Test and document what happens for non-existent storage index.

docs/proposed/http-storage-node-protocol.rst

@@ -635,6 +635,9 @@ indicated storage index. For example::
 
   [1, 5]
 
+An unknown storage index results in empty list, so that lack of existence of
+storage index is not leaked.
+
 ``GET /v1/immutable/:storage_index/:share_number``
 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 

src/allmydata/storage/http_server.py

@@ -266,8 +266,6 @@ class HTTPServer(object):
         List shares for the given storage index.
         """
         storage_index = si_a2b(storage_index.encode("ascii"))
-
-        # TODO in future ticket, handle KeyError as 404
         share_numbers = list(self._storage_server.get_buckets(storage_index).keys())
         return self._cbor(request, share_numbers)
 

src/allmydata/test/test_storage_http.py

@@ -413,6 +413,14 @@ class ImmutableHTTPAPITests(SyncTestCase):
         # Now shares 1 and 3 exist:
         self.assertEqual(result_of(im_client.list_shares(storage_index)), {1, 3})
 
+    def test_list_shares_unknown_storage_index(self):
+        """
+        Listing unknown storage index's shares results in empty list of shares.
+        """
+        im_client = StorageClientImmutables(self.http.client)
+        storage_index = b"".join(bytes([i]) for i in range(16))
+        self.assertEqual(result_of(im_client.list_shares(storage_index)), set())
+
     def test_multiple_shares_uploaded_to_different_place(self):
         """
         If a storage index has multiple shares, uploads to different shares are