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/google.golang.org/appengine/internal/net.go

@@ -0,0 +1,56 @@
+// Copyright 2014 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package internal
+
+// This file implements a network dialer that limits the number of concurrent connections.
+// It is only used for API calls.
+
+import (
+	"log"
+	"net"
+	"runtime"
+	"sync"
+	"time"
+)
+
+var limitSem = make(chan int, 100) // TODO(dsymonds): Use environment variable.
+
+func limitRelease() {
+	// non-blocking
+	select {
+	case <-limitSem:
+	default:
+		// This should not normally happen.
+		log.Print("appengine: unbalanced limitSem release!")
+	}
+}
+
+func limitDial(network, addr string) (net.Conn, error) {
+	limitSem <- 1
+
+	// Dial with a timeout in case the API host is MIA.
+	// The connection should normally be very fast.
+	conn, err := net.DialTimeout(network, addr, 500*time.Millisecond)
+	if err != nil {
+		limitRelease()
+		return nil, err
+	}
+	lc := &limitConn{Conn: conn}
+	runtime.SetFinalizer(lc, (*limitConn).Close) // shouldn't usually be required
+	return lc, nil
+}
+
+type limitConn struct {
+	close sync.Once
+	net.Conn
+}
+
+func (lc *limitConn) Close() error {
+	defer lc.close.Do(func() {
+		limitRelease()
+		runtime.SetFinalizer(lc, nil)
+	})
+	return lc.Conn.Close()
+}