Match Foolscap behavior for slot_readv of unknown storage index.

src/allmydata/storage_client.py

@@ -1200,7 +1200,13 @@ class _HTTPStorageServer(object):
         # If shares list is empty, that means list all shares, so we need
         # to do a query to get that.
         if not shares:
-            shares = yield mutable_client.list_shares(storage_index)
+            try:
+                shares = yield mutable_client.list_shares(storage_index)
+            except ClientException as e:
+                if e.code == http.NOT_FOUND:
+                    shares = set()
+                else:
+                    raise
 
         # Start all the queries in parallel:
         for share_number in shares:

src/allmydata/test/test_istorageserver.py

@@ -854,6 +854,22 @@ class IStorageServerMutableAPIsTestsMixin(object):
             {0: [b"abcdefg"], 1: [b"0123456"], 2: [b"9876543"]},
         )
 
+    @inlineCallbacks
+    def test_slot_readv_unknown_storage_index(self):
+        """
+        With unknown storage index, ``IStorageServer.slot_readv()`` TODO.
+        """
+        storage_index = new_storage_index()
+        reads = yield self.storage_client.slot_readv(
+            storage_index,
+            shares=[],
+            readv=[(0, 7)],
+        )
+        self.assertEqual(
+            reads,
+            {},
+        )
+
     @inlineCallbacks
     def create_slot(self):
         """Create a slot with sharenum 0."""