new connhelper: kube-pod

Fix #769
Relates to tonistiigi/buildx#22

Usage:

  $ kubectl run --generator=run-pod/v1 --image moby/buildkit:master-rootless bk -- --oci-worker-no-process-sandbox
  $ export BUILDKIT_HOST=kube-pod://bk

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

README.md

@@ -271,6 +271,18 @@ buildctl build --help
 The images can be also built locally using `./hack/dockerfiles/test.Dockerfile` (or `./hack/dockerfiles/test.buildkit.Dockerfile` if you already have BuildKit).
 Run `make images` to build the images as `moby/buildkit:local` and `moby/buildkit:local-rootless`.
 
+#### Connection helpers
+
+If you are running `moby/buildkit:master` or `moby/buildkit:master-rootless` as a Docker/Kubernetes container, you can use special `BUILDKIT_HOST` URL for connecting to the BuildKit daemon in the container:
+
+```
+export BUILDKIT_HOST=docker://<container>
+```
+
+```
+export BUILDKIT_HOST=kube-pod://<pod>
+```
+
 ### Opentracing support
 
 BuildKit supports opentracing for buildkitd gRPC API and buildctl commands. To capture the trace to [Jaeger](https://github.com/jaegertracing/jaeger), set `JAEGER_TRACE` environment variable to the collection address.

client/connhelper/kubepod/kubepod.go

@@ -0,0 +1,77 @@
+// Package kubepod provides connhelper for kube-pod://<pod>
+package kubepod
+
+import (
+	"context"
+	"net"
+	"net/url"
+	"regexp"
+
+	"github.com/docker/cli/cli/connhelper/commandconn"
+	"github.com/moby/buildkit/client/connhelper"
+	"github.com/pkg/errors"
+)
+
+func init() {
+	connhelper.Register("kube-pod", Helper)
+}
+
+// Helper returns helper for connecting to a Kubernetes pod.
+// Requires BuildKit v0.5.0 or later in the pod.
+func Helper(u *url.URL) (*connhelper.ConnectionHelper, error) {
+	sp, err := SpecFromURL(u)
+	if err != nil {
+		return nil, err
+	}
+	return &connhelper.ConnectionHelper{
+		ContextDialer: func(ctx context.Context, addr string) (net.Conn, error) {
+			return commandconn.New(ctx, "kubectl", "--context="+sp.Context, "--namespace="+sp.Namespace,
+				"exec", "--container="+sp.Container, "-i", sp.Pod, "--", "buildctl", "dial-stdio")
+		},
+	}, nil
+}
+
+// Spec
+type Spec struct {
+	Context   string
+	Namespace string
+	Pod       string
+	Container string
+}
+
+// SpecFromURL creates Spec from URL.
+// URL is like kube-pod://<pod>?context=<context>&namespace=<namespace>&container=<container> .
+// Only <pod> part is mandatory.
+func SpecFromURL(u *url.URL) (*Spec, error) {
+	q := u.Query()
+	sp := Spec{
+		Context:   q.Get("context"),
+		Namespace: q.Get("namespace"),
+		Pod:       u.Hostname(),
+		Container: q.Get("container"),
+	}
+	if sp.Context != "" && !validKubeIdentifier(sp.Context) {
+		return nil, errors.Errorf("unsupported context name: %q", sp.Context)
+	}
+	if sp.Namespace != "" && !validKubeIdentifier(sp.Namespace) {
+		return nil, errors.Errorf("unsupported namespace name: %q", sp.Namespace)
+	}
+	if sp.Pod == "" {
+		return nil, errors.New("url lacks pod name")
+	}
+	if !validKubeIdentifier(sp.Pod) {
+		return nil, errors.Errorf("unsupported pod name: %q", sp.Pod)
+	}
+	if sp.Container != "" && !validKubeIdentifier(sp.Container) {
+		return nil, errors.Errorf("unsupported container name: %q", sp.Container)
+	}
+	return &sp, nil
+}
+
+var kubeIdentifierRegexp = regexp.MustCompile(`^[-a-z0-9.]+$`)
+
+// validKubeIdentifier: https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#names
+// The length is not checked because future version of Kube may support longer identifiers.
+func validKubeIdentifier(s string) bool {
+	return kubeIdentifierRegexp.MatchString(s)
+}

client/connhelper/kubepod/kubepod_test.go

@@ -0,0 +1,34 @@
+package kubepod
+
+import (
+	"net/url"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSpecFromURL(t *testing.T) {
+	cases := map[string]*Spec{
+		"kube-pod://podname": {
+			Pod: "podname",
+		},
+		"kube-pod://podname?container=containername&namespace=nsname&context=ctxname": {
+			Context: "ctxname", Namespace: "nsname", Pod: "podname", Container: "containername",
+		},
+		"kube-pod://":                     nil,
+		"kube-pod://unsupported_pod_name": nil,
+	}
+	for s, expected := range cases {
+		u, err := url.Parse(s)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got, err := SpecFromURL(u)
+		if expected != nil {
+			require.NoError(t, err)
+			require.EqualValues(t, expected, got, s)
+		} else {
+			require.Error(t, err, s)
+		}
+	}
+}

cmd/buildctl/main.go

@@ -5,6 +5,7 @@ import (
 	"os"
 
 	_ "github.com/moby/buildkit/client/connhelper/dockercontainer"
+	_ "github.com/moby/buildkit/client/connhelper/kubepod"
 	bccommon "github.com/moby/buildkit/cmd/buildctl/common"
 	"github.com/moby/buildkit/util/apicaps"
 	"github.com/moby/buildkit/util/appdefaults"