Issue #12062: In the `io` module, fix a flushing bug when doing a certain
type of I/O sequence on a file opened in read+write mode (namely: reading,
seeking a bit forward, writing, then seeking before the previous write but
still within buffered data, and writing again).
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 0bad56d..8af8a64 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1462,6 +1462,32 @@
self.assertEqual(s,
b"A" + b"B" * overwrite_size + b"A" * (9 - overwrite_size))
+ def test_write_rewind_write(self):
+ # Various combinations of reading / writing / seeking backwards / writing again
+ def mutate(bufio, pos1, pos2):
+ assert pos2 >= pos1
+ # Fill the buffer
+ bufio.seek(pos1)
+ bufio.read(pos2 - pos1)
+ bufio.write(b'\x02')
+ # This writes earlier than the previous write, but still inside
+ # the buffer.
+ bufio.seek(pos1)
+ bufio.write(b'\x01')
+
+ b = b"\x80\x81\x82\x83\x84"
+ for i in range(0, len(b)):
+ for j in range(i, len(b)):
+ raw = self.BytesIO(b)
+ bufio = self.tp(raw, 100)
+ mutate(bufio, i, j)
+ bufio.flush()
+ expected = bytearray(b)
+ expected[j] = 2
+ expected[i] = 1
+ self.assertEqual(raw.getvalue(), expected,
+ "failed result for i=%d, j=%d" % (i, j))
+
def test_truncate_after_read_or_write(self):
raw = self.BytesIO(b"A" * 10)
bufio = self.tp(raw, 100)