(Merge 3.1) Issue #11277: mmap.mmap() calls fcntl(fd, F_FULLFSYNC) on Mac OS X

to get around a mmap bug with sparse files. Patch written by Steffen Daode
Nurpmeso.

Doc/library/mmap.rst

@@ -94,6 +94,10 @@ memory but does not update the underlying file.
    defaults to 0.  *offset* must be a multiple of the PAGESIZE or
    ALLOCATIONGRANULARITY.
 
+   To ensure validity of the created memory mapping the file specified
+   by the descriptor *fileno* is internally automatically synchronized
+   with physical backing store on Mac OS X and OpenVMS.
+
    This example shows a simple way of using :class:`mmap`::
 
       import mmap

Misc/NEWS

@@ -70,6 +70,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11277: mmap.mmap() calls fcntl(fd, F_FULLFSYNC) on Mac OS X to get
+  around a mmap bug with sparse files. Patch written by Steffen Daode Nurpmeso.
+
 - Issue #10761: Fix tarfile.extractall failure  when symlinked files are
   present. Initial patch by Scott Leerssen.
 

Modules/mmapmodule.c

@@ -23,6 +23,9 @@
 
 #ifndef MS_WINDOWS
 #define UNIX
+# ifdef __APPLE__
+#  include <fcntl.h>
+# endif
 #endif
 
 #ifdef MS_WINDOWS
@@ -1170,6 +1173,12 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
                             "mmap invalid access parameter.");
     }
 
+#ifdef __APPLE__
+    /* Issue #11277: fsync(2) is not enough on OS X - a special, OS X specific
+       fcntl(2) is necessary to force DISKSYNC and get around mmap(2) bug */
+    if (fd != -1)
+        (void)fcntl(fd, F_FULLFSYNC);
+#endif
 #ifdef HAVE_FSTAT
 #  ifdef __VMS
     /* on OpenVMS we must ensure that all bytes are written to the file */