[Samba] stat between reads

[Volker Lendecke]

On Fri, Mar 22, 2013 at 02:28:34PM -0700, Jeremy Allison wrote:
> On Fri, Mar 22, 2013 at 02:07:29PM -0700, Jeremy Allison wrote:
> > On Fri, Mar 22, 2013 at 05:24:20PM +0100, Volker Lendecke wrote:
> > > If I see it right, we could avoid most of those calls.
> > > First, they are only relevant to see whether we can do
> > > sendfile. That choice is racy, we first look and have to
> > > deal with the mess if we have a short read afterwards
> > > anyway.
> > > 
> > > Jeremy, what do you think? Only do that stat call in the
> > > sendfile if-branch, and there only if the read call in
> > > question would go beyond what we currently have in
> > > st.st_ex_size?
> > 
> > Yes.... we could certainly make that change. It's not
> > relevent in the non-sendfile read path but we'd have to be
> > careful about not doing it by checking the existing
> > fsp->fsp_name->st.st_ex_size, as I don't think we
> > update that on an ftruncate call.
> > 
> > My worry would be (to one single smbd):
> > 
> > open 1mb file
> > ftruncate to 10k
> > readX of 50k at offset 0.
> > 
> > Unless the ftruncate call updates fsp->fsp_name->st.st_ex_size
> > then we'll return 10k of real data + 40k of zeros if
> > sendfile is turned on, rather than a correct short read of
> > 10k.
> > 
> > Let me look at the code some and revisit this.
> 
> Yep, I was right. This would be a problem (ftruncate
> and other file-size changing calls don't automatically
> update the st_ex_size on an fsp).
> 
> > Moving the fstat and ISREG check to the use_sendfile()
> > path is an obviously correct no brainer though.
> 
> And here is that patch. Should apply cleanly to
> 4.0.x and 3.6.x (with a few offsets). I'm testing
> here and will propose as an official optimization
> if 'make test' passes locally.

From a quick glance this looks exactly like what I had in
mind. Thanks!

Volker

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