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.

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
-// the given unit. The root argument indicates the effective base
+// UnitDestination builds the appropriate absolute file path for
+// 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"
 	}
 
-	return path.Join(root, dir, "systemd", u.Group(), u.Name)
+	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 {