WithLease: always return context and done fn

We should never return a nil context because of the way this function is
typically used... e.g.

```
  ctx, done, err := containerd.WithLease(ctx)
```

If there is an error `ctx` will be nil and any error handling may cause
an NPE if it tries to use `ctx`.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/commands/images/mount.go

@@ -1,3 +1,19 @@
+/*
+   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 images
 
 import (
@@ -15,22 +31,6 @@ import (
 	"github.com/urfave/cli"
 )
 
-/*
-   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.
-*/
-
 var mountCommand = cli.Command{
 	Name:      "mount",
 	Usage:     "mount an image to a target path",

cmd/ctr/commands/images/unmount.go

@@ -1,16 +1,3 @@
-package images
-
-import (
-	"fmt"
-
-	"github.com/containerd/containerd/cmd/ctr/commands"
-	"github.com/containerd/containerd/errdefs"
-	"github.com/containerd/containerd/leases"
-	"github.com/containerd/containerd/mount"
-	"github.com/pkg/errors"
-	"github.com/urfave/cli"
-)
-
 /*
    Copyright The containerd Authors.
 
@@ -27,6 +14,19 @@ import (
    limitations under the License.
 */
 
+package images
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/leases"
+	"github.com/containerd/containerd/mount"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
 var unmountCommand = cli.Command{
 	Name:        "unmount",
 	Usage:       "unmount the image from the target",

lease.go

@@ -25,11 +25,11 @@ import (
 
 // WithLease attaches a lease on the context
 func (c *Client) WithLease(ctx context.Context, opts ...leases.Opt) (context.Context, func(context.Context) error, error) {
+	nop := func(context.Context) error { return nil }
+
 	_, ok := leases.FromContext(ctx)
 	if ok {
-		return ctx, func(context.Context) error {
-			return nil
-		}, nil
+		return ctx, nop, nil
 	}
 
 	ls := c.LeasesService()
@@ -44,7 +44,7 @@ func (c *Client) WithLease(ctx context.Context, opts ...leases.Opt) (context.Con
 
 	l, err := ls.Create(ctx, opts...)
 	if err != nil {
-		return nil, nil, err
+		return ctx, nop, err
 	}
 
 	ctx = leases.WithLease(ctx, l.ID)