Treat a CloudFetch link without bytesNum as size 0

The Thrift reader doesn't enforce required fields, so a missing size must
not fail the fetch; it only disables the sizing heuristic.

Signed-off-by: Vu Anh Phung <vu.phung@databricks.com>
Co-authored-by: Isaac <no-reply@databricks.com>

Author: vuanhphung

src/databricks/sql/result_set.py

@@ -286,8 +286,11 @@ def __init__(
 
     def _note_link_sizes(self, links) -> None:
         for link in links:
-            if link.bytesNum > self._max_link_bytes:
-                self._max_link_bytes = link.bytesNum
+            # The reader doesn't enforce required fields; a missing size only
+            # disables the sizing heuristic.
+            size = link.bytesNum or 0
+            if size > self._max_link_bytes:
+                self._max_link_bytes = size
 
     def _fetch_max_bytes(self) -> int:
         # The server packs CloudFetch links into a FetchResults reply up to maxBytes of

tests/unit/test_fetches.py

@@ -499,6 +499,17 @@ def test_caps_at_four_buffers(self):
 
         self.assertEqual(calls[0]["max_bytes"], 4 * self.BUFFER)
 
+    def test_links_without_size_keep_buffer_size(self):
+        rs, calls = self._make_result_set(
+            initial_link_sizes=[None],
+            fetched_queues=[self._cloudfetch_queue(None), self._cloudfetch_queue()],
+        )
+
+        rs._fill_results_buffer()
+        rs._fill_results_buffer()
+
+        self.assertEqual([c["max_bytes"] for c in calls], [self.BUFFER, self.BUFFER])
+
     def test_small_buffer_size_caps_batch(self):
         rs, calls = self._make_result_set(
             initial_link_sizes=[20 * self.MIB],