[issue11877] Change os.fsync() to support physical backing store syncs

[Steffen Daode Nurpmeso]

Steffen Daode Nurpmeso <sdaoden at googlemail.com> added the comment:

I don't agree with you and i don't believe it is implemented like
that.  But it seems i am the only one on this issue who sees it
like that.  Thus i apply 11877.6.diff.

> Declaring variables as "auto" is not necessary in C code and not
> used anywhere else in Python's source code

Changed.

> Steffen, you changed the default to doing a "full sync" in your
> last patch

Changed all through.

> The reason being that Apple and NetBSD folks should know what
> they're doing [.]
> People requiring write durability will probably manage to find
> this full_sync parameter.

This sounds logical.  I've changed the doc in os.rst so that it
includes a note on *which* operating systems actually do something
dependend on that argument.
I should have done that from the very beginning.

> Finally, depending on the workload, it could have a significant
> performance impact.

:-)

----------
Added file: http://bugs.python.org/file21953/11877.6.diff

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11877>
_______________________________________
-------------- next part --------------
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -798,7 +798,7 @@
    Availability: Unix.
 
 
-.. function:: fsync(fd)
+.. function:: fsync(fd, full_fsync=False)
 
    Force write of file with filedescriptor *fd* to disk.  On Unix, this calls the
    native :c:func:`fsync` function; on Windows, the MS :c:func:`_commit` function.
@@ -807,6 +807,13 @@
    ``f.flush()``, and then do ``os.fsync(f.fileno())``, to ensure that all internal
    buffers associated with *f* are written to disk.
 
+   The POSIX standart only requires that :c:func:`fsync` must transfer the
+   buffered data to the storage device, not that the data is actually
+   written by the device itself.  On operating systems where it is
+   necessary and possible the optional *full_fsync* argument can be used to
+   initiate additional steps to synchronize the physical backing store.
+   At the time of this writing this affects Apple Mac OS X and NetBSD.
+
    Availability: Unix, and Windows.
 
 
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -2121,13 +2121,46 @@
 
 #ifdef HAVE_FSYNC
 PyDoc_STRVAR(posix_fsync__doc__,
-"fsync(fildes)\n\n\
-force write of file with filedescriptor to disk.");
-
-static PyObject *
-posix_fsync(PyObject *self, PyObject *fdobj)
-{
-    return posix_fildes(fdobj, fsync);
+"fsync(fildes, full_fsync=False)\n\n"
+"force write of file buffers with fildes to disk;\n"
+"full_fsync forces flush of disk caches in case fsync() alone is not enough.");
+
+static PyObject *
+posix_fsync(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    PyObject *fdobj;
+    int full_fsync = 0;
+    static char *keywords[] = {"fd", "full_fsync", NULL };
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O|i", keywords,
+                                     &fdobj, &full_fsync))
+        return NULL;
+
+    /* See issue 11877 discussion */
+# if ((defined __APPLE__ && defined F_FULLFSYNC) || \
+      (defined __NetBSD__ && defined FDISKSYNC))
+    if (full_fsync != 0) {
+        int res, fd = PyObject_AsFileDescriptor(fdobj);
+        if (fd < 0)
+            return NULL;
+        if (!_PyVerify_fd(fd))
+            return posix_error();
+
+        Py_BEGIN_ALLOW_THREADS
+#  if defined __APPLE__
+        res = fcntl(fd, F_FULLFSYNC);
+#  elif defined __NetBSD__
+        res = fsync_range(fd, FFILESYNC|FDISKSYNC, 0, 0);
+#  endif
+        Py_END_ALLOW_THREADS
+
+        if (res < 0)
+            return posix_error();
+        Py_INCREF(Py_None);
+        return Py_None;
+    } else
+# endif
+        return posix_fildes(fdobj, fsync);
 }
 #endif /* HAVE_FSYNC */
 
@@ -9472,7 +9505,8 @@
     {"fchdir",          posix_fchdir, METH_O, posix_fchdir__doc__},
 #endif
 #ifdef HAVE_FSYNC
-    {"fsync",       posix_fsync, METH_O, posix_fsync__doc__},
+    {"fsync",           (PyCFunction)posix_fsync, METH_VARARGS|METH_KEYWORDS,
+                        posix_fsync__doc__},
 #endif
 #ifdef HAVE_SYNC
     {"sync",        posix_sync, METH_NOARGS, posix_sync__doc__},