metadata: improve deleting a non-empty namespace's error message

Deleting a non-empty namespace fails with

> namespace must be empty: failed precondition

This change improves the error message by listing the types of
the objects in the namespace that prevent deletion.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

metadata/namespaces_test.go

@@ -0,0 +1,85 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package metadata
+
+import (
+	"context"
+	"testing"
+
+	"github.com/containerd/containerd/containers"
+	"github.com/containerd/containerd/namespaces"
+	"github.com/gogo/protobuf/types"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.etcd.io/bbolt"
+)
+
+func TestCreateDelete(t *testing.T) {
+	ctx, db, cleanup := testDB(t)
+	defer cleanup()
+
+	subtests := []struct {
+		name     string
+		create   func(t *testing.T, ctx context.Context)
+		validate func(t *testing.T, err error)
+	}{
+		{
+			name:   "empty",
+			create: func(t *testing.T, ctx context.Context) {},
+			validate: func(t *testing.T, err error) {
+				require.NoError(t, err)
+			},
+		},
+		{
+			name: "not-empty",
+			create: func(t *testing.T, ctx context.Context) {
+				store := NewContainerStore(db)
+				_, err := store.Create(ctx, containers.Container{
+					ID:      "c1",
+					Runtime: containers.RuntimeInfo{Name: "rt"},
+					Spec:    &types.Any{},
+				})
+				require.NoError(t, err)
+			},
+			validate: func(t *testing.T, err error) {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), "still has containers")
+			},
+		},
+	}
+
+	for _, subtest := range subtests {
+		ns := subtest.name
+		ctx = namespaces.WithNamespace(ctx, ns)
+
+		t.Run(subtest.name, func(t *testing.T) {
+			err := db.Update(func(tx *bbolt.Tx) error {
+				store := NewNamespaceStore(tx)
+				return store.Create(ctx, ns, nil)
+			})
+			require.NoError(t, err)
+
+			subtest.create(t, ctx)
+
+			err = db.Update(func(tx *bbolt.Tx) error {
+				store := NewNamespaceStore(tx)
+				return store.Delete(ctx, ns)
+			})
+			subtest.validate(t, err)
+		})
+	}
+}