Re: Can't pipe output of pasta --version

[Stefano Brivio <sbrivio@redhat.com>]

[Cc: Paul as author of d0006fa78]

On Wed, 28 May 2025 12:25:10 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, May 27, 2025 at 05:21:19PM +0000, John Radley (jradxl2) wrote:
> > Hello,I can't pipe output like this:-
> > $ pasta --version | grep pasta<blank!>
> > but this works:-
> > $ sudo apt install expect
> > $ unbuffer pasta --version | grep pasta
> > pasta 2025_05_12.8ec1341-9-g3262c9b
> > 
> > If this is the intended action, please can you tell me why?  
> 
> Ouch.  No, that's not intended.
> 
> I initially assumed this was because we were sending the version
> information to stderr instead of stdout, but that's not the case.
> 
> This is a bug introduced by d0006fa78 ("treewide: use _exit() over
> exit()").  For --version we're displaying the information with
> fprintf(), then immediately exiting with _exit().  When writing to a
> pipe, stdout is buffered.  Normally the buffer would be flushed before
> exiting, but using _exit() bypasses that.
> 
> We could fix this specific problem by adding an fflush(stdout) before
> the _exit().  However, I worry that there might be other bugs we have
> because we're not running libc installed exit handlers here and
> elsewhere.  I was already a bit dubious about using _exit() by
> default, and now I really don't think it was a good idea.

Well, that's something we looked into (of course...), and (I?) concluded
that we never needed to flush stdio, which is the only otherwise useful
thing glibc handlers would do for us, and which kind of holds... except
for this case. And maybe for --help or any "early" exit.

So we could fflush() as well in those cases, and just in those
cases, with the added benefit that we don't need to add additional
system calls to the permitted seccomp set, because if we exit early, we
didn't install the seccomp filter yet.

> That said, I don't immediately have a better idea of how to address
> the problem d0006fa78 was aiming to fix in the first place.
> 
> Well... I'm on holiday at the moment, so I won't be fixing it.
> Laurent & Stefano, I hope the analysis above is helpful.

I'm on holiday too, I'll fix this later this week or next week, unless
Paul is particularly inspired (or John wants to look into the issue a
bit further and contribute a patch...?).

-- 
Stefano