Add a helper to LeaseInfo for computing size

This lets some code LBYL and avoid writing if the lease won't fit in the
immutable share in the space available.

src/allmydata/storage/lease.py

@@ -13,6 +13,9 @@ if PY2:
 
 import struct, time
 
+# struct format for representation of a lease in an immutable share
+IMMUTABLE_FORMAT = ">L32s32sL"
+
 class LeaseInfo(object):
     def __init__(self, owner_num=None, renew_secret=None, cancel_secret=None,
                  expiration_time=None, nodeid=None):
@@ -39,12 +42,19 @@ class LeaseInfo(object):
         (self.owner_num,
          self.renew_secret,
          self.cancel_secret,
-         self.expiration_time) = struct.unpack(">L32s32sL", data)
+         self.expiration_time) = struct.unpack(IMMUTABLE_FORMAT, data)
         self.nodeid = None
         return self
 
+    def immutable_size(self):
+        """
+        :return int: The size, in bytes, of the representation of this lease in an
+            immutable share file.
+        """
+        return struct.calcsize(IMMUTABLE_FORMAT)
+
     def to_immutable_data(self):
-        return struct.pack(">L32s32sL",
+        return struct.pack(IMMUTABLE_FORMAT,
                            self.owner_num,
                            self.renew_secret, self.cancel_secret,
                            int(self.expiration_time))

src/allmydata/test/test_storage.py

@@ -117,6 +117,29 @@ class FakeStatsProvider(object):
     def register_producer(self, producer):
         pass
 
+
+class LeaseInfoTests(unittest.TestCase):
+    """
+    Tests for ``LeaseInfo``.
+    """
+    @given(
+        strategies.tuples(
+            strategies.integers(min_value=0, max_value=2 ** 31 - 1),
+            strategies.binary(min_size=32, max_size=32),
+            strategies.binary(min_size=32, max_size=32),
+            strategies.integers(min_value=0, max_value=2 ** 31 - 1),
+            strategies.binary(min_size=20, max_size=20),
+        ),
+    )
+    def test_immutable_size(self, initializer_args):
+        """
+        ``LeaseInfo.immutable_size`` returns the length of the result of
+        ``LeaseInfo.to_immutable_data``.
+        """
+        info = LeaseInfo(*initializer_args)
+        self.assertEqual(len(info.to_immutable_data()), info.immutable_size())
+
+
 class Bucket(unittest.TestCase):
     def make_workdir(self, name):
         basedir = os.path.join("storage", "Bucket", name)