Added a simple utility for master election and pod creation.

contrib/pod-master/Dockerfile

+# Copyright 2015 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.
+
+FROM scratch
+MAINTAINER Brendan Burns <bburns@google.com>
+ADD podmaster podmaster
+ENTRYPOINT ["/podmaster"]

contrib/pod-master/Makefile

+all: push
+
+# Set this to the *next* version to prevent accidentally overwriting the existing image.
+# Next tag=1.2
+# Usage:
+#  tag with the current git hash:
+#   make TAG=`git log -1 --format="%H"`
+#  tag with a formal version
+#   make TAG=1.2
+
+podmaster: podmaster.go
+	CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -ldflags '-w' ./podmaster.go
+
+container: podmaster
+	docker build -t gcr.io/google_containers/podmaster:$(TAG) .
+
+push: container
+	gcloud preview docker push gcr.io/google_containers/podmaster:$(TAG)
+
+clean:
+	rm -f podmaster

contrib/pod-master/podmaster.go

+/*
+Copyright 2015 The Kubernetes Authors 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.
+*/
+
+// podmaster is a simple utility, it attempts to acquire and maintain a lease-lock from etcd using compare-and-swap.
+// if it is the master, it copies a source file into a destination file.  If it is not the master, it makes sure it is removed.
+//
+// typical usage is to copy a Pod manifest from a staging directory into the kubelet's directory, for example:
+//   podmaster --etcd-servers=http://127.0.0.1:4001 --key=scheduler --source-file=/kubernetes/kube-scheduler.manifest --dest-file=/manifests/kube-scheduler.manifest
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/tools"
+
+	"github.com/coreos/go-etcd/etcd"
+	"github.com/golang/glog"
+	"github.com/spf13/pflag"
+)
+
+type Config struct {
+	etcdServers string
+	key         string
+	whoami      string
+	ttl         uint64
+	src         string
+	dest        string
+	sleep       time.Duration
+}
+
+// runs the election loop. never returns.
+func (c *Config) leaseAndUpdateLoop(etcdClient *etcd.Client) {
+	for {
+		master, err := c.acquireOrRenewLease(etcdClient)
+		if err != nil {
+			glog.Errorf("Error in master election: %v", err)
+			continue
+		}
+		if err := c.update(master); err != nil {
+			glog.Errorf("Error updating files: %v", err)
+		}
+		time.Sleep(c.sleep)
+	}
+}
+
+// acquireOrRenewLease either races to acquire a new master lease, or update the existing master's lease
+// returns true if we have the lease, and an error if one occurs.
+// TODO: use the master election utility once it is merged in.
+func (c *Config) acquireOrRenewLease(etcdClient *etcd.Client) (bool, error) {
+	result, err := etcdClient.Get(c.key, false, false)
+	if tools.IsEtcdNotFound(err) {
+		// there is no current master, try to become master, create will fail if the key already exists
+		_, err := etcdClient.Create(c.key, c.whoami, c.ttl)
+		if err != nil {
+			return false, err
+		}
+		return true, nil
+	}
+	if result.Node.Value == c.whoami {
+		glog.Infof("key already exists, we are the master (%s)", result.Node.Value)
+		// we extend our lease @ 1/2 of the existing TTL, this ensures the master doesn't flap around
+		if result.Node.Expiration.Sub(time.Now()) < time.Duration(c.ttl/2)*time.Second {
+			_, err := etcdClient.CompareAndSwap(c.key, c.whoami, c.ttl, c.whoami, result.Node.ModifiedIndex)
+			if err != nil {
+				return false, err
+			}
+		}
+		return true, nil
+	}
+	glog.Infof("key already exists, the master is %s, sleeping.", result.Node.Value)
+	return false, nil
+}
+
+// update enacts the policy, copying a file if we are the master, and it doesn't exist.
+// deleting a file if we aren't the master and it does.
+func (c *Config) update(master bool) error {
+	exists, err := exists(c.dest)
+	if err != nil {
+		return err
+	}
+	switch {
+	case master && !exists:
+		return copyFile(c.src, c.dest)
+	case !master && exists:
+		return os.Remove(c.dest)
+	}
+	return nil
+}
+
+// exists tests to see if a file exists.
+func exists(file string) (bool, error) {
+	_, err := os.Stat(file)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		} else {
+			return false, err
+		}
+	}
+	return true, nil
+}
+
+func copyFile(src, dest string) error {
+	data, err := ioutil.ReadFile(src)
+	if err != nil {
+		return err
+	}
+	return ioutil.WriteFile(dest, data, 0755)
+}
+
+func initFlags(c *Config) {
+	pflag.StringVar(&c.etcdServers, "etcd-servers", "", "The comma-seprated list of etcd servers to use")
+	pflag.StringVar(&c.key, "key", "", "The key to use for the lock")
+	pflag.StringVar(&c.whoami, "whoami", "", "The name to use for the reservation.  If empty use os.Hostname")
+	pflag.Uint64Var(&c.ttl, "ttl-secs", 30, "The time to live for the lock.")
+	pflag.StringVar(&c.src, "source-file", "", "The source file to copy from.")
+	pflag.StringVar(&c.dest, "dest-file", "", "The destination file to copy to.")
+	pflag.DurationVar(&c.sleep, "sleep", 5*time.Second, "The length of time to sleep between checking the lock.")
+}
+
+func validateFlags(c *Config) {
+	if len(c.etcdServers) == 0 {
+		glog.Fatalf("--etcd-servers=<server-list> is required")
+	}
+	if len(c.key) == 0 {
+		glog.Fatalf("--key=<some-key> is required")
+	}
+	if len(c.src) == 0 {
+		glog.Fatalf("--source-file=<some-file> is required")
+	}
+	if len(c.dest) == 0 {
+		glog.Fatalf("--dest-file=<some-file> is required")
+	}
+	if len(c.whoami) == 0 {
+		hostname, err := os.Hostname()
+		if err != nil {
+			glog.Fatalf("Failed to get hostname: %v", err)
+		}
+		c.whoami = hostname
+		glog.Infof("--whoami is empty, defaulting to %s", c.whoami)
+	}
+}
+
+func main() {
+	c := Config{}
+	initFlags(&c)
+	pflag.Parse()
+	validateFlags(&c)
+
+	machines := strings.Split(c.etcdServers, ",")
+	etcdClient := etcd.NewClient(machines)
+
+	c.leaseAndUpdateLoop(etcdClient)
+}