Re: [PATCH v3 5/9] log: Don't use O_APPEND at all

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 30 Oct 2024 13:33:43 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Oct 29, 2024 at 11:23:29AM +0100, Stefano Brivio wrote:
> > On Tue, 29 Oct 2024 20:32:40 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Tue, Oct 29, 2024 at 09:48:50AM +0100, Stefano Brivio wrote:  
> > > > On Tue, 29 Oct 2024 15:20:56 +1100
> > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > >     
> > > > > On Mon, Oct 28, 2024 at 11:00:40AM +0100, Stefano Brivio wrote:    
> > > > > > We open the log file with O_APPEND, but switch it off before seeking,
> > > > > > and turn it back on afterwards.
> > > > > > 
> > > > > > We never seek when O_APPEND is on, so we don't actually need it, as
> > > > > > its only function is to override the offset for writes so that they
> > > > > > are always performed at the end regardless of the current offset
> > > > > > (which is at the end anyway, for us).      
> > > > > 
> > > > > Sorry, this sounded fishy to me on the call, but I figured I was just
> > > > > missing something.  But looking at this the reasoning doesn't make
> > > > > sense to me.
> > > > > 
> > > > > We don't seek with O_APPEND, but we do write(), which is exactly where
> > > > > it matters. AIUI the point of O_APPEND is that if you have multiple
> > > > > processes writing to the same file, they won't clobber each others
> > > > > writes because of a stale file pointer.    
> > > > 
> > > > That's not the reason why I originally added it though: it was there
> > > > because I thought I would lseek() to do the rotation and possibly end
> > > > up with the cursor somewhere before the end. Then restart writing, and
> > > > the write would happen in the middle of the file:    
> > > 
> > > I don't entirely follow.  I see why you disable O_APPEND across the
> > > rotation, but I'm not clear on why it's opened with O_APPEND in the
> > > first place, if it's not for the typical logging reason.  
> > 
> > I initially opened it with O_APPEND because I _thought_ I would set the
> > offset to a possibly inconsistent value around the rotation.
> > 
> > Then I dropped O_APPEND around the rotation, forgetting about the
> > initial reason why I added it at all. So it makes no sense to have
> > O_APPEND at all.  
> 
> Ok, that makes sense.
> 
> Except that maybe there is a reason to use O_APPEND (the multiple
> writer thing), even if it's not the one you thought of initially.
> 
> [snip]
> > > > > Of course the rotation process *can* clobber things (which is exactly
> > > > > why I was always a bit sceptical of this "in place" rotation, not that
> > > > > we really have other options).    
> > > > 
> > > > Why would it clobber things? logfile_rotate_fallocate() and
> > > > logfile_rotate_move() take care of cutting cleanly at a line boundary,
> > > > and tests check that.    
> > > 
> > > I mean that in the case that there are multiple writers, the rotation
> > > breaks that "no data loss, and probably readable-ish" property of
> > > O_APPEND.  
> > 
> > Ah, sure. But I think that supporting multiple writers would need more
> > work anyway (at least adding a prefix as you mentioned).  
> 
> That's fair.  I wonder if it might make sense to flock() the logfile,
> to (somewhat) enforce that only one process uses it at a time.

...but if it kind of works for multiple writers, we shouldn't prevent
that usage, right?

On the other hand, I don't think we should try to make that usage all
nice and supported because we would need a prefix, which, in case of a
single writer, just adds noise and size. And I don't think we want to
detect if there are multiple writers...

So, all in all, I would choose to spend no effort and leave like it is,
until somebody comes up with a use case in one direction or the other.

-- 
Stefano