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

[Steffen Daode Nurpmeso]

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

> > Even PEP 3151 won't help.
> 
> I don't understand. If the syscall supposed to flush the disk's buffer
> cache fails - be it fcntl() or sync_file_range() - I think the error
> should be propagated, not silently ignored and replaced with another
> syscall which doesn't have the same semantic. That's all.

I'm with you theoretically - of course errors should be propagated
to users so that they can act accordingly.
And this is exactly what the patches do, *unless* it is an error
which is not produced by the native fsync(2) call:

-- >8 --
?0%0[steffen at sherwood tmp]$ cat t.c 
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
int main(void) {
	int r  = fcntl(2, F_FULLFSYNC);
	fprintf(stderr, "1. %d: %d, %s\n", r, errno, strerror(errno));
    errno = 0;
	r = fsync(2);
	fprintf(stderr, "2. %d: %d, %s\n", r, errno, strerror(errno));
    return 0;
}
?0%0[steffen at sherwood tmp]$ gcc -o t t.c && ./t
1. -1: 25, Inappropriate ioctl for device
2. 0: 0, Unknown error: 0
?0%0[steffen at sherwood tmp]$ grep -F 25 /usr/include/sys/errno.h 
#define ENOTTY      25      /* Inappropriate ioctl for device */
-- >8 --

So in fact the patches do what is necessary to make the changed
version act just as the plain systemcall.

> > - I favour haypos fullsync() approach
> Are you willing to update your patch accordingly?

Both patches still apply onto the tip of friday noon:
http://bugs.python.org/file22016/11877.9.diff,
http://bugs.python.org/file22046/11877-standalone.1.diff.

Again: i personally would favour os.fsync(fd, fullsync=True), because
that is the only way to put reliability onto unaware facilities
unaware (e.g. my S-Postman replaces os.fsync() with a special function
so that reliability is injected in- and onto Python's mailbox.py,
which calls plain os.fsync()), but i've been convinced that this is
impossible to do.  It seems to be impossible to change os.fsync()
at all, because it has a standartized function prototype.

So what do you mean?  Shall i rewrite 11877-standalone.1.diff to
always offer fullsync() whenever there is fsync()?  This sounds to
be a useful change, because testing hasattr() of the one would
imply availability of the other.

> >   + Aaarrg!  I'm a liar!!  I lie about - data integrity!!!
> Well, actually, some hard disks lie about this too :-)

Yeah.  But hey:
"I feel save in New York City.  I feel save in New York City."
Nice weekend - and may the juice be with you!

--Steffen
Ciao, sdaoden(*)(gmail.com)
ASCII ribbon campaign           ( ) More nuclear fission plants
  against HTML e-mail            X    can serve more coloured
    and proprietary attachments / \     and sounding animations

----------

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11877>
_______________________________________