feat: OpenRC and runit init backends behind an Init interface

Extract init-neutral unit resolution into service/, add initsys.Init with systemd, openrc and runit backends, selected by 'init:' config or autodetect. OpenRC and runit are root only; rootless units run as system services as that user. Add docs and generated examples for all three.

initsys/initsys.go

@@ -0,0 +1,77 @@
+// Package initsys selects the init system backend that installs and controls
+// unitdore services.
+package initsys
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/warkanum/unitdore/config"
+	"github.com/warkanum/unitdore/openrc"
+	"github.com/warkanum/unitdore/runit"
+	"github.com/warkanum/unitdore/systemd"
+)
+
+// Backend names.
+const (
+	Systemd = "systemd"
+	OpenRC  = "openrc"
+	Runit   = "runit"
+)
+
+// Init is implemented by every init system backend. u.User selects the scope;
+// backends that cannot do per-user managers run user units as system services.
+type Init interface {
+	Name() string
+	ServiceName(u config.Unit, prefix, suffix string) string
+	UnitPath(u config.Unit, prefix, suffix string) (string, error)
+	Generate(u config.Unit, prefix, suffix string) (string, error)
+	Install(u config.Unit, prefix, suffix string) error
+	Uninstall(u config.Unit, prefix, suffix string) error
+	Enable(u config.Unit, prefix, suffix string) error
+	Disable(u config.Unit, prefix, suffix string) error
+	Start(u config.Unit, prefix, suffix string) error
+	Stop(u config.Unit, prefix, suffix string) error
+	// Status returns active | inactive | failed | activating | deactivating | unknown.
+	Status(u config.Unit, prefix, suffix string) string
+	IsInstalled(u config.Unit, prefix, suffix string) bool
+	// SupportsLinger reports whether user units need systemd-style linger.
+	SupportsLinger() bool
+}
+
+// New returns the backend named in cfg.Init, or the detected one.
+func New(cfg *config.Config) (Init, error) {
+	name := cfg.Init
+	if name == "" {
+		name = Detect("/")
+	}
+	switch name {
+	case Systemd:
+		return systemd.Backend{}, nil
+	case OpenRC:
+		return openrc.Backend{}, nil
+	case Runit:
+		return runit.Backend{ServiceDir: cfg.ServiceDir}, nil
+	default:
+		return nil, fmt.Errorf("unknown init %q (want systemd, openrc or runit)", name)
+	}
+}
+
+// Detect guesses the running init system below root. Falls back to systemd.
+func Detect(root string) string {
+	has := func(p string) bool {
+		_, err := os.Stat(filepath.Join(root, p))
+		return err == nil
+	}
+	switch {
+	case has("run/systemd/system"):
+		return Systemd
+	case has("run/openrc"), has("sbin/openrc-run"):
+		return OpenRC
+	case has("run/runit"), has("etc/runit"):
+		return Runit
+	default:
+		return Systemd
+	}
+}