Re: PATCH: job-control

[Sven Wischnowsky <wischnow@informatik.hu-berlin.de>]

Bart Schaefer wrote:

> On Jan 31, 11:00am, Sven Wischnowsky wrote:
> } Subject: Re: PATCH: job-control
> }
> } Bart Schaefer wrote:
> } 
> } > If it really is somehow the case the "it found out that the pipe-leader
> } > was suspended too late," then it seems to me that the while() condition
> } > in waitjob() is what needs fixing, or we still have a race condition:
> } > the ^Z could suspend the pipe-leader between the child_block() and the
> } > while() test within waitjob().  All that this change has done is shrink
> } > the window.
> } 
> } No, the important bit is the child_unblock() which makes the signal
> } handler be run for all pending signals (we are blocking child signals
> } during most of the execution code), so that the job and process
> } infos are updated.
> 
> Yes, that's exactly my point.  waitjob() should enter the body of the
> while() loop -- thus calling child_suspend() and allowing the job and
> process info to be updated -- when there are any jobs that the shell
> "believes" are still in a runnable state.  It should never be the case
> that the job info has to be updated by a signal handler in order for
> the shell to discover that there may be runnable jobs; in that case it
> can mean only that (a) the setup of the info for those jobs is wrong
> to begin with, or (b) there's a condition in which the loop should be 
> entered but that is not tested.
> 
> The other possibility is that child_suspend() isn't sufficient to get
> the job info updated, but that would imply a much more serious problem.
> 
> } Without the patch this happened only when a
> } execpline() finished (shortly before that). In the test case there
> } were two of them active and we need to know that the leader was
> } suspended in the inner one but since child-signals were only delivered 
> } after the call to waitjobs(), we could see that only in the outer
> } execpline().
> 
> When you say "we need to know that the leader was suspended in the
> inner one," what does that mean code-wise?  What is it that we "see
> only in the outer execpline()"?

Ok, I've done some testing with the child_unblock() commented out
again and now I remember...

As a reminder, the problem was with this invocation:

  foo() {
    local a
    while read a; do :; done
    less "$@"
  }
  cat builtin.c | f glob.c

and hitting ^Z while the cat is still running.

In this case, when we hit the call to waitjobs() for the first time,
the job tested below it (the one for the loop) has no processes and
hence we don't even enter waitjob(). But some lines below the call to
waitjobs() we need the updated job-entry for the list_pipe_job.

Hm, efore I tried again, I was about to suggest to do the signal
magic only if there are any processes because it's rather expensive,
but obviously that would be wrong. But removing those two child_* and
adding:

  if (list_pipe_job && jobtab[list_pipe_job].procs &&
      !(jobtab[list_pipe_job].stat & STAT_STOPPED))
      child_suspend(0);

before the if (!list_pipe_child && ...) fixes the problem, too and is
almost certainly better. Can anyone see a problem with this?

Bye
 Sven


--
Sven Wischnowsky                         wischnow@informatik.hu-berlin.de