+	bo, err := NewBackupOperation(
+		ctx,
+		Options{},
+		w,
+		sw,
+		acct,
+		bsel.Selector)
+	require.NoError(t, err)
+	require.NoError(t, bo.Run(ctx))
+	require.NotEmpty(t, bo.Results.BackupID)
+
+	rsel := selectors.NewExchangeRestore()
+	rsel.Include(rsel.Users(selectors.None()))
+
+	ro, err := NewRestoreOperation(
+		ctx,
+		Options{},
+		w,
+		sw,
+		acct,
+		bo.Results.BackupID,
+		rsel.Selector)
+	require.NoError(t, err)
+	require.Error(t, ro.Run(ctx), "restoreOp.Run() should have 0 results")
+}