bpo-36051: Drop GIL during large bytes.join() (GH-17757)

Improve multi-threaded performance by dropping the GIL in the fast path
of bytes.join. To avoid increasing overhead for small joins, it is only
done if the output size exceeds a threshold.

1 files changed, 6 insertions, 2 deletions

diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index ddcf367..770e2c5 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -547,9 +547,13 @@ class BaseBytesTest:
         self.assertEqual(dot_join([bytearray(b"ab"), b"cd"]), b"ab.:cd")
         self.assertEqual(dot_join([b"ab", bytearray(b"cd")]), b"ab.:cd")
         # Stress it with many items
-        seq = [b"abc"] * 1000
-        expected = b"abc" + b".:abc" * 999
+        seq = [b"abc"] * 100000
+        expected = b"abc" + b".:abc" * 99999
         self.assertEqual(dot_join(seq), expected)
+        # Stress test with empty separator
+        seq = [b"abc"] * 100000
+        expected = b"abc" * 100000
+        self.assertEqual(self.type2test(b"").join(seq), expected)
         self.assertRaises(TypeError, self.type2test(b" ").join, None)
         # Error handling and cleanup when some item in the middle of the
         # sequence has the wrong type.