kubeadm implement preflight checks

Includes checks for verifying services exist and are enabled, ports are
open, directories do not exist or are empty, and required binaries are
in the path.

Checks that user running kubeamd init and join is root and will only execute
command if user is root. Moved away from using kubectl error handling to
having kubeadm handle its own errors. This should allow kubeadm to have
more meaningful errors, exit codes, and logging for specific kubeadm use
cases.

pkg/util/initsystem/initsystem.go

@@ -0,0 +1,83 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+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 initsystem
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+type InitSystem interface {
+
+	// ServiceExists ensures the service is defined for this init system.
+	ServiceExists(service string) bool
+
+	// ServiceIsEnabled ensures the service is enabled to start on each boot.
+	ServiceIsEnabled(service string) bool
+
+	// ServiceIsActive ensures the service is running, or attempting to run. (crash looping in the case of kubelet)
+	ServiceIsActive(service string) bool
+}
+
+type SystemdInitSystem struct{}
+
+func (sysd SystemdInitSystem) ServiceExists(service string) bool {
+	args := []string{"status", service}
+	outBytes, _ := exec.Command("systemctl", args...).Output()
+	output := string(outBytes)
+	if strings.Contains(output, "Loaded: not-found") {
+		return false
+	}
+	return true
+}
+
+func (sysd SystemdInitSystem) ServiceIsEnabled(service string) bool {
+	args := []string{"is-enabled", service}
+	_, err := exec.Command("systemctl", args...).Output()
+	if err != nil {
+		fmt.Println(err)
+		return false
+	}
+	return true
+}
+
+// ServiceIsActive will check is the service is "active". In the case of
+// crash looping services (kubelet in our case) status will return as
+// "activating", so we will consider this active as well.
+func (sysd SystemdInitSystem) ServiceIsActive(service string) bool {
+	args := []string{"is-active", service}
+	// Ignoring error here, command returns non-0 if in "activating" status:
+	outBytes, _ := exec.Command("systemctl", args...).Output()
+	output := strings.TrimSpace(string(outBytes))
+	if output == "active" || output == "activating" {
+		return true
+	}
+	return false
+}
+
+// getInitSystem returns an InitSystem for the current system, or nil
+// if we cannot detect a supported init system for pre-flight checks.
+// This indicates we will skip init system checks, not an error.
+func GetInitSystem() InitSystem {
+	// Assume existence of systemctl in path implies this is a systemd system:
+	_, err := exec.LookPath("systemctl")
+	if err == nil {
+		return &SystemdInitSystem{}
+	}
+	return nil
+}