catching output from asynchronous processes

[Ralf Wildenhues]

Hi Ole, bug-parallel readers,

you asked whether I could reproduce output loss with background shell
processes when O_APPEND is not used.  With the following snippet, I
can reliably produce a failure on an otherwise idle two-way GNU/Linux
system:

$ sh -c 'n=1
         while :; do
           { echo 1 & echo 2; wait; } > file
           test `wc -l < file` -eq 2 || { echo $? $n; break; }
           n=$((n+1))
         done'

with sh being either bash 4.1.5 or dash 0.5.5.1.  On this system, $n
is usually a few hundred, with the lowest in a number of tries being
81 and the highest in the 500s.  The failure of course does not occur
with

$ sh -c 'n=1
         while :; do
           : > file
           { echo 1 & echo 2; wait; } >> file
           test `wc -l < file` -eq 2 || { echo $? $n; break; }
           n=$((n+1))
         done'

and the counting is of course only for statistics and has nothing to do
with exposing the issue.  Similarly, the failure of course does not
occur if you pipe the output into another process, or into a named pipe.

This is probably very hard to reproduce on a single processor system.

Cheers,
Ralf