How to replace STDOUT with an IO::Tee object?

I have a very large program that writes a lot of things to STDOUT, I'd like to be able to feed all of that output to both STDOUT and to an arbitrary FILE. I've decided to use IO::Tee to create the shared writeable handle:

use IO::Tee;

pipe(my $reader, my $writer);
my $new_stdout = IO::Tee->new(\*STDOUT, $writer);

*{STDOUT} = $new_stdout;

print "foo";

print(do { local $\; <$reader> });

However, this causes a deep-recursion and crashes the program. So, instead I can not reference *STDOUT, and it creates it:

use IO::Tee;

pipe(my $reader, my $writer);
my $new_stdout = IO::Tee->new(*STDOUT, $writer);

*{STDOUT} = $new_stdout;

print "foo";

print(do { local $\; <$reader> });

This creates the warning: Undefined value assigned to typeglob at ... line 42, and when I use Data::Printer to describe $new_stdout, it is undef. How can I do this?

Solution

Almost all code that accepts a file handle accepts it in many forms:

A reference to a glob with an IO object (\*STDOUT)
A glob with an IO object (*STDOUT)
An IO object (*STDOUT{IO})

Here is no exception. Instead of a passing a reference to a glob you later modify, you can solve this problem by passing the IO object originally assigned to STDOUT. This makes the later modification of *STDOUT moot.

Replace

IO::Tee->new( \*STDOUT, $writer )

with

IO::Tee->new( *STDOUT{IO}, $writer )

That leaves you with a number of other problems, though.

You'll need to empty the pipe before the end of the program or risk becoming deadlocked.
You'll need to close the pipe at some point if you're going to continue reading from the pipe until EOF.
You might want to disable buffering for the new handle ($new_stdout->autoflush;).

[just pipe its output to tee] would be ideal... But we're hoping to be able to package this as a library, such that we can e.g use Stdout::Replace

package Stdout::Replace;

use IPC::Open3 qw( open3 );

my $pid = open3( local *CHILD_STDIN, ">&STDOUT", ">&STDERR", "tee", "file.out" );
open( STDOUT, ">&", *CHILD_STDIN );  # dup2
close( CHILD_STDIN );
STDOUT->autoflush();

END {
   close( STDOUT );
   waitpid( $pid, 0 );
}

1;

Demo:

$ cat file.out
cat: file.out: No such file or directory

$ perl -I lib -Mv5.14 -e'use Stdout::Replace; say "foo";'
foo

$ cat file.out
foo

You can dup2 CHILD_STDIN onto STDERR too, if you so desire.

You can save and restore the original STDOUT as follows:

open( my $orig_stdout, ">&", *STDOUT );
...
open( STDOUT, ">&", $orig_stdout );