In reality we cannot handle a Deferred here

... yet?  This code is invoked from an `__init__` where async is always
tricky.  Maybe we can invert the relationship someday.

src/allmydata/interfaces.py

@@ -3114,7 +3114,7 @@ class IFoolscapStoragePlugin(IPlugin):
             server portion of this plugin on the currently active connection,
             or ``None`` if no connection has been established yet.
 
-        :rtype: ``Deferred`` firing with ``IStorageServer``
+        :rtype: ``IStorageServer``
         """
 
     def get_client_resource(configuration):

src/allmydata/storage_client.py

@@ -446,7 +446,7 @@ class _FoolscapStorage(object):
     permutation_seed = attr.ib()
     tubid = attr.ib()
 
-    storage_server = attr.ib()
+    storage_server = attr.ib(validator=attr.validators.provides(IStorageServer))
 
     _furl = attr.ib()
     _short_description = attr.ib()

src/allmydata/test/test_storage_client.py

@@ -15,6 +15,9 @@ from testtools.matchers import (
     AfterPreprocessing,
 )
 
+from zope.interface import (
+    implementer,
+)
 from zope.interface.verify import (
     verifyObject,
 )
@@ -59,6 +62,7 @@ from allmydata.storage_client import (
 )
 from allmydata.interfaces import (
     IConnectionStatus,
+    IStorageServer,
 )
 
 SOME_FURL = b"pb://abcde@nowhere/fake"
@@ -381,6 +385,9 @@ class FoolscapStorageServers(unittest.TestCase):
         """
         Instances of ``_FoolscapStorage`` provide ``IFoolscapStorageServer``.
         """
+        @implementer(IStorageServer)
+        class NotStorageServer(object):
+            pass
         self.assertTrue(
             verifyObject(
                 IFoolscapStorageServer,
@@ -388,7 +395,7 @@ class FoolscapStorageServers(unittest.TestCase):
                     u"server-id",
                     SOME_FURL,
                     {u"permutation-seed-base32": base32.b2a(b"permutationseed")},
-                    object(),
+                    NotStorageServer(),
                 ),
             ),
         )