sh: Forget about terminated background processes sooner.

Unless $! has been referenced for a particular job or $! still contains that
job's pid, forget about it after it has terminated. If $! has been
referenced, remember the job until the wait builtin has reported its
completion (either with the pid as parameter or without parameters).

In interactive mode, jobs are forgotten after termination has been reported,
which happens before primary prompts and through the jobs builtin. Even
then, though, remember a job if $! has been referenced.

This is similar to what is suggested by POSIX and should fix most memory
leaks (which also tend to cause sh to use more CPU time) with long running
scripts that start background jobs.

Caveats:
* Repeatedly referencing $! without ever doing 'wait', like
    while :; do foo & echo started foo: $!; sleep 60; done
  will still use a lot of memory and CPU time in the long run.
* The jobs and jobid builtins do not cause a job to be remembered for longer
  like expanding $! does.

PR:		bin/55346

1 files changed, 5 insertions, 1 deletions

diff --git a/bin/sh/sh.1 b/bin/sh/sh.1
index aed995f..1498564 100644
--- a/bin/sh/sh.1
+++ b/bin/sh/sh.1
@@ -32,7 +32,7 @@
 .\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
 .\" $FreeBSD$
 .\"
-.Dd May 24, 2010
+.Dd June 29, 2010
 .Dt SH 1
 .Os
 .Sh NAME
@@ -1106,6 +1106,10 @@ command executed from the current shell.
 For a
 pipeline, the process ID is that of the last command in the
 pipeline.
+If this parameter is referenced, the shell will remember
+the process ID and its exit status until the
+.Ic wait
+built-in command reports completion of the process.
 .It Li $0
 (zero) Expands to the name of the shell or shell script.
 .El