upload: add a think-of-the-compatibility note to UploadResults

diff --git a/src/allmydata/immutable/upload.py b/src/allmydata/immutable/upload.py
index 28edda49aadbd22d70db73ef84ff47d94049cf0f..ed377daf26b4244981e2f90f4b64ea1718492cc0 100644 (file)
--- a/src/allmydata/immutable/upload.py
+++ b/src/allmydata/immutable/upload.py
@@ -46,6 +46,11 @@ class UploadResults(Copyable, RemoteCopy):
     typeToCopy = "allmydata.upload.UploadResults.tahoe.allmydata.com"
     copytype = typeToCopy
 
+    # also, think twice about changing the shape of any existing attribute,
+    # because instances of this class are sent from the helper to its client,
+    # so changing this may break compatibility. Consider adding new fields
+    # instead of modifying existing ones.
+
     def __init__(self):
         self.timings = {} # dict of name to number of seconds
         self.sharemap = dictutil.DictOfSets() # {shnum: set(serverid)}