[GR-26868] _struct: fix native format sizing and overflow handling for unsigned longs

PullRequest: graalpython/1347

Author: cosminbasca

graalpython/com.oracle.graal.python.test/src/tests/test_struct.py

@@ -145,3 +145,17 @@ def test_pack_unpack():
 def test_pack_nan():
     import math
     assert struct.pack('<d', math.nan) == b'\x00\x00\x00\x00\x00\x00\xf8\x7f'
+
+
+def test_pack_large_long():
+    for fmt in ('l', 'q'):
+        assert struct.pack(fmt, 0) == b'\x00' * struct.calcsize(fmt)
+        assert struct.unpack(fmt, b'\x00' * struct.calcsize(fmt)) == (0,)
+        assert struct.pack(fmt, -1) == b'\xff' * struct.calcsize(fmt)
+        assert struct.unpack(fmt, b'\xff' * struct.calcsize(fmt)) == (-1,)
+
+    for fmt in ('L', 'Q'):
+        assert struct.pack(fmt, 0) == b'\x00' * struct.calcsize(fmt)
+        assert struct.unpack(fmt, b'\x00' * struct.calcsize(fmt)) == (0,)
+        assert struct.pack(fmt, 18446744073709551615) == b'\xff\xff\xff\xff\xff\xff\xff\xff'
+        assert struct.unpack(fmt, b'\xff\xff\xff\xff\xff\xff\xff\xff') == (18446744073709551615,)