PATCH: 3.1.4: Insidious exit status bug

[Peter Stephenson <pws@ibmth.df.unipi.it>]

Bart wrote:
> zagzig% echo yyy | fgrep -q `echo xxx` && echo ok
> ok

First:  getoutput():  OK, I'll stop worrying about a $(...) setting
lastval.  It's not necessary in the middle of an ordinary command, but
when the rest of the code is working it's harmless.

Second:  Phew.  Here's what was happening.

1) The `echo yyy' was added to the process list for the current job;
that job finished straight away since it was short.  (It had to fork
even for the builtin because of the pipeline.)

2) The shell handled the arguments for fgrep, called the `echo xxx',
and waited for it.  As `echo yyy' was already finished, it was
harvested, too.  As it was (at that point) the last job in the
pipeline, the job had the STAT_DONE flag set.

3) The fgrep was added to the job table, making a second process in
the pipeline, but the STAT_DONE flag was not unset.

4) The shell now waited for all jobs to finish.  However, in waitjobs(),
the current status is updated (via printjob()) to check whether
everything has already finished.  As the STAT_DONE flag was set on the
job, it was wiped.

5) The fgrep process was harvested anyway, but it didn't have a job
table entry corresponding to it any more, so the shell didn't know it
was supposed to use the return status for lastval/$?.

Benissimo.  Here's the simplest fix: turn off the STAT_DONE flag
explicitly when adding a new process.  As noted in the comment, it's
important that the shell doesn't try to check job statuses between the
wait() which harvested the first process, and the time the second
process is added, else the bug will reappear.  At the moment things
look OK.  (Maybe specific processes like $(...) should have a
waitpid() for their own process, rather than wreaking havoc on the job
table?)

For a more sophisticated fix:  add a flags field to struct process,
record there by an extra argument to addproc() whether the process is
the last in the pipeline, don't set STAT_DONE if that process isn't
yet in the procs list.  I'll do that if there's a preference.

(Quite likely applies to 3.0.5 too, could certainly be done by hand
without much effort.)

*** Src/jobs.c.cout	Thu Jul  9 12:04:42 1998
--- Src/jobs.c	Fri Sep 25 16:40:34 1998
***************
*** 658,663 ****
--- 658,670 ----
  	/* first process for this job */
  	jobtab[thisjob].procs = pn;
      }
+     /* If the first process in the job finished before any others were *
+      * added, maybe STAT_DONE got set incorrectly.  This can happen if *
+      * a $(...) was waited for and the last existing job in the        *
+      * pipeline was already finished.  We need to be very careful that *
+      * there was no call to printjob() between then and now, else      *
+      * the job will already have been deleted from the table.          */
+     jobtab[thisjob].stat &= ~STAT_DONE;
  }
  
  /* Check if we have files to delete.  We need to check this to see *

-- 
Peter Stephenson <pws@ibmth.df.unipi.it>       Tel: +39 050 844536
WWW:  http://www.ifh.de/~pws/
Gruppo Teorico, Dipartimento di Fisica
Piazza Torricelli 2, 56100 Pisa, Italy