Fix VCDiff decoding after an empty base payload

ably/types/mixins.py

@@ -83,7 +83,7 @@ def decode(data, encoding='', cipher=None, context=None):
                     log.error('Message cannot be decoded as no VCDiff decoder available')
                     raise AblyException('VCDiff decoder not available', 40019, 40019)
 
-                if not context.base_payload:
+                if context.base_payload is None:
                     log.error('VCDiff decoding requires base payload')
                     raise AblyException('VCDiff decode failure', 40018, 40018)
 

test/uts/rest/unit/encoding/empty_delta_base_test.py

@@ -0,0 +1,53 @@
+import pytest
+
+from ably import AblyVCDiffDecoder
+from ably.types.message import Message
+from ably.types.mixins import DecodingContext
+from ably.util.exceptions import AblyException
+
+# A VCDIFF window containing only an ADD instruction, independent of source bytes.
+DELTA = bytes.fromhex('d6c3c40000000b0500050100') + b'hello' + bytes([6])
+
+
+def delta_message():
+    return {
+        'id': 'next',
+        'data': DELTA,
+        'encoding': 'utf-8/vcdiff',
+        'extras': {'delta': {'from': 'initial'}},
+    }
+
+
+@pytest.mark.parametrize('base', ['', b'', bytearray(), b'previous'])
+def test_delta_decodes_from_present_base(base):
+    context = DecodingContext(base, 'initial', AblyVCDiffDecoder())
+
+    message = Message.from_encoded(delta_message(), context=context)
+
+    assert message.data == 'hello'
+    assert context.base_payload == b'hello'
+    assert context.last_message_id == 'next'
+
+
+def test_delta_still_rejects_missing_base():
+    context = DecodingContext(None, 'initial', AblyVCDiffDecoder())
+
+    with pytest.raises(AblyException) as error:
+        Message.from_encoded(delta_message(), context=context)
+
+    assert error.value.code == 40018
+    assert context.base_payload is None
+    assert context.last_message_id == 'initial'
+
+
+def test_batch_chains_delta_after_empty_full_message():
+    context = DecodingContext(vcdiff_decoder=AblyVCDiffDecoder())
+
+    messages = Message.from_encoded_array([
+        {'id': 'initial', 'data': '', 'encoding': 'utf-8'},
+        delta_message(),
+    ], context=context)
+
+    assert [message.data for message in messages] == ['', 'hello']
+    assert context.base_payload == b'hello'
+    assert context.last_message_id == 'next'