[Python-Dev] small PATCH to fix os.fsync on OS X

[Ian Charnas]

Hello heroes and heroins of the python universe, lend me your ears and
I will tell you a tale that will make your platters spin!

As noted by SQLite [1] and MySQL [2] developers, fsync on OS X does
not actually flush a file to disk... instead OS X provides "fullfsync"
which actually flushes the file descriptor to disk.  This simple patch
modifies python's "fsync" to take advantage of fullfsync on platforms
that support it:
http://bugs.python.org/issue3512

While some people might suggest that fullfsync should be exposed as a
method on the 'os' module for platforms that support it, I believe
this is not the best approach.  Because the docstring for os.fsync
says that it forces the file to be written to disk, I think it's up to
python to ensure that functionality is really there on all platforms.
Currently on OS X, fullfsync is needed to provide this functionality,
so I think python's os.fsync should use fullfsync on OS X.

Looking at 'svn blame' output for Modules/posixmodule.c, I see that
guido wrote the original python fsync, ronald.oussoren added all the
OS X specific stuff to posixmodule.c, and recent commits have been
made to that file by martin.v.loewis. christian.heimes,
facundo.batista, and gregory.p.smith.  Would any of you fine folks
like to take a look at this?  I believe it's a *very* small patch.

Many thanks,  -Ian Charnas

[1] SQLite uses fullfsync on all platforms that define it:
http://www.sqlite.org/cvstrac/fileview?f=sqlite/src/os_unix.c

[2] MySQL uses fullfsync only on the darwin platform and only when
F_FULLFSYNC is defined as 51, which seems to be short-sighted in that
this symbol may change value in future versions of OS X.  To see this
code, download a mysql 5.x source snapshot and open up
mysql-<version-number>/innobase/os/os0file.c