Support plugins in `docker inspect`

This fix tries to address the proposal raised in 28946
to support plugins in `docker inspect`.

The command `docker inspect` already supports
"container", "image", "node", "network", "service", "volume", "task".
However, `--type plugin` is not supported yet at the moment.

This fix address this issue by adding the support of `--type plugin`
for `docker inspect`.

An additional integration test has been added to cover the changes.

This fix fixes 28946.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

errors.go

@@ -255,3 +255,24 @@ func IsErrSecretNotFound(err error) bool {
 	_, ok := err.(secretNotFoundError)
 	return ok
 }
+
+// pluginNotFoundError implements an error returned when a plugin is not in the docker host.
+type pluginNotFoundError struct {
+	name string
+}
+
+// NotFound indicates that this error type is of NotFound
+func (e pluginNotFoundError) NotFound() bool {
+	return true
+}
+
+// Error returns a string representation of a pluginNotFoundError
+func (e pluginNotFoundError) Error() string {
+	return fmt.Sprintf("Error: No such plugin: %s", e.name)
+}
+
+// IsErrPluginNotFound returns true if the error is caused
+// when a plugin is not found in the docker host.
+func IsErrPluginNotFound(err error) bool {
+	return IsErrNotFound(err)
+}

plugin_inspect.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"io/ioutil"
+	"net/http"
 
 	"github.com/docker/docker/api/types"
 	"golang.org/x/net/context"
@@ -13,6 +14,9 @@ import (
 func (cli *Client) PluginInspectWithRaw(ctx context.Context, name string) (*types.Plugin, []byte, error) {
 	resp, err := cli.get(ctx, "/plugins/"+name, nil, nil)
 	if err != nil {
+		if resp.statusCode == http.StatusNotFound {
+			return nil, nil, pluginNotFoundError{name}
+		}
 		return nil, nil, err
 	}