immutable: prevent clients from reading past the end of share data, which would allow them to learn the cancellation secret

Declare explicitly that we prevent this problem in the server's version dict.
fixes #1528 (there are two patches that are each a sufficient fix to #1528 and this is one of them)

src/allmydata/storage/immutable.py

@@ -83,11 +83,9 @@ class ShareFile:
     def read_share_data(self, offset, length):
         precondition(offset >= 0)
         # reads beyond the end of the data are truncated. Reads that start
-        # beyond the end of the data return an empty string. I wonder why
+        # beyond the end of the data return an empty string.
-        # Python doesn't do the following computation for me?
         seekpos = self._data_offset+offset
-        fsize = os.path.getsize(self.home)
+        actuallength = max(0, min(length, self._lease_offset-seekpos))
-        actuallength = max(0, min(length, fsize-seekpos))
         if actuallength == 0:
             return ""
         f = open(self.home, 'rb')

src/allmydata/storage/server.py

@@ -222,6 +222,7 @@ class StorageServer(service.MultiService, Referenceable):
                     { "maximum-immutable-share-size": remaining_space,
                       "tolerates-immutable-read-overrun": True,
                       "delete-mutable-shares-with-zero-length-writev": True,
+                      "prevents-read-past-end-of-share-data": True,
                       },
                     "application-version": str(allmydata.__full_version__),
                     }