Get rid of the "no such storage index" edge case, since it's not really necessary.

src/allmydata/storage/http_server.py

@@ -660,10 +660,7 @@ class HTTPServer(object):
     )
     def enumerate_mutable_shares(self, request, authorization, storage_index):
         """List mutable shares for a storage index."""
-        try:
-            shares = self._storage_server.enumerate_mutable_shares(storage_index)
-        except KeyError:
-            raise _HTTPError(http.NOT_FOUND)
+        shares = self._storage_server.enumerate_mutable_shares(storage_index)
         return self._send_encoded(request, shares)
 
 

src/allmydata/storage/server.py

@@ -691,15 +691,12 @@ class StorageServer(service.MultiService):
         return share
 
     def enumerate_mutable_shares(self, storage_index: bytes) -> set[int]:
-        """List all share numbers for the given mutable.
-
-        Raises ``KeyError`` if the storage index is not known.
-        """
+        """Return all share numbers for the given mutable."""
         si_dir = storage_index_to_dir(storage_index)
         # shares exist if there is a file for them
         bucketdir = os.path.join(self.sharedir, si_dir)
         if not os.path.isdir(bucketdir):
-            raise KeyError("Not found")
+            return set()
         result = set()
         for sharenum_s in os.listdir(bucketdir):
             try:

src/allmydata/storage_client.py

@@ -1200,13 +1200,7 @@ 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:
-            try:
-                shares = yield mutable_client.list_shares(storage_index)
-            except ClientException as e:
-                if e.code == http.NOT_FOUND:
-                    shares = set()
-                else:
-                    raise
+            shares = yield mutable_client.list_shares(storage_index)
 
         # Start all the queries in parallel:
         for share_number in shares:

src/allmydata/test/test_storage.py

@@ -1317,14 +1317,14 @@ class MutableServer(unittest.TestCase):
 
     def test_enumerate_mutable_shares(self):
         """
-        ``StorageServer.enumerate_mutable_shares()`` returns a set of share numbers
-        for the given storage index, or raises ``KeyError`` if it does not exist at all.
+        ``StorageServer.enumerate_mutable_shares()`` returns a set of share
+        numbers for the given storage index, or an empty set if it does not
+        exist at all.
         """
         ss = self.create("test_enumerate_mutable_shares")
 
         # Initially, nothing exists:
-        with self.assertRaises(KeyError):
-            ss.enumerate_mutable_shares(b"si1")
+        empty = ss.enumerate_mutable_shares(b"si1")
 
         self.allocate(ss, b"si1", b"we1", b"le1", [0, 1, 4, 2], 12)
         shares0_1_2_4 = ss.enumerate_mutable_shares(b"si1")
@@ -1336,8 +1336,8 @@ class MutableServer(unittest.TestCase):
         ss.slot_testv_and_readv_and_writev(b"si1", secrets, {2: ([], [], 0)}, [])
         shares0_1_4 = ss.enumerate_mutable_shares(b"si1")
         self.assertEqual(
-            (shares0_1_2_4, shares0_1_4),
-            ({0, 1, 2, 4}, {0, 1, 4})
+            (empty, shares0_1_2_4, shares0_1_4),
+            (set(), {0, 1, 2, 4}, {0, 1, 4})
         )
 
     def test_bad_magic(self):