David it seem you walked my road already... :-)

I'm actually on a research project, so I do not care too much about the
issues on existing programs. I'm going to change/break them anyway.
Also, as far as I can foresee, it should be viable to fix such programs in
a partially automated way (eg via sed and a new "ocreate" library function
that mimic the current behaviour).

But reading that thread I can't actually see why the OEXCL path has been
taken instead of eliminating the race mapping the syscall to the 9p message.
I mean except backward compatibility.

Maybe it was found a performance issue in some more common use case?
Or a worse race prevented by the current semantic?


For example I've found pretty cryptic this message from David:
http://marc.info/?l=9fans&m=111558704718797&w=2

I'm surprised I haven't yet seen "What about union directories?"
>
> If create(2) is changed then it could succeed even though a
> file with that name exists in the union.  Then the above:
>
> if ((fd = create(file, mode, perm)) < 0) {
> 	error...
> }
>
> Would need to become:
>
> if ((fd = open(file, mode|OTRUNC)) < 0 ||
>     (fd = create(file, mode, perm)) < 0 ||
>     (fd = open(file, mode|OTRUNC)) < 0 ||
> 	error...
> }
>
> This is precisely the current create(2) call and the nasty
> race is clear.
>
>
Why the initial open() would be needed if create(2) always send a Tcreate?


Giacomo


2016-11-30 14:53 GMT+01:00 Charles Forsyth <charles.forsyth@gmail.com>:

>
> On 30 November 2016 at 13:32, <cinap_lenrek@felloff.net> wrote:
>
>> interesting, the thread starts here:
>>
>> http://marc.info/?l=9fans&m=111558704718788&w=2
>>
>
>
> I suspect the discussion predated 9P2000 and the introduction of the OEXCL
> option.
>