How to detect exported subs overwrites? - perl

Question

Having a next code:
use strict;
use warnings;
use Devel::Peek;
use YAML;
my $s = {a=>'b'};
print Dump($s);
it prints YAML output:
---
a: b
now changing the order of the modules.
use strict;
use warnings;
use YAML;
use Devel::Peek;
my $s = {a=>'b'};
print Dump($s);
it prints:
SV = IV(0x7ff5d2829308) at 0x7ff5d2829318
REFCNT = 1
FLAGS = (PADMY,ROK)
RV = 0x7ff5d2803438
SV = PVHV(0x7ff5d2808d20) at 0x7ff5d2803438
REFCNT = 1
FLAGS = (SHAREKEYS)
ARRAY = 0x7ff5d243acf0 (0:7, 1:1)
hash quality = 100.0%
KEYS = 1
FILL = 1
MAX = 7
Elt "a" HASH = 0x274d838f
SV = PV(0x7ff5d2804070) at 0x7ff5d2828a00
REFCNT = 1
FLAGS = (POK,IsCOW,pPOK)
PV = 0x7ff5d240e2d0 "b"\0
CUR = 1
LEN = 16
COW_REFCNT = 1
Use of uninitialized value in print at yy line 8.
Both module exports a function Dump so, the last wins.
I have enabled warnings, but it doesn't warn me about the exported functions redefine (overwrite?). It is possible detect and show a warning for such redefines?

Answer 1

Most interesting question. The problem, I think, lies in the fact that Exporter.pm doesn't have warnings enabled. Here's a simple set of files that demonstrates the behaviour you described:
Foo.pm:
package Foo;
use base 'Exporter';
our #EXPORT = qw(Baz);
sub Baz {
print "Hello from Foo::Baz\n";
}
Bar.pm:
package Bar;
use base 'Exporter';
our #EXPORT = qw(Baz);
sub Baz {
print "Hi from Bar::Baz\n";
}
import-redefine.pl:
use strict;
use warnings;
use Foo;
use Bar;
Baz();
Sample run:
C:\Users\Lona\Desktop\pm>perl import-redefine.pl
Hi from Bar::Baz
Reverse the use statements, as follows:
use strict;
use warnings;
use Bar;
use Foo;
Baz();
And run again:
C:\Users\Lona\Desktop\pm>perl import-redefine.pl
Hello from Foo::Baz
I've come up with the following solution, that redefines Exporter.pm's default import method:
BEGIN {
require Exporter; # We'll need Exporter.pm loaded.
my $old_import = \&Exporter::import; # Save copy of original Exporter::import.
no strict 'refs'; # We'll be using some hacks that will
no warnings 'redefine'; # raise errors and warnings. Suppress those.
*Exporter::import = sub { # Our enhancement of Exporter::import.
use Carp;
my $pkg = shift;
my $callpkg = caller($Exporter::ExportLevel + 1);
my #exports = #_ > 0 # Which subs to export?
? #_ # Those provided as 'use MODULE' arguments...
: #{"$pkg\::EXPORT"} # Or thosedefined in the module's #EXPORT?
;
foreach my $sub (#exports) { # For each of the exportees...
if (exists ${"$callpkg\::"}{$sub}) { # ... check if it exists...
carp "Subroutine $callpkg\::$sub redefined by import"; # and throw a warning if needed.
}
$old_import->($pkg, #_); # Call the original Exporter::import.
}
}
}
To use this, but it somewhere in your main script file, above the use MODULE statements:
use strict;
use warnings;
BEGIN {
require Exporter; # We'll need Exporter.pm loaded.
my $old_import = \&Exporter::import; # Save copy of original Exporter::import.
no strict 'refs'; # We'll be using some hacks that will
no warnings 'redefine'; # raise errors and warnings. Suppress those.
*Exporter::import = sub { # Our enhancement of Exporter::import.
use Carp;
my $pkg = shift;
my $callpkg = caller($Exporter::ExportLevel + 1);
my #exports = #_ > 0 # Which subs to export?
? #_ # Those provided as 'use MODULE' arguments...
: #{"$pkg\::EXPORT"} # Or thosedefined in the module's #EXPORT?
;
foreach my $sub (#exports) { # For each of the exportees...
if (exists ${"$callpkg\::"}{$sub}) { # ... check if it exists...
carp "Subroutine $callpkg\::$sub redefined by import"; # and throw a warning if needed.
}
$old_import->($pkg, #_); # Call the original Exporter::import.
}
}
}
use Foo;
use Bar;
Baz();
And run it:
C:\Users\Lona\Desktop\pm>perl import-redefine.pl
Subroutine main::Baz redefined by import at import-redefine.pl line 21.
main::__ANON__("Bar") called at import-redefine.pl line 30
main::BEGIN() called at import-redefine.pl line 30
eval {...} called at import-redefine.pl line 30
Hi from Bar::Baz