Issue #12062: 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 --cc Lib/test/test_io.py
Simple merge

diff --cc Misc/NEWS
index 751f1e307c164c9ddffa29009f7ccef7185892d3,6b0a6b50f8368c585a0a56a9248bcc00e25180cf..620d3d66abe948b7ecc96a454e683c182f798ad4
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -147,9 -90,11 +147,14 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #12062: 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).
+ 
 +- Issue #9971: Write an optimized implementation of BufferedReader.readinto().
 +  Patch by John O'Connor.
 +
  - Issue #1028: Tk returns invalid Unicode null in %A: UnicodeDecodeError.
    With Tk < 8.5 _tkinter.c:PythonCmd() raised UnicodeDecodeError, caused
    IDLE to exit.  Converted to valid Unicode null in PythonCmd().

diff --cc Modules/_io/bufferedio.c
Simple merge