Re: [bug-gawk] Behavior of fflush with SIGPIPE on stdout [PATCH]

[FERRIEUX Alexandre - IMT/OLN]

On 25/03/2017 15:04, Andrew J. Schorr wrote:
> On Sat, Mar 25, 2017 at 02:50:19PM +0100, FERRIEUX Alexandre - IMT/OLN wrote:
> >  >$ uname -a
> >  >CYGWIN_NT-10.0 ti47 2.5.0(0.297/5/3) 2016-04-11 09:58 x86_64 Cygwin
> >  >
> >  >$ false | head -1; echo $?
> >  >1
> >  >
> >  >$ cat /etc/services | head -1 ; echo $?
> >  ># Copyright (c) 1993-2004 Microsoft Corp.
> >  >0
> >  >
> >  >I don't know why that's the case.
> >
> >  You forgot "set -o pipefail".
>
> I did not forget. If I had forgotten, the example with "false" above
> would not have worked:
>
> $ false | head -1; echo $?
> 0

Ah yes, sorry :)


> $ set -o pipefail
>
> $ false | head -1; echo $?
> 1
>
> $ cat /etc/services | head -1; echo $?
> # Copyright (c) 1993-2004 Microsoft Corp.
> 0

Yes, that may happen if the output from "cat" fits within a pipe buffer: a single write() completes without blocking ; 
then "head" exits after reading part or all of it (depending on stdin buffer settings), but it's way too late for any 
feedback like sending a SIGPIPE, since nobody's writing at the time.

Use the command "yes" instead of "cat /etc/services". It never stops.

> >  Here is a variant that doesn't need it:
> >
> >    (cat /etc/services;echo $?>&2) | head -1
> >    # /etc/services:
> >    141
>
> Strangely, that does not work on my Cygwin system:
>
> $ (cat /etc/services;echo $?>&2) | head -1
> 0
> # Copyright (c) 1993-2004 Microsoft Corp.
>
> I guess I may be experiencing a Cygwin bug of some sort. Did you actually
> test on Cygwin?

Nope, I don't have any Windows. But I tend to hope that the development of such a venerable pillar of the unix universe 
as Awk, will not take Windows (even Cygwin) as a reference. Windows may not get what a file descriptor or exit status 
means, we still need gawk to get it right on Unix.

You may wonder why the detailed exit status matters.
First, of course, a script may inspect $? and act accordingly.
But a much more prevalent use is the following:
        - I'm debugging a big contraption made of dozens of pipe-connected 
processes (not necessarily written by me)
        - half of them are Awk processes
        - At some point, things go awry, some of the processes exiting 
prematurely, and the others following by starvation.

In this situation, I need to understand why each one died: EOF, SIGPIPE, or 
intentional exit ?

To this effect, "strace -f" gives all the answers in one single command. This is true of all unix filters that use the 
default SIGPIPE behavior (without even setting a handler): cut, sed, grep, old awk. If this stops being true for shiny 
gawk for no better reason than "Windows people don't need it", that's tough.