R-124 recipe root namespace as PBS spells it; R-118 no root size for an absent drive; R-269 rotated-out token rejected at once; R-317 dnsmasq install probed by its unit (burn-down round 2)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/lanresolver/lanresolver.go

@@ -101,11 +101,35 @@ func NewManager(runner proxmox.Runner, hostIP string, upstreams []string, logger
 	}
 }
 
+// hostRoot is the filesystem root the install probe resolves against: "/" in production; a test
+// points it at a fixture tree so the REAL probe runs against files it controls.
+var hostRoot = "/"
+
+// dnsmasqUnitPaths are where the `dnsmasq` package ships its systemd unit (Debian; /lib is the
+// pre-usrmerge spelling). R-317: probe the UNIT, never /usr/sbin/dnsmasq — that binary belongs to
+// `dnsmasq-base`, so a host carrying dnsmasq-base without dnsmasq used to skip the install and then
+// `systemctl enable --now dnsmasq` failed against a unit that is not there (resolver never up).
+// Pinned by TestEnsureDnsmasq_BinaryWithoutUnitInstalls.
+var dnsmasqUnitPaths = []string{
+	"usr/lib/systemd/system/dnsmasq.service",
+	"lib/systemd/system/dnsmasq.service",
+}
+
+// dnsmasqUnitInstalled reports whether the dnsmasq service unit (the `dnsmasq` package) is present.
+func dnsmasqUnitInstalled() bool {
+	for _, p := range dnsmasqUnitPaths {
+		if _, err := os.Stat(filepath.Join(hostRoot, p)); err == nil {
+			return true
+		}
+	}
+	return false
+}
+
 // EnsureDnsmasq makes dnsmasq present + enabled and writes the host base config. Idempotent: it
 // installs the package only when absent, and writes the base drop-in only when its content changes.
 func (m *Manager) EnsureDnsmasq(ctx context.Context) error {
-	if _, err := os.Stat("/usr/sbin/dnsmasq"); err != nil { // metadata read, no privilege needed
-		m.logger.Info("lanresolver: dnsmasq absent — installing")
+	if !dnsmasqUnitInstalled() { // metadata read, no privilege needed
+		m.logger.Info("lanresolver: dnsmasq service unit absent — installing")
 		if out, errOut, ierr := m.runner.Run(ctx, "apt-get", "install", "-y", "-q", "dnsmasq"); ierr != nil {
 			return fmt.Errorf("install dnsmasq: %s: %w", strings.TrimSpace(string(errOut))+string(out), ierr)
 		}