VM_CONTACT: check d ignores root-only dirs; exit 9 points to U6

backend/deploy/VM_CONTACT.md

@@ -490,7 +490,9 @@ From #131 (`e74ce1e`) a rerun over an existing install is upgrade-safe:
 - Before anything is overwritten, the previous install is copied to a root-only
   directory under `/var/backups/vm-mcp/`; the installer prints the path. That is an
   extra copy alongside U1's backup, not a replacement for it.
-- If that backup fails the installer exits 9 with nothing changed.
+- If that backup fails the installer exits 9 before changing any installed file. U4
+  has already stopped the units by then, so the MCP path is down: use U6 to restore
+  the prior activation.
 
 Exit order in the pinned installer: 3 (token, or a settings change without the
 override) and 7 (Yama precheck) come before the installer's backup; 9 means that
@@ -753,7 +755,7 @@ The pinned `e74ce1e` is the merge of The-Interdependency/skill-lib#131 (upgrade-
 installer reruns, see U4), on top of #130 (`acd5e9f`, a first-use context notice from
 vm-mcp; its privileged components record a one-time marker in a new root-only state
 directory), #127 (`023ae12`) and #123 (`24029ca`).
- #127 hardens the installer and makes the server check its token
+#127 hardens the installer and makes the server check its token
 descriptor: fd 0 must be a regular file owned by root with no group or other bits. The
 installer's `/etc/credstore/vm-mcp-token` (root, `0600`) satisfies that. From #123 on,
 vm-mcp refuses every HTTP request, loopback included, that lacks
@@ -1006,7 +1008,9 @@ false `PASS`.
 ```bash
 (
 ok() { if [ "$2" = "$3" ]; then echo "PASS $1"; else echo "FAIL $1 (want $2, got $3)"; fi; }
-w=$(sudo -n -u vmmcp find /opt/vm-mcp -writable) || w=READ-FAILED
+# Directories vmmcp cannot search (root-only __pycache__) are pruned: nothing in them is
+# reachable by vmmcp. Any other find error still sets READ-FAILED.
+w=$(sudo -n -u vmmcp find /opt/vm-mcp -type d ! -executable -prune -o -writable -print) || w=READ-FAILED
 ok "nothing vmmcp-writable" 0 "$(grep -c . <<<"$w" || true)"
 o=$(sudo -n find /opt/vm-mcp ! -user root) || o=READ-FAILED
 ok "everything root-owned" 0 "$(grep -c . <<<"$o" || true)"
@@ -1017,8 +1021,12 @@ stat -c '%U %a %n' /opt /opt/vm-mcp /opt/vm-mcp/.venv/pyvenv.cfg
 ```
 
 Expect three `PASS` lines. All three paths must be owned by `root` with no group or
-world write bit (for example `755` or `644`). If `find` reports a directory it can't read, the check
-prints `FAIL`. Look at that directory's permissions before treating the result as a pass.
+world write bit (for example `755` or `644`). The first check skips directories that
+`vmmcp` cannot enter, such as the root-only `__pycache__`, because `vmmcp` can reach
+nothing inside them; those directories themselves are not writable by `vmmcp` either.
+If `find` still reports an error (for example a directory `vmmcp` can enter but not
+list), the check prints `FAIL`. Look at that directory's permissions before treating
+the result as a pass.
 
 ### e. HTTP authentication
 

backend/tests/test_vm_contact_runbook.py

@@ -8,8 +8,10 @@
 from __future__ import annotations
 
 from pathlib import Path
+import os
 import re
 import subprocess
+import tempfile
 import unittest
 
 RUNBOOK = Path(__file__).resolve().parents[1] / "deploy" / "VM_CONTACT.md"
@@ -103,5 +105,41 @@ def test_broker_receipt_check_only_counts_this_run(self):
         self.assertIn("CHECK_PORT, CHECK_PROFILE, CHECK_SOCKET, TOKEN_FILE, CHECK_ROOT = load_target()", block)
 
 
+    def _check_d_writable(self):
+        lines = [text for _, block in self.blocks for text in block.splitlines()
+                 if text.startswith("w=$(sudo -n -u vmmcp find /opt/vm-mcp")]
+        self.assertEqual(len(lines), 1)
+        return lines[0]
+
+    def test_check_d_prunes_unsearchable_dirs_and_keeps_read_failures(self):
+        line = self._check_d_writable()
+        self.assertTrue(line.endswith("|| w=READ-FAILED"))
+        self.assertIn("-type d ! -executable -prune -o -writable -print", line)
+
+    @unittest.skipIf(os.geteuid() == 0, "permission bits do not bind root")
+    def test_check_d_find_behaviour(self):
+        line = self._check_d_writable().replace("sudo -n -u vmmcp ", "")
+        script = line.replace("/opt/vm-mcp", '"$1"') + '\nprintf "%s" "$w"\n'
+        with tempfile.TemporaryDirectory() as tmp:
+            root = Path(tmp, "opt")
+            (root / "__pycache__").mkdir(parents=True)
+            (root / "server.py").write_text("x")
+            (root / "server.py").chmod(0o444)
+            (root / "__pycache__").chmod(0o000)
+            run = lambda: subprocess.run(["bash", "-c", script, "_", str(root)],
+                                         text=True, capture_output=True, check=False).stdout
+            try:
+                root.chmod(0o555)
+                self.assertEqual("", run(), "an unsearchable dir must not read as a failure")
+                (root / "server.py").chmod(0o644)
+                self.assertEqual(str(root / "server.py"), run().strip())
+                (root / "server.py").chmod(0o444)
+                (root / "__pycache__").chmod(0o100)  # enterable but not listable
+                self.assertEqual("READ-FAILED", run())
+            finally:
+                root.chmod(0o755)
+                (root / "__pycache__").chmod(0o755)
+
+
 if __name__ == "__main__":
     unittest.main()