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

[Stefano Brivio <sbrivio@redhat.com>]

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.

> > $ cat append.c
> > #include <stdio.h>
> > #include <unistd.h>
> > #include <string.h>
> > #include <fcntl.h>
> > 
> > int main(int argc, char **argv)
> > {
> > 	int flags = O_CREAT | O_TRUNC | O_WRONLY | ((argc == 3) ? O_APPEND : 0);
> > 	int fd = open(argv[1], flags, S_IRUSR | S_IWUSR);
> > 	char buf[BUFSIZ];
> > 
> > 	memset(buf, 'a', BUFSIZ);
> > 	write(fd, buf, 10);
> > 	lseek(fd, 1, SEEK_SET);
> > 	memset(buf, 'b', BUFSIZ);
> > 	write(fd, buf, 10);
> > 	write(fd, (char *){ "\n" }, 1);
> > 
> > 	return 0;
> > }
> > $ gcc -o append{,.c}
> > $ ./append test append
> > $ cat test
> > aaaaaaaaaabbbbbbbbbb
> > $ ./append test
> > $ cat test
> > abbbbbbbbbb
> >   
> > > That's usually not
> > > _necessary_ for us as such, but it's perhaps valuable since it reduces
> > > the likelihood of data loss if somehow you do get two instances
> > > logging to the same file.  
> > 
> > The result will be completely unreadable anyway, so I don't think it
> > matters for us.  
> 
> Not necessarily.  It certainly can get garbled, but individual writes
> of reasonable size - such as a single log line will generally complete
> atomically.  With a text logging format, that's not ideal but often
> pretty decipherable.  Particularly if each writer includes a prefix
> identifying itself.
> 
> > > 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).

Well, anyway, if you think this might add a regression with multiple
writers, I can add an extra flag to output_file_open() and keep
O_APPEND for the log file. But I really struggle to see the actual use
case.

-- 
Stefano