[Build Failure] TestBSON.test_encode_size_limit MemoryError on Windows

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    • Type: Build Failure
    • Resolution: Fixed
    • Priority: Major - P3
    • 4.19.0
    • Affects Version/s: None
    • Component/s: Tests
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    • Python Drivers
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      Name of Failure:

      test.test_bson.TestBSON.test_encode_size_limit fails with MemoryError instead of the expected ValueError.

      Link to task:

      test-standard-v8.0-python3.15-sync-noauth-ssl-replica-set (execution 0)|https://spruce.corp.mongodb.com/task/mongo_python_driver_test_win32_test_standard_v8.0_python3.15_sync_noauth_ssl_replica_set1bb24c934407bead62045c5497af5b6f75b32e6a26_10_01_23_53_50/tests?execution=0&sorts=STATUS%3AASC

      Context of when and why the failure occurred:

      The test (from PYTHON-5996) needs ~3GiB peak memory (1GiB string + 1GiB UTF-8 copy + 1GiB encode buffer). On 4GB windows-64-vsMulti-small hosts, allocation fails before the size-limit check can raise the expected ValueError. Fails 38% of the time on test-win32 (8% across all variants) over the last 30 days.

      Proposed fix: Skip the test on hosts with < 6GiB total memory.

      It is also failing on the v4.18 branch, so we'll need to backport the fix.

      Stack trace:

      2026/10/01 19:26:54.319 FAILURE: MemoryError ()
2026/10/01 19:26:54.319 self = <test.test_bson.TestBSON testMethod=test_encode_size_limit>
2026/10/01 19:26:54.319     @unittest.skipUnless(bson.has_c(), "This test requires the C extension")
2026/10/01 19:26:54.319     def test_encode_size_limit(self):
2026/10/01 19:26:54.319         # PYTHON-5996: encoding must raise when a document's encoded size
2026/10/01 19:26:54.319         # exceeds the BSON size limit.
2026/10/01 19:26:54.319         big_value = "a" * (1 << 30)
2026/10/01 19:26:54.319         with self.assertRaises(ValueError):
2026/10/01 19:26:54.319 >           encode({"a": big_value, "b": big_value, "c": big_value})
2026/10/01 19:26:54.319 test\test_bson.py:700:
2026/10/01 19:26:54.319 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2026/10/01 19:26:54.319     if not isinstance(codec_options, CodecOptions):
2026/10/01 19:26:54.319         raise CODEC_OPTIONS_TYPE_ERROR
2026/10/01 19:26:54.319
2026/10/01 19:26:54.319 >       return dict_to_bson(document, check_keys, codec_options)
2026/10/01 19:26:54.319 E       MemoryError
2026/10/01 19:26:54.319 bson\_init_.py:1085: MemoryError

            Assignee:
            Steve Silvester
            Reporter:
            Steve Silvester
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              Created:
              Updated:
              Resolved: