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_test.go

@@ -0,0 +1,52 @@
+package initsys
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/warkanum/unitdore/config"
+)
+
+func TestDetect(t *testing.T) {
+	tests := []struct {
+		name  string
+		paths []string
+		want  string
+	}{
+		{"systemd", []string{"run/systemd/system"}, Systemd},
+		{"openrc", []string{"run/openrc"}, OpenRC},
+		{"openrc binary", []string{"sbin/openrc-run"}, OpenRC},
+		{"runit", []string{"etc/runit"}, Runit},
+		{"systemd wins", []string{"run/systemd/system", "etc/runit"}, Systemd},
+		{"fallback", nil, Systemd},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			root := t.TempDir()
+			for _, p := range tt.paths {
+				if err := os.MkdirAll(filepath.Dir(filepath.Join(root, p)), 0755); err != nil {
+					t.Fatal(err)
+				}
+				if err := os.WriteFile(filepath.Join(root, p), nil, 0644); err != nil {
+					t.Fatal(err)
+				}
+			}
+			if got := Detect(root); got != tt.want {
+				t.Errorf("Detect() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestNew(t *testing.T) {
+	for _, name := range []string{Systemd, OpenRC, Runit} {
+		ini, err := New(&config.Config{Init: name})
+		if err != nil || ini.Name() != name {
+			t.Errorf("New(%q) = %v, %v", name, ini, err)
+		}
+	}
+	if _, err := New(&config.Config{Init: "sysv"}); err == nil {
+		t.Error("New() should reject an unknown init")
+	}
+}