refactor(*): rework cloudconfig for better extensibility and consistency

This change creates a few simple interfaces for coreos-specific
configuration options and moves things to them wherever possible; so if
an option needs to write a file, or create a unit, it is acted on
exactly the same way as every other file/unit that needs to be written
during the cloud configuration process.

initialize/config.go

@@ -1,6 +1,7 @@
 package initialize
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"path"
@@ -10,18 +11,35 @@ import (
 	"github.com/coreos/coreos-cloudinit/system"
 )
 
+// CloudConfigFile represents a CoreOS specific configuration option that can generate
+// an associated system.File to be written to disk
+type CloudConfigFile interface {
+	// File should either return (*system.File, error), or (nil, nil) if nothing
+	// needs to be done for this configuration option.
+	File(root string) (*system.File, error)
+}
+
+// CloudConfigUnit represents a CoreOS specific configuration option that can generate
+// an associated system.Unit to be created/enabled appropriately
+type CloudConfigUnit interface {
+	// Unit should either return (*system.Unit, error), or (nil, nil) if nothing
+	// needs to be done for this configuration option.
+	Unit(root string) (*system.Unit, error)
+}
+
+// CloudConfig encapsulates the entire cloud-config configuration file and maps directly to YAML
 type CloudConfig struct {
 	SSHAuthorizedKeys []string `yaml:"ssh_authorized_keys"`
 	Coreos            struct {
 		Etcd   EtcdEnvironment
-		Update map[string]string
-		Units  []system.Unit
 		OEM    OEMRelease
+		Update UpdateConfig
+		Units  []system.Unit
 	}
 	WriteFiles     []system.File `yaml:"write_files"`
 	Hostname       string
 	Users          []system.User
-	ManageEtcHosts string `yaml:"manage_etc_hosts"`
+	ManageEtcHosts EtcHosts `yaml:"manage_etc_hosts"`
 }
 
 func NewCloudConfig(contents string) (*CloudConfig, error) {
@@ -42,6 +60,9 @@ func (cc CloudConfig) String() string {
 	return stringified
 }
 
+// Apply renders a CloudConfig to an Environment. This can involve things like
+// configuring the hostname, adding new users, writing various configuration
+// files to disk, and manipulating systemd services.
 func Apply(cfg CloudConfig, env *Environment) error {
 	if cfg.Hostname != "" {
 		if err := system.SetHostname(cfg.Hostname); err != nil {
@@ -50,14 +71,6 @@ func Apply(cfg CloudConfig, env *Environment) error {
 		log.Printf("Set hostname to %s", cfg.Hostname)
 	}
 
-	if cfg.Coreos.OEM.ID != "" {
-		if err := WriteOEMRelease(&cfg.Coreos.OEM, env.Root()); err != nil {
-			return err
-		}
-		log.Printf("Wrote /etc/oem-release to filesystem")
-	}
-
-	if len(cfg.Users) > 0 {
 	for _, user := range cfg.Users {
 		if user.Name == "" {
 			log.Printf("User object has no 'name' field, skipping")
@@ -100,7 +113,6 @@ func Apply(cfg CloudConfig, env *Environment) error {
 			}
 		}
 	}
-	}
 
 	if len(cfg.SSHAuthorizedKeys) > 0 {
 		err := system.AuthorizeSSHKeys("core", env.SSHKeyName(), cfg.SSHAuthorizedKeys)
@@ -111,7 +123,26 @@ func Apply(cfg CloudConfig, env *Environment) error {
 		}
 	}
 
-	if len(cfg.WriteFiles) > 0 {
+	for _, ccf := range []CloudConfigFile{cfg.Coreos.OEM, cfg.Coreos.Update, cfg.ManageEtcHosts} {
+		f, err := ccf.File(env.Root())
+		if err != nil {
+			return err
+		}
+		if f != nil {
+			cfg.WriteFiles = append(cfg.WriteFiles, *f)
+		}
+	}
+
+	for _, ccu := range []CloudConfigUnit{cfg.Coreos.Etcd, cfg.Coreos.Update} {
+		u, err := ccu.Unit(env.Root())
+		if err != nil {
+			return err
+		}
+		if u != nil {
+			cfg.Coreos.Units = append(cfg.Coreos.Units, *u)
+		}
+	}
+
 	for _, file := range cfg.WriteFiles {
 		file.Path = path.Join(env.Root(), file.Path)
 		if err := system.WriteFile(&file); err != nil {
@@ -119,25 +150,9 @@ func Apply(cfg CloudConfig, env *Environment) error {
 		}
 		log.Printf("Wrote file %s to filesystem", file.Path)
 	}
-	}
 
-	if len(cfg.Coreos.Etcd) > 0 {
-		if err := WriteEtcdEnvironment(cfg.Coreos.Etcd, env.Root()); err != nil {
-			log.Fatalf("Failed to write etcd config to filesystem: %v", err)
-		}
-
-		log.Printf("Wrote etcd config file to filesystem")
-	}
-
-	if s, ok := cfg.Coreos.Update["reboot-strategy"]; ok {
-		if err := WriteLocksmithConfig(s, env.Root()); err != nil {
-			log.Fatalf("Failed to write locksmith config to filesystem: %v", err)
-		}
-		log.Printf("Wrote locksmith config file to filesystem")
-	}
-
-	if len(cfg.Coreos.Units) > 0 {
 	commands := make(map[string]string, 0)
+	reload := false
 	for _, unit := range cfg.Coreos.Units {
 		dst := system.UnitDestination(&unit, env.Root())
 		if unit.Content != "" {
@@ -146,6 +161,14 @@ func Apply(cfg CloudConfig, env *Environment) error {
 				return err
 			}
 			log.Printf("Placed unit %s at %s", unit.Name, dst)
+			reload = true
+		}
+
+		if unit.Mask {
+			log.Printf("Masking unit file %s", unit.Name)
+			if err := system.MaskUnit(unit.Name, env.Root()); err != nil {
+				return err
+			}
 		}
 
 		if unit.Enable {
@@ -162,15 +185,15 @@ func Apply(cfg CloudConfig, env *Environment) error {
 
 		if unit.Group() == "network" {
 			commands["systemd-networkd.service"] = "restart"
-			} else {
+		} else if unit.Command != "" {
-				if unit.Command != "" {
 			commands[unit.Name] = unit.Command
 		}
 	}
-		}
 
+	if reload {
 		if err := system.DaemonReload(); err != nil {
-			log.Fatalf("Failed systemd daemon-reload: %v", err)
+			return errors.New(fmt.Sprintf("failed systemd daemon-reload: %v", err))
+		}
 	}
 
 	for unit, command := range commands {
@@ -181,17 +204,6 @@ func Apply(cfg CloudConfig, env *Environment) error {
 		}
 		log.Printf("Result of '%s %s': %s", command, unit, res)
 	}
-	}
-
-	if cfg.ManageEtcHosts != "" {
-
-		if err := WriteEtcHosts(cfg.ManageEtcHosts, env.Root()); err != nil {
-			log.Fatalf("Failed to write /etc/hosts to filesystem: %v", err)
-		}
-
-		log.Printf("Wrote /etc/hosts file to filesystem")
-
-	}
 
 	return nil
 }

initialize/env.go

@@ -45,3 +45,16 @@ func (self *Environment) Apply(data string) string {
 	}
 	return data
 }
+
+// normalizeSvcEnv standardizes the keys of the map (environment variables for a service)
+// by replacing any dashes with underscores and ensuring they are entirely upper case.
+// For example, "some-env" --> "SOME_ENV"
+func normalizeSvcEnv(m map[string]string) map[string]string {
+	out := make(map[string]string, len(m))
+	for key, val := range m {
+		key = strings.ToUpper(key)
+		key = strings.Replace(key, "-", "_", -1)
+		out[key] = val
+	}
+	return out
+}

initialize/etcd.go

@@ -3,26 +3,14 @@ package initialize
 import (
 	"errors"
 	"fmt"
-	"path"
-	"strings"
 
 	"github.com/coreos/coreos-cloudinit/system"
 )
 
 type EtcdEnvironment map[string]string
 
-func (ec EtcdEnvironment) normalized() map[string]string {
+func (ee EtcdEnvironment) String() (out string) {
-	out := make(map[string]string, len(ec))
+	norm := normalizeSvcEnv(ee)
-	for key, val := range ec {
-		key = strings.ToUpper(key)
-		key = strings.Replace(key, "-", "_", -1)
-		out[key] = val
-	}
-	return out
-}
-
-func (ec EtcdEnvironment) String() (out string) {
-	norm := ec.normalized()
 
 	if val, ok := norm["DISCOVERY_URL"]; ok {
 		delete(norm, "DISCOVERY_URL")
@@ -40,23 +28,23 @@ func (ec EtcdEnvironment) String() (out string) {
 	return
 }
 
-// Write an EtcdEnvironment to the appropriate path on disk for etcd.service
+// Unit creates a Unit file drop-in for etcd, using any configured
-func WriteEtcdEnvironment(env EtcdEnvironment, root string) error {
+// options and adding a default MachineID if unset.
-	if _, ok := env["name"]; !ok {
+func (ee EtcdEnvironment) Unit(root string) (*system.Unit, error) {
+	if _, ok := ee["name"]; !ok {
 		if machineID := system.MachineID(root); machineID != "" {
-			env["name"] = machineID
+			ee["name"] = machineID
 		} else if hostname, err := system.Hostname(); err == nil {
-			env["name"] = hostname
+			ee["name"] = hostname
 		} else {
-			return errors.New("Unable to determine default etcd name")
+			return nil, errors.New("Unable to determine default etcd name")
 		}
 	}
 
-	file := system.File{
+	return &system.Unit{
-		Path: path.Join(root, "run", "systemd", "system", "etcd.service.d", "20-cloudinit.conf"),
+		Name:    "etcd.service",
-		RawFilePermissions: "0644",
+		Runtime: true,
-		Content: env.String(),
+		DropIn:  true,
-	}
+		Content: ee.String(),
-
+	}, nil
-	return system.WriteFile(&file)
 }

initialize/etcd_test.go

@@ -3,9 +3,10 @@ package initialize
 import (
 	"io/ioutil"
 	"os"
-	"os/exec"
 	"path"
 	"testing"
+
+	"github.com/coreos/coreos-cloudinit/system"
 )
 
 func TestEtcdEnvironment(t *testing.T) {
@@ -69,8 +70,18 @@ func TestEtcdEnvironmentWrittenToDisk(t *testing.T) {
 	}
 	defer os.RemoveAll(dir)
 
-	if err := WriteEtcdEnvironment(ec, dir); err != nil {
+	u, err := ec.Unit(dir)
-		t.Fatalf("Processing of EtcdEnvironment failed: %v", err)
+	if err != nil {
+		t.Fatalf("Generating etcd unit failed: %v", err)
+	}
+	if u == nil {
+		t.Fatalf("Returned nil etcd unit unexpectedly")
+	}
+
+	dst := system.UnitDestination(u, dir)
+	os.Stderr.WriteString("writing to " + dir + "\n")
+	if err := system.PlaceUnit(u, dst); err != nil {
+		t.Fatalf("Writing of EtcdEnvironment failed: %v", err)
 	}
 
 	fullPath := path.Join(dir, "run", "systemd", "system", "etcd.service.d", "20-cloudinit.conf")
@@ -100,7 +111,7 @@ Environment="ETCD_PEER_BIND_ADDR=127.0.0.1:7002"
 }
 
 func TestEtcdEnvironmentWrittenToDiskDefaultToMachineID(t *testing.T) {
-	ec := EtcdEnvironment{}
+	ee := EtcdEnvironment{}
 	dir, err := ioutil.TempDir(os.TempDir(), "coreos-cloudinit-")
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
@@ -113,8 +124,18 @@ func TestEtcdEnvironmentWrittenToDiskDefaultToMachineID(t *testing.T) {
 		t.Fatalf("Failed writing out /etc/machine-id: %v", err)
 	}
 
-	if err := WriteEtcdEnvironment(ec, dir); err != nil {
+	u, err := ee.Unit(dir)
-		t.Fatalf("Processing of EtcdEnvironment failed: %v", err)
+	if err != nil {
+		t.Fatalf("Generating etcd unit failed: %v", err)
+	}
+	if u == nil {
+		t.Fatalf("Returned nil etcd unit unexpectedly")
+	}
+
+	dst := system.UnitDestination(u, dir)
+	os.Stderr.WriteString("writing to " + dir + "\n")
+	if err := system.PlaceUnit(u, dst); err != nil {
+		t.Fatalf("Writing of EtcdEnvironment failed: %v", err)
 	}
 
 	fullPath := path.Join(dir, "run", "systemd", "system", "etcd.service.d", "20-cloudinit.conf")
@@ -131,8 +152,3 @@ Environment="ETCD_NAME=node007"
 		t.Fatalf("File has incorrect contents")
 	}
 }
-
-func rmdir(path string) error {
-	cmd := exec.Command("rm", "-rf", path)
-	return cmd.Run()
-}

initialize/locksmith.go

@@ -2,8 +2,6 @@ package initialize
 
 import (
 	"bufio"
-	"fmt"
-	"io/ioutil"
 	"os"
 	"path"
 	"strings"
@@ -13,73 +11,79 @@ import (
 
 const locksmithUnit = "locksmithd.service"
 
-// addStrategy creates an `/etc/coreos/update.conf` file with the requested
+type UpdateConfig map[string]string
-// strategy via rewriting the file on disk or by starting from
+
-// `/usr/share/coreos/update.conf`.
+func (uc UpdateConfig) strategy() string {
-func addStrategy(strategy string, root string) error {
+	s, _ := uc["reboot-strategy"]
+	return s
+}
+
+// File creates an `/etc/coreos/update.conf` file with the requested
+// strategy, by either rewriting the existing file on disk, or starting
+// from `/usr/share/coreos/update.conf`
+func (uc UpdateConfig) File(root string) (*system.File, error) {
+
+	// If no reboot-strategy is set, we don't need to generate a new config
+	if _, ok := uc["reboot-strategy"]; !ok {
+		return nil, nil
+	}
+
+	var out string
+
 	etcUpdate := path.Join(root, "etc", "coreos", "update.conf")
 	usrUpdate := path.Join(root, "usr", "share", "coreos", "update.conf")
 
-	// Ensure /etc/coreos/ exists before attempting to write a file in it
-	os.MkdirAll(path.Join(root, "etc", "coreos"), 0755)
-
-	tmp, err := ioutil.TempFile(path.Join(root, "etc", "coreos"), ".update.conf")
-	if err != nil {
-		return err
-	}
-	err = tmp.Chmod(0644)
-	if err != nil {
-		return err
-	}
-
 	conf, err := os.Open(etcUpdate)
 	if os.IsNotExist(err) {
 		conf, err = os.Open(usrUpdate)
-		if err != nil {
-			return err
 	}
+	if err != nil {
+		return nil, err
 	}
 
 	scanner := bufio.NewScanner(conf)
 
 	sawStrat := false
-	stratLine := "REBOOT_STRATEGY="+strategy
+	stratLine := "REBOOT_STRATEGY=" + uc.strategy()
 	for scanner.Scan() {
 		line := scanner.Text()
 		if strings.HasPrefix(line, "REBOOT_STRATEGY=") {
 			line = stratLine
 			sawStrat = true
 		}
-		fmt.Fprintln(tmp, line)
+		out += line
+		out += "\n"
 		if err := scanner.Err(); err != nil {
-			return err
+			return nil, err
 		}
 	}
 
 	if !sawStrat {
-		fmt.Fprintln(tmp, stratLine)
+		out += stratLine
+		out += "\n"
 	}
-
+	return &system.File{
-	return os.Rename(tmp.Name(), etcUpdate)
+		Path:               path.Join("etc", "coreos", "update.conf"),
+		RawFilePermissions: "0644",
+		Content:            out,
+	}, nil
 }
 
-// WriteLocksmithConfig updates the `update.conf` file with a REBOOT_STRATEGY for locksmith.
+// Unit generates a locksmith system.Unit for the cloud-init initializer to
-func WriteLocksmithConfig(strategy string, root string) error {
+// act on appropriately
-	cmd := "restart"
+func (uc UpdateConfig) Unit(root string) (*system.Unit, error) {
-	if strategy == "off" {
+	u := &system.Unit{
-		err := system.MaskUnit(locksmithUnit, root)
+		Name:    locksmithUnit,
-		if err != nil {
+		Enable:  true,
-			return err
+		Command: "restart",
+		Mask:    false,
 	}
-		cmd = "stop"
+
-	} else {
+	if uc.strategy() == "off" {
-		return addStrategy(strategy, root)
+		u.Enable = false
+		u.Command = "stop"
+		u.Mask = true
 	}
-	if err := system.DaemonReload(); err != nil {
+
-		return err
+	return u, nil
-	}
-	if _, err := system.RunUnitCommand(cmd, locksmithUnit); err != nil {
-		return err
-	}
-	return nil
 }

initialize/locksmith_test.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path"
 	"testing"
+
+	"github.com/coreos/coreos-cloudinit/system"
 )
 
 const (
@@ -42,9 +44,19 @@ func TestLocksmithEnvironmentWrittenToDisk(t *testing.T) {
 				t.Fatal(err)
 			}
 		}
+		uc := &UpdateConfig{"reboot-strategy": "etcd-lock"}
 
-		if err := WriteLocksmithConfig("etcd-lock", dir); err != nil {
+		f, err := uc.File(dir)
-			t.Fatalf("Processing of LocksmithEnvironment failed: %v", err)
+		if err != nil {
+			t.Fatalf("Processing UpdateConfig failed: %v", err)
+		}
+		if f == nil {
+			t.Fatalf("UpdateConfig generated nil file unexpectedly")
+		}
+
+		f.Path = path.Join(dir, f.Path)
+		if err := system.WriteFile(f); err != nil {
+			t.Fatalf("Error writing update config: %v", err)
 		}
 
 		fullPath := path.Join(dir, "etc", "coreos", "update.conf")
@@ -76,9 +88,17 @@ func TestLocksmithEnvironmentMasked(t *testing.T) {
 	defer os.RemoveAll(dir)
 	setupFixtures(dir)
 
-	if err := WriteLocksmithConfig("off", dir); err != nil {
+	uc := &UpdateConfig{"reboot-strategy": "off"}
-		t.Fatalf("Processing of LocksmithEnvironment failed: %v", err)
+
+	u, err := uc.Unit(dir)
+	if err != nil {
+		t.Fatalf("Processing UpdateConfig failed: %v", err)
 	}
+	if u == nil {
+		t.Fatalf("UpdateConfig generated nil unit unexpectedly")
+	}
+
+	system.MaskUnit(u.Name, dir)
 
 	fullPath := path.Join(dir, "etc", "systemd", "system", "locksmithd.service")
 	target, err := os.Readlink(fullPath)

initialize/manage_etc_hosts.go

@@ -11,8 +11,10 @@ import (
 
 const DefaultIpv4Address = "127.0.0.1"
 
-func generateEtcHosts(option string) (out string, err error) {
+type EtcHosts string
-	if option != "localhost" {
+
+func (eh EtcHosts) generateEtcHosts() (out string, err error) {
+	if eh != "localhost" {
 		return "", errors.New("Invalid option to manage_etc_hosts")
 	}
 
@@ -26,19 +28,19 @@ func generateEtcHosts(option string) (out string, err error) {
 
 }
 
-// Write an /etc/hosts file
+func (eh EtcHosts) File(root string) (*system.File, error) {
-func WriteEtcHosts(option string, root string) error {
+	if eh == "" {
-
+		return nil, nil
-	etcHosts, err := generateEtcHosts(option)
-	if err != nil {
-		return err
 	}
 
-	file := system.File{
+	etcHosts, err := eh.generateEtcHosts()
-		Path:               path.Join(root, "etc", "hosts"),
+	if err != nil {
+		return nil, err
+	}
+
+	return &system.File{
+		Path:               path.Join("etc", "hosts"),
 		RawFilePermissions: "0644",
 		Content:            etcHosts,
-	}
+	}, nil
-
-	return system.WriteFile(&file)
 }

initialize/manage_etc_hosts_test.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path"
 	"testing"
+
+	"github.com/coreos/coreos-cloudinit/system"
 )
 
 func TestCloudConfigManageEtcHosts(t *testing.T) {
@@ -25,14 +27,9 @@ manage_etc_hosts: localhost
 }
 
 func TestManageEtcHostsInvalidValue(t *testing.T) {
-	dir, err := ioutil.TempDir(os.TempDir(), "coreos-cloudinit-")
+	eh := EtcHosts("invalid")
-	if err != nil {
+	if f, err := eh.File(""); err == nil || f != nil {
-		t.Fatalf("Unable to create tempdir: %v", err)
+		t.Fatalf("EtcHosts File succeeded with invalid value!")
-	}
-	defer rmdir(dir)
-
-	if err := WriteEtcHosts("invalid", dir); err == nil {
-		t.Fatalf("WriteEtcHosts succeeded with invalid value: %v", err)
 	}
 }
 
@@ -41,10 +38,22 @@ func TestEtcHostsWrittenToDisk(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
-	defer rmdir(dir)
+	defer os.RemoveAll(dir)
 
-	if err := WriteEtcHosts("localhost", dir); err != nil {
+	eh := EtcHosts("localhost")
-		t.Fatalf("WriteEtcHosts failed: %v", err)
+
+	f, err := eh.File(dir)
+	if err != nil {
+		t.Fatalf("Error calling File on EtcHosts: %v", err)
+	}
+	if f == nil {
+		t.Fatalf("manageEtcHosts returned nil file unexpectedly")
+	}
+
+	f.Path = path.Join(dir, f.Path)
+
+	if err := system.WriteFile(f); err != nil {
+		t.Fatalf("Error writing EtcHosts: %v", err)
 	}
 
 	fullPath := path.Join(dir, "etc", "hosts")

initialize/oem.go

@@ -16,7 +16,7 @@ type OEMRelease struct {
 	BugReportURL string `yaml:"bug-report-url"`
 }
 
-func (oem *OEMRelease) String() string {
+func (oem OEMRelease) String() string {
 	fields := []string{
 		fmt.Sprintf("ID=%s", oem.ID),
 		fmt.Sprintf("VERSION_ID=%s", oem.VersionID),
@@ -28,12 +28,14 @@ func (oem *OEMRelease) String() string {
 	return strings.Join(fields, "\n") + "\n"
 }
 
-func WriteOEMRelease(oem *OEMRelease, root string) error {
+func (oem OEMRelease) File(root string) (*system.File, error) {
-	file := system.File{
+	if oem.ID == "" {
-		Path:               path.Join(root, "etc", "oem-release"),
+		return nil, nil
-		RawFilePermissions: "0644",
-		Content:            oem.String(),
 	}
 
-	return system.WriteFile(&file)
+	return &system.File{
+		Path:               path.Join("etc", "oem-release"),
+		RawFilePermissions: "0644",
+		Content:            oem.String(),
+	}, nil
 }

initialize/oem_test.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path"
 	"testing"
+
+	"github.com/coreos/coreos-cloudinit/system"
 )
 
 func TestOEMReleaseWrittenToDisk(t *testing.T) {
@@ -21,8 +23,17 @@ func TestOEMReleaseWrittenToDisk(t *testing.T) {
 	}
 	defer os.RemoveAll(dir)
 
-	if err := WriteOEMRelease(&oem, dir); err != nil {
+	f, err := oem.File(dir)
-		t.Fatalf("Processing of EtcdEnvironment failed: %v", err)
+	if err != nil {
+		t.Fatalf("Processing of OEMRelease failed: %v", err)
+	}
+	if f == nil {
+		t.Fatalf("OEMRelease returned nil file unexpectedly")
+	}
+
+	f.Path = path.Join(dir, f.Path)
+	if err := system.WriteFile(f); err != nil {
+		t.Fatalf("Writing of OEMRelease failed: %v", err)
 	}
 
 	fullPath := path.Join(dir, "etc", "oem-release")

system/systemd.go

@@ -17,12 +17,17 @@ import (
 // never be used as a true MachineID
 const fakeMachineID = "42000000000000000000000000000042"
 
+// Name for drop-in service configuration files created by cloudconfig
+const cloudConfigDropIn = "20-cloudinit.conf"
+
 type Unit struct {
 	Name    string
+	Mask    bool
 	Enable  bool
 	Runtime bool
 	Content string
 	Command string
+	DropIn  bool // drop-in configuration? if so, a cloudinit.conf will be generated
 }
 
 func (u *Unit) Type() string {
@@ -42,8 +47,8 @@ func (u *Unit) Group() (group string) {
 
 type Script []byte
 
-// UnitDestination builds the appropriate absolte file path for
+// UnitDestination builds the appropriate absolute file path for
-// the given unit. The root argument indicates the effective base
+// the given Unit. The root argument indicates the effective base
 // directory of the system (similar to a chroot).
 func UnitDestination(u *Unit, root string) string {
 	dir := "etc"
@@ -51,7 +56,11 @@ func UnitDestination(u *Unit, root string) string {
 		dir = "run"
 	}
 
+	if u.DropIn {
+		return path.Join(root, dir, "systemd", u.Group(), fmt.Sprintf("%s.d", u.Name), cloudConfigDropIn)
+	} else {
 		return path.Join(root, dir, "systemd", u.Group(), u.Name)
+	}
 }
 
 // PlaceUnit writes a unit file at the provided destination, creating

system/systemd_test.go

@@ -60,6 +60,30 @@ Address=10.209.171.177/19
 	}
 }
 
+func TestUnitDestination(t *testing.T) {
+	dir := "/some/dir"
+	name := "foobar.service"
+
+	u := Unit{
+		Name:   name,
+		DropIn: false,
+	}
+
+	dst := UnitDestination(&u, dir)
+	expectDst := path.Join(dir, "etc", "systemd", "system", "foobar.service")
+	if dst != expectDst {
+		t.Errorf("UnitDestination returned %s, expected %s", dst, expectDst)
+	}
+
+	u.DropIn = true
+
+	dst = UnitDestination(&u, dir)
+	expectDst = path.Join(dir, "etc", "systemd", "system", "foobar.service.d", cloudConfigDropIn)
+	if dst != expectDst {
+		t.Errorf("UnitDestination returned %s, expected %s", dst, expectDst)
+	}
+}
+
 func TestPlaceMountUnit(t *testing.T) {
 	u := Unit{
 		Name:    "media-state.mount",
@@ -123,6 +147,7 @@ func TestMachineID(t *testing.T) {
 		t.Fatalf("File has incorrect contents")
 	}
 }
+
 func TestMaskUnit(t *testing.T) {
 	dir, err := ioutil.TempDir(os.TempDir(), "coreos-cloudinit-")
 	if err != nil {