Issue #12102: Document that buffered files must be flushed before being used

with mmap. Patch by Steffen Daode Nurpmeso.

Doc/ACKS.txt

@@ -141,6 +141,7 @@ docs@python.org), and we'll be glad to correct the problem.
    * Sjoerd Mullender
    * Dale Nagata
    * Michal Nowikowski
+   * Steffen Daode Nurpmeso
    * Ng Pheng Siong
    * Koray Oner
    * Tomas Oppelstrup

Doc/library/mmap.rst

@@ -23,6 +23,12 @@ file object, use its :meth:`fileno` method to obtain the correct value for the
 :func:`os.open` function, which returns a file descriptor directly (the file
 still needs to be closed when done).
 
+.. note::
+   If you want to create a memory-mapping for a writable, buffered file, you
+   should :func:`~io.IOBase.flush` the file first.  This is necessary to ensure
+   that local modifications to the buffers are actually available to the
+   mapping.
+
 For both the Unix and Windows versions of the constructor, *access* may be
 specified as an optional keyword parameter. *access* accepts one of three
 values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY`

Misc/NEWS

@@ -37,6 +37,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12102: Document that buffered files must be flushed before being used
+  with mmap. Patch by Steffen Daode Nurpmeso.
+
 - Issue #12560: Build libpython.so on OpenBSD. Patch by Stefan Sperling.
 
 - Issue #1813: Fix codec lookup and setting/getting locales under Turkish