test: probe hardlink staging in deployment preflight

tests/test_deployment_preflight.py

@@ -1,6 +1,7 @@
 import importlib.util
 import sys
 import unittest
+from unittest import mock
 from pathlib import Path
 
 
@@ -49,6 +50,24 @@ class DeploymentPreflightTests(unittest.TestCase):
             "/data/qb/.archive-control-routes",
         )
 
+    def test_runs_hardlink_probe_with_qb_and_route_roots(self):
+        completed = __import__("subprocess").CompletedProcess(
+            args=[], returncode=0, stdout="hard-link staging probe passed\n", stderr=""
+        )
+        with mock.patch.object(preflight.subprocess, "run", return_value=completed) as run:
+            preflight.run_hardlink_probe("client", "/data/qb", "/data/qb/routes")
+        args = run.call_args.args[0]
+        self.assertEqual(args[:5], ["docker", "exec", "client", "python", "-c"])
+        self.assertEqual(args[-2:], ["/data/qb", "/data/qb/routes"])
+
+    def test_hardlink_probe_failure_is_actionable(self):
+        completed = __import__("subprocess").CompletedProcess(
+            args=[], returncode=1, stdout="", stderr="[Errno 18] Invalid cross-device link"
+        )
+        with mock.patch.object(preflight.subprocess, "run", return_value=completed):
+            with self.assertRaisesRegex(preflight.CheckFailure, "hard-link staging probe failed"):
+                preflight.run_hardlink_probe("client", "/data/qb", "/data/qb/routes")
+
 
 if __name__ == "__main__":
     unittest.main()