Reject sets which apparently weren't ever passed in

Only the documentation was wrong, whew.

src/_zkapauthorizer/storage_common.py

@@ -55,7 +55,7 @@ def required_passes(bytes_per_pass, share_sizes):
     :param int bytes_per_pass: The number of bytes the storage of which for
         one lease period one pass covers.
 
-    :param set[int] share_sizes: The sizes of the shared which will be stored.
+    :param list[int] share_sizes: The sizes of the shared which will be stored.
 
     :return int: The number of passes required to cover the storage cost.
     """
@@ -64,6 +64,12 @@ def required_passes(bytes_per_pass, share_sizes):
             sum(share_sizes, 0) / bytes_per_pass,
         ),
     )
+    if not isinstance(share_sizes, list):
+        raise TypeError(
+            "Share sizes must be a list of integers, got {!r} instead".format(
+                share_sizes,
+            ),
+        )
     # print("required_passes({}, {}) == {}".format(bytes_per_pass, share_sizes, result))
     return result
 

src/_zkapauthorizer/tests/test_storage_protocol.py

@@ -33,6 +33,7 @@ from testtools.matchers import (
     HasLength,
     IsInstance,
     AfterPreprocessing,
+    raises,
 )
 from testtools.twistedsupport import (
     succeeded,
@@ -49,7 +50,9 @@ from hypothesis import (
     assume,
 )
 from hypothesis.strategies import (
+    sets,
     tuples,
+    integers,
 )
 
 from twisted.python.filepath import (
@@ -101,6 +104,7 @@ from ..api import (
 from ..storage_common import (
     slot_testv_and_readv_and_writev_message,
     get_implied_data_length,
+    required_passes,
 )
 from ..model import (
     Pass,
@@ -148,6 +152,21 @@ class LocalRemote(object):
         )
 
 
+class RequiredPassesTests(TestCase):
+    """
+    Tests for ``required_passes``.
+    """
+    @given(integers(min_value=1), sets(integers(min_value=0)))
+    def test_incorrect_types(self, bytes_per_pass, share_sizes):
+        """
+        ``required_passes`` raises ``TypeError`` if passed a ``set`` for
+        ``share_sizes``.
+        """
+        self.assertThat(
+            lambda: required_passes(bytes_per_pass, share_sizes),
+            raises(TypeError),
+        )
+
 class ShareTests(TestCase):
     """
     Tests for interaction with shares.