Reopening STDOUT to in-memory scalar interferes with "-|" piping

[jimav]

If STDOUT is redirected to an in-memory scalar, then in a subsequent subprocess created with open(my $from_kid, "-|") STDOUT does not refer to the pipe, but to something with fileno==-1. Anything written to STDOUT in the child is lost.

The OS file descriptor 1 does seem to refer to the pipe: Output from an exec'd program gets through correctly.

#!/usr/bin/env perl
use strict; use warnings;

open my $orig_STDOUT, ">&", *STDOUT or die "dup STDOUT: $!";
my $buffer = "";

# Redirect STDOUT to a string
# --> COMMENT OUT THIS LINE AND THE PROBLEM DISAPPEARS
close STDOUT; open STDOUT, ">", \$buffer or die "redir to string failed; $!";

# Creatw piped subprocess
my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
if ($child_pid == 0) {
  # CHILD
  print STDOUT "Child: print to STDOUT *** THIS GETS LOST ***\n";
  STDOUT->flush();
  exec "date"; # output from external command works
  die "exec failed";
}
# PARENT
while (<$from_kid>) { print "GOT from kid: $_"; }

# Dump what was captured
close STDOUT;
open STDOUT, ">&", $orig_STDOUT or die "restore orig failed: $!";
print "Captured text:\n", $buffer, "<END>\n";
print "Done.\n";

perl 5.36.0 perl-V_output.txt

[jkeenan]

FWIW, here's what I got when running the program above (on perl-5.38.0):

$ perl gh-21881-pipe.pl 
Captured text:
GOT from kid: Sat Jan 27 05:22:16 PM EST 2024
<END>
Done.

And here's what I got when running the same program but having commented out line 9, as directed.

$ perl xgh-21881-pipe.pl 
GOT from kid: Child: print to STDOUT *** THIS GETS LOST ***
GOT from kid: Sat Jan 27 05:22:53 PM EST 2024
Captured text:
<END>
Done.

[tonycoz]

The problem is with a pipe open perl just updates the fds for STDOUT and doesn't touch the layers, including the scalar layer in this case.

This is fine when starting exec()ing a new program, since the fds are what the new program looks at, but perl still sees the scalar layer and continues to write via that rather than the unix layer which does the actual write() system call.