[Toybox] default SIGPIPE handler that calls _exit(0)?

[Rich Felker]

On Mon, Apr 27, 2015 at 09:17:31AM -0700, enh wrote:
> On Mon, Apr 27, 2015 at 9:11 AM, Rich Felker <dalias at libc.org> wrote:
> > On Mon, Apr 27, 2015 at 08:36:53AM -0700, enh wrote:
> >> On Sun, Apr 26, 2015 at 7:30 PM, Rich Felker <dalias at libc.org> wrote:
> >> > On Sat, Apr 25, 2015 at 12:14:44PM -0700, enh wrote:
> >> >> what's the plan wrt SIGPIPE? the desktop is pretty inconsistent. many
> >> >> (but not all) commands install a signal handler that does _exit(0).
> >> >> others (coreutils 8.21's ls, say) do nothing. normally "what you do
> >> >> about SIGPIPE" isn't a problem but on Android that leads to a crash
> >> >> report and people filing "ls crashed" bugs against me. (our default
> >> >> shell PS setup is also noisy about crashes.)
> >> >
> >> > Why not just *block* SIGPIPE (with sigprocmask) so that the write
> >> > returns an error (EPIPE) and the program applies the same logic it
> >> > would for any other write error?
> >>
> >> toybox does a little better than toolbox there thanks to xwrite, but
> >> i've yet never met anyone who checks the return value of printf...
> >>
> >> plus there's the question of whether giving up because you're writing
> >> to a broken pipe is an error exit or not. you could add a special case
> >> to xwrite (and add xprintf and xputs and...), but since the whole idea
> >> is that code shouldn't have to care, it's easier just to install a
> >> signal handler that does _exit(0).
> >
> > _exit(0) is wrong and broken. It reports success when the command did
> > not succeed.
> 
> coreutils seems divided on this issue.

Could you provide some examples? I think POSIX is pretty clear that
you can't return success if the output was not successfully written.
As I already mentioned doing so results in data loss. I really doubt
coreutils is doing this, but well, GNU software is not always a good
example of doing the Right Thing...

> > Failure of the caller to recognize that it didn't succeed
> > can result in data loss. If anything the right behavior for the signal
> > handler would be _exit(1) not _exit(0). But I think xwrite/xprintf
> > solves the issue more elegantly. Note that even if you don't check
> > every printf, you still get the error status from ferror and/or fflush
> > at close time (if you check them) but that might result in a lot of
> > wasted cpu time if you continue processing/writing after a pipe broke.
> 
> it would also result in things like top(1) not exiting.

If top is using stdio, it's easy to add a single ferror(stdout) check
to the main loop. If it's using write(), it _must_ be checking for
errors anyway since write can always return with a short/partial
write. But top is unlikely to be hooked up to a pipe or socket anyway;
it normally needs a terminal.

Rich

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