introduce and use LeaseInfo.is_cancel_secret

src/allmydata/storage/immutable.py

@@ -209,7 +209,7 @@ class ShareFile(object):
         leases = list(self.get_leases())
         num_leases_removed = 0
         for i,lease in enumerate(leases):
-            if timing_safe_compare(lease.cancel_secret, cancel_secret):
+            if lease.is_cancel_secret(cancel_secret):
                 leases[i] = None
                 num_leases_removed += 1
         if not num_leases_removed:

src/allmydata/storage/lease.py

@@ -80,6 +80,16 @@ class LeaseInfo(object):
         """
         return timing_safe_compare(self.renew_secret, candidate_secret)
 
+    def is_cancel_secret(self, candidate_secret):
+        # type: (bytes) -> bool
+        """
+        Check a string to see if it is the correct cancel secret.
+
+        :return: ``True`` if it is the correct cancel secret, ``False``
+            otherwise.
+        """
+        return timing_safe_compare(self.cancel_secret, candidate_secret)
+
     def get_grant_renew_time_time(self):
         # hack, based upon fixed 31day expiration period
         return self._expiration_time - 31*24*60*60

src/allmydata/storage/mutable.py

@@ -371,7 +371,7 @@ class MutableShareFile(object):
         with open(self.home, 'rb+') as f:
             for (leasenum,lease) in self._enumerate_leases(f):
                 accepting_nodeids.add(lease.nodeid)
-                if timing_safe_compare(lease.cancel_secret, cancel_secret):
+                if lease.is_cancel_secret(cancel_secret):
                     self._write_lease_record(f, leasenum, blank_lease)
                     modified += 1
                 else:

src/allmydata/test/test_storage.py

@@ -3047,3 +3047,22 @@ class ShareFileTests(unittest.TestCase):
         sf.add_lease(lease)
         (loaded_lease,) = sf.get_leases()
         self.assertTrue(loaded_lease.is_renew_secret(renew_secret))
+
+    def test_cancel_secret(self):
+        """
+        A lease loaded from a share file can have its cancel secret verified.
+        """
+        renew_secret = b"r" * 32
+        cancel_secret = b"c" * 32
+        expiration_time = 2 ** 31
+
+        sf = self.get_sharefile()
+        lease = LeaseInfo(
+            owner_num=0,
+            renew_secret=renew_secret,
+            cancel_secret=cancel_secret,
+            expiration_time=expiration_time,
+        )
+        sf.add_lease(lease)
+        (loaded_lease,) = sf.get_leases()
+        self.assertTrue(loaded_lease.is_cancel_secret(cancel_secret))