perl: one parent, many children - single pipe reader in parent? - perl

Question

is it possible in perl to establish a pipe in such a way that the parent has only one READER pipe and many children write to it as they come to life/exit?
The typical cookbook code is:
#!/usr/bin/perl -w
# pipe2 - use pipe and fork so child can send to parent
use IO::Handle;
pipe(READER, WRITER);
WRITER->autoflush(1);
if ($pid = fork) {
close WRITER;
chomp($line = <READER>);
print "Parent Pid $$ just read this: `$line'\n";
// do what you need
} else {
die "cannot fork: $!" unless defined $pid;
close READER;
print WRITER "Child Pid $$ is sending this\n";
close WRITER; # this will happen anyway
exit;
}
Let's instead assume a case where I need my parent "READER" to get messages from multiple children, is it possible to do this without keeping a list of pipes, one per child? I can't close WRITER in the parent because the next child won't get a valid handle to write to. I also need the parent to continue its regular operation and not block on any client data from the pipe.
Pseudocode of what I need:
# parent code
pipe (READER, WRITER)
fork_random_number_of_children(READER,WRITER)
on_some_tick => {
my $data = read_from(READER, non_blocking)
if (data) print "Hey some child sent me: $data"
else print "No data, going back life"
do_other_things_before_next_tick()
}
child_job(R,W) { # lets assume this is called for each child fork
close (R); # no problem, its a copy
sleep (random duration)
print W, "Message from child with pid $$"
exit 0
}

Answer 1

I don't think it's necessary to close WRITER in the parent. It may be a good practice, but since you can't reuse the same pipe for new child processes after you close it, that's a good excuse not to do it. If you keep WRITER open until you are done launching all your child processes, you can reuse the pipe with multiple child processes. Here's a proof of concept:
use IO::Handle;
use POSIX ':sys_wait_h';
pipe(READER,WRITER);
WRITER->autoflush(1);
sub child_process {
my $stage = shift;
close READER; # also a best but optional practice
srand($$);
do {
sleep 1 + 5*rand();
print WRITER "Child Pid $$ ($stage) is sending this\n";
} while (rand > 0.5);
exit;
}
# initial set of children
for (my $i=0; $i<5; $i++) {
if (fork() == 0) {
child_process("LAUNCH");
}
}
# parent
my ($rin,$rout) = ('');
vec($rin,fileno(READER),1) = 1;
while (1) {
# non-blocking read on pipe
my $read_avail = select($rout=$rin, undef, undef, 0.0);
if ($read_avail < 0) {
if (!$!{EINTR}) {
warn "READ ERROR: $read_avail $!\n";
last;
}
} elsif ($read_avail > 0) {
chomp(my $line = <READER>);
print "Read in Parent $$: '$line'\n";
} else {
print STDERR "No input ... do other stuff\n";
# start some run-time child processes
if (time-$^T > 5 && time-$^T < 10) {
# launch a few more children in the middle of the program
if (fork() == 0) {
child_process("RUN");
}
}
sleep 1;
}
last if waitpid(-1,&WNOHANG) < 0; # no more children are alive
}
close WRITER; # now it is safe to do this ...