bpo-38256: Fix binascii.crc32() when inputs are 4+GiB (GH-32000)

When compiled with `USE_ZLIB_CRC32` defined (`configure` sets this on POSIX systems), `binascii.crc32(...)` failed to compute the correct value when the input data was >= 4GiB. Because the zlib crc32 API is limited to a 32-bit length.

This lines it up with the `zlib.crc32(...)` implementation that doesn't have that flaw.

**Performance:** This also adopts the same GIL releasing for larger inputs logic that `zlib.crc32` has, and causes the Windows build to always use zlib's crc32 instead of our slow C code as zlib is a required build dependency on Windows.

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/test/test_binascii.py b/Lib/test/test_binascii.py
index b5aa847..7087d7a 100644
--- a/Lib/test/test_binascii.py
+++ b/Lib/test/test_binascii.py
@@ -4,7 +4,7 @@ import unittest
 import binascii
 import array
 import re
-from test.support import warnings_helper
+from test.support import bigmemtest, _1G, _4G, warnings_helper
 
 
 # Note: "*_hex" functions are aliases for "(un)hexlify"
@@ -441,6 +441,14 @@ class BytearrayBinASCIITest(BinASCIITest):
 class MemoryviewBinASCIITest(BinASCIITest):
     type2test = memoryview
 
+class ChecksumBigBufferTestCase(unittest.TestCase):
+    """bpo-38256 - check that inputs >=4 GiB are handled correctly."""
+
+    @bigmemtest(size=_4G + 4, memuse=1, dry_run=False)
+    def test_big_buffer(self, size):
+        data = b"nyan" * (_1G + 1)
+        self.assertEqual(binascii.crc32(data), 1044521549)
+
 
 if __name__ == "__main__":
     unittest.main()