new driver: kubernetes

Tested with `kind` and GKE.

Note: "nodes" shown in `docker buildx ls` are unrelated to Kubernetes "nodes".
Probably buildx should come up with an alternative term.

Usage:

  $ kind create cluster
  $ export KUBECONFIG="$(kind get kubeconfig-path --name="kind")"

  $ docker buildx create --driver kubernetes --driver-opt replicas=3 --use
  $ docker buildx build -t foo --load .

`--load` loads the image into the local Docker.

Driver opts:

  - `image=IMAGE` - Sets the container image to be used for running buildkit.
  - `namespace=NS` - Sets the Kubernetes namespace. Defaults to the current namespace.
  - `replicas=N` - Sets the number of `Pod` replicas. Defaults to 1.
  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. Defaults to false.
  - `loadbalance=(sticky|random)` - Load-balancing strategy. If set to "sticky", the pod is chosen using the hash of the context path. Defaults to "sticky"

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/github.com/googleapis/gnostic/compiler/extension-handler.go

@@ -0,0 +1,101 @@
+// Copyright 2017 Google Inc. All Rights Reserved.
+//
+// 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 compiler
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+
+	"strings"
+
+	"errors"
+
+	"github.com/golang/protobuf/proto"
+	"github.com/golang/protobuf/ptypes/any"
+	ext_plugin "github.com/googleapis/gnostic/extensions"
+	yaml "gopkg.in/yaml.v2"
+)
+
+// ExtensionHandler describes a binary that is called by the compiler to handle specification extensions.
+type ExtensionHandler struct {
+	Name string
+}
+
+// HandleExtension calls a binary extension handler.
+func HandleExtension(context *Context, in interface{}, extensionName string) (bool, *any.Any, error) {
+	handled := false
+	var errFromPlugin error
+	var outFromPlugin *any.Any
+
+	if context != nil && context.ExtensionHandlers != nil && len(*(context.ExtensionHandlers)) != 0 {
+		for _, customAnyProtoGenerator := range *(context.ExtensionHandlers) {
+			outFromPlugin, errFromPlugin = customAnyProtoGenerator.handle(in, extensionName)
+			if outFromPlugin == nil {
+				continue
+			} else {
+				handled = true
+				break
+			}
+		}
+	}
+	return handled, outFromPlugin, errFromPlugin
+}
+
+func (extensionHandlers *ExtensionHandler) handle(in interface{}, extensionName string) (*any.Any, error) {
+	if extensionHandlers.Name != "" {
+		binary, _ := yaml.Marshal(in)
+
+		request := &ext_plugin.ExtensionHandlerRequest{}
+
+		version := &ext_plugin.Version{}
+		version.Major = 0
+		version.Minor = 1
+		version.Patch = 0
+		request.CompilerVersion = version
+
+		request.Wrapper = &ext_plugin.Wrapper{}
+
+		request.Wrapper.Version = "v2"
+		request.Wrapper.Yaml = string(binary)
+		request.Wrapper.ExtensionName = extensionName
+
+		requestBytes, _ := proto.Marshal(request)
+		cmd := exec.Command(extensionHandlers.Name)
+		cmd.Stdin = bytes.NewReader(requestBytes)
+		output, err := cmd.Output()
+
+		if err != nil {
+			fmt.Printf("Error: %+v\n", err)
+			return nil, err
+		}
+		response := &ext_plugin.ExtensionHandlerResponse{}
+		err = proto.Unmarshal(output, response)
+		if err != nil {
+			fmt.Printf("Error: %+v\n", err)
+			fmt.Printf("%s\n", string(output))
+			return nil, err
+		}
+		if !response.Handled {
+			return nil, nil
+		}
+		if len(response.Error) != 0 {
+			message := fmt.Sprintf("Errors when parsing: %+v for field %s by vendor extension handler %s. Details %+v", in, extensionName, extensionHandlers.Name, strings.Join(response.Error, ","))
+			return nil, errors.New(message)
+		}
+		return response.Value, nil
+	}
+	return nil, nil
+}