gh-139090: add os.RWF_DONTCACHE

Author: benjaminp

Doc/library/os.rst

@@ -1501,6 +1501,7 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
 
    - :data:`RWF_HIPRI`
    - :data:`RWF_NOWAIT`
+   - :data:`RWF_DONTCACHE`
 
    Return the total number of bytes actually read which can be less than the
    total capacity of all the objects.
@@ -1546,6 +1547,15 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
    .. versionadded:: 3.7
 
 
+.. data:: RWF_DONTCACHE
+
+   Use uncached buffered IO.
+
+   .. availability:: Linux >= 6.14
+
+   .. versionadded:: next
+
+
 .. function:: ptsname(fd, /)
 
    Return the name of the slave pseudo-terminal device associated with the
@@ -1587,6 +1597,7 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
    - :data:`RWF_DSYNC`
    - :data:`RWF_SYNC`
    - :data:`RWF_APPEND`
+   - :data:`RWF_DONTCACHE`
 
    Return the total number of bytes actually written.
 

Misc/NEWS.d/next/Library/2025-09-17-21-52-30.gh-issue-139090.W7vbhF.rst

@@ -0,0 +1 @@
+Add :data:`os.RWF_DONTCACHE` constant for Linux 6.14+.

Modules/posixmodule.c

@@ -11756,6 +11756,7 @@ The flags argument contains a bitwise OR of zero or more of the following flags:
 
 - RWF_HIPRI
 - RWF_NOWAIT
+- RWF_DONTCACHE
 
 Using non-zero flags requires Linux 4.6 or newer.
 [clinic start generated code]*/
@@ -12413,6 +12414,7 @@ The flags argument contains a bitwise OR of zero or more of the following flags:
 - RWF_DSYNC
 - RWF_SYNC
 - RWF_APPEND
+- RWF_DONTCACHE
 
 Using non-zero flags requires Linux 4.7 or newer.
 [clinic start generated code]*/
@@ -17646,6 +17648,9 @@ all_ins(PyObject *m)
 #ifdef RWF_NOWAIT
     if (PyModule_AddIntConstant(m, "RWF_NOWAIT", RWF_NOWAIT)) return -1;
 #endif
+#ifdef RWF_DONTCACHE
+    if (PyModule_AddIntConstant(m, "RWF_DONTCACHE", RWF_DONTCACHE)) return -1;
+#endif
 #ifdef RWF_APPEND
     if (PyModule_AddIntConstant(m, "RWF_APPEND", RWF_APPEND)) return -1;
 #endif