fix: generate message and annotation ids with the URL-safe base64 alphabet

An id travels in a URL path, where the standard alphabet's `+` and `/` are
not safe to carry. RSL1k1 asks only for "base64-encoding a sequence of at
least 9 bytes" and names no alphabet, so both encodings conform.

`test_idempotent_library_generated` decodes a generated id, and needs the
matching decoder to do so.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: owenpearson

ably/rest/annotations.py

@@ -130,7 +130,7 @@ async def __send_annotation(self, annotation: Annotation, params: dict | None =
         # RSAN1c4: Generate random ID if not provided (for idempotent publishing)
         # Spec: base64-encode at least 9 random bytes, append ':0'
         if not annotation.id and self.__client_options.idempotent_rest_publishing:
-            random_id = base64.b64encode(os.urandom(9)).decode('ascii') + ':0'
+            random_id = base64.urlsafe_b64encode(os.urandom(9)).decode('ascii') + ':0'
             annotation = annotation._copy_with(id=random_id)
 
         # RSAN1c3: encrypt the data payload on an encrypted channel

ably/rest/channel.py

@@ -60,7 +60,7 @@ def __publish_request_body(self, messages):
         if self.ably.options.idempotent_rest_publishing:
             # RSL1k1
             if all(message.id is None for message in messages):
-                base_id = base64.b64encode(os.urandom(12)).decode()
+                base_id = base64.urlsafe_b64encode(os.urandom(12)).decode()
                 for serial, message in enumerate(messages):
                     message.id = f'{base_id}:{serial}'
 

test/ably/rest/restchannelpublish_test.py

@@ -503,7 +503,7 @@ def test_idempotent_library_generated(self):
         message = Message('name', 'data')
         request_body = channel._Channel__publish_request_body(messages=[message])
         base_id, serial = request_body['id'].split(':')
-        assert len(base64.b64decode(base_id)) >= 9
+        assert len(base64.urlsafe_b64decode(base_id)) >= 9
         assert serial == '0'
 
     # RSL1k2