stages/files: add support for runtime systemd units

Teach `writeSystemdUnit()` how to create runtime units in `/run`. This
only adds *internal* support for our own purposes. It does not extend
the Ignition spec to allow users to specify it. Though we could do this
too later on.

internal/exec/stages/files/units.go

@@ -22,7 +22,7 @@ import (
 // createUnits creates the units listed under systemd.units and networkd.units.
 func (s stage) createUnits(config types.Config) error {
 	for _, unit := range config.Systemd.Units {
-		if err := s.writeSystemdUnit(unit); err != nil {
+		if err := s.writeSystemdUnit(unit, false); err != nil {
 			return err
 		}
 		if unit.Enable {
@@ -71,20 +71,25 @@ func (s stage) createUnits(config types.Config) error {
 // writeSystemdUnit creates the specified unit and any dropins for that unit.
 // If the contents of the unit or are empty, the unit is not created. The same
 // applies to the unit's dropins.
-func (s stage) writeSystemdUnit(unit types.Unit) error {
+func (s stage) writeSystemdUnit(unit types.Unit, runtime bool) error {
+	// use a different DestDir if it's runtime so it affects our /run
+	u := s.Util
+	if runtime {
+		u.DestDir = "/"
+	}
+
 	return s.Logger.LogOp(func() error {
 		for _, dropin := range unit.Dropins {
 			if dropin.Contents == "" {
 				continue
 			}
-
-			f, err := util.FileFromSystemdUnitDropin(unit, dropin)
+			f, err := util.FileFromSystemdUnitDropin(unit, dropin, runtime)
 			if err != nil {
 				s.Logger.Crit("error converting systemd dropin: %v", err)
 				return err
 			}
 			if err := s.Logger.LogOp(
-				func() error { return s.PerformFetch(f) },
+				func() error { return u.PerformFetch(f) },
 				"writing systemd drop-in %q at %q", dropin.Name, f.Path,
 			); err != nil {
 				return err
@@ -95,13 +100,13 @@ func (s stage) writeSystemdUnit(unit types.Unit) error {
 			return nil
 		}
 
-		f, err := util.FileFromSystemdUnit(unit)
+		f, err := util.FileFromSystemdUnit(unit, runtime)
 		if err != nil {
 			s.Logger.Crit("error converting unit: %v", err)
 			return err
 		}
 		if err := s.Logger.LogOp(
-			func() error { return s.PerformFetch(f) },
+			func() error { return u.PerformFetch(f) },
 			"writing unit %q at %q", unit.Name, f.Path,
 		); err != nil {
 			return err

internal/exec/util/path.go

@@ -22,6 +22,14 @@ func SystemdUnitsPath() string {
 	return filepath.Join("etc", "systemd", "system")
 }
 
+func SystemdRuntimeUnitsPath() string {
+	return filepath.Join("run", "systemd", "system")
+}
+
+func SystemdRuntimeUnitWantsPath(unitName string) string {
+	return filepath.Join("run", "systemd", "system", unitName+".wants")
+}
+
 func NetworkdUnitsPath() string {
 	return filepath.Join("etc", "systemd", "network")
 }
@@ -30,6 +38,10 @@ func SystemdDropinsPath(unitName string) string {
 	return filepath.Join("etc", "systemd", "system", unitName+".d")
 }
 
+func SystemdRuntimeDropinsPath(unitName string) string {
+	return filepath.Join("run", "systemd", "system", unitName+".d")
+}
+
 func NetworkdDropinsPath(unitName string) string {
 	return filepath.Join("etc", "systemd", "network", unitName+".d")
 }

internal/exec/util/unit.go

@@ -31,13 +31,21 @@ const (
 	DefaultPresetPermissions os.FileMode = 0644
 )
 
-func FileFromSystemdUnit(unit types.Unit) (*FetchOp, error) {
+func FileFromSystemdUnit(unit types.Unit, runtime bool) (*FetchOp, error) {
 	u, err := url.Parse(dataurl.EncodeBytes([]byte(unit.Contents)))
 	if err != nil {
 		return nil, err
 	}
+
+	var path string
+	if runtime {
+		path = SystemdRuntimeUnitsPath()
+	} else {
+		path = SystemdUnitsPath()
+	}
+
 	return &FetchOp{
-		Path: filepath.Join(SystemdUnitsPath(), string(unit.Name)),
+		Path: filepath.Join(path, string(unit.Name)),
 		Url:  *u,
 		Mode: configUtil.IntToPtr(int(DefaultFilePermissions)),
 	}, nil
@@ -55,13 +63,21 @@ func FileFromNetworkdUnit(unit types.Networkdunit) (*FetchOp, error) {
 	}, nil
 }
 
-func FileFromSystemdUnitDropin(unit types.Unit, dropin types.SystemdDropin) (*FetchOp, error) {
+func FileFromSystemdUnitDropin(unit types.Unit, dropin types.SystemdDropin, runtime bool) (*FetchOp, error) {
 	u, err := url.Parse(dataurl.EncodeBytes([]byte(dropin.Contents)))
 	if err != nil {
 		return nil, err
 	}
+
+	var path string
+	if runtime {
+		path = SystemdRuntimeDropinsPath(string(unit.Name))
+	} else {
+		path = SystemdDropinsPath(string(unit.Name))
+	}
+
 	return &FetchOp{
-		Path: filepath.Join(SystemdDropinsPath(string(unit.Name)), string(dropin.Name)),
+		Path: filepath.Join(path, string(dropin.Name)),
 		Url:  *u,
 		Mode: configUtil.IntToPtr(int(DefaultFilePermissions)),
 	}, nil
@@ -94,6 +110,27 @@ func (u Util) EnableUnit(unit types.Unit) error {
 	return u.appendLineToPreset(fmt.Sprintf("enable %s", unit.Name))
 }
 
+// presets link in /etc, which doesn't make sense for runtime units
+// Related: https://github.com/coreos/ignition/issues/588
+func (u Util) EnableRuntimeUnit(unit types.Unit, target string) error {
+	// we want to affect /run, which will be carried into the pivot,
+	// not a directory named /$DestDir/run
+	u.DestDir = "/"
+
+	link := types.Link{
+		Node: types.Node{
+			Filesystem: "root",
+			// XXX(jl): make Wants/Required a parameter
+			Path: filepath.Join(SystemdRuntimeUnitWantsPath(target), string(unit.Name)),
+		},
+		LinkEmbedded1: types.LinkEmbedded1{
+			Target: filepath.Join("/", SystemdRuntimeUnitsPath(), string(unit.Name)),
+		},
+	}
+
+	return u.WriteLink(link)
+}
+
 func (u Util) DisableUnit(unit types.Unit) error {
 	return u.appendLineToPreset(fmt.Sprintf("disable %s", unit.Name))
 }