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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 28 May 2025 19:31:42 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> On 2025-05-28 03:45, Stefano Brivio via user wrote:
> > [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...?).  
> 
> I'll take a look tomorrow.

Patch with review thread now at:

  https://archives.passt.top/passt-dev/20250529170858.185281-1-jmaloy@redhat.com/

-- 
Stefano