File Coverage

inc/Sub/Uplevel.pm

Criterion	Covered	Total	%
statement	29	66	43.9
branch	6	30	20.0
condition	0	5	0.0
subroutine	8	12	66.6
pod	1	1	100.0
total	44	114	38.6

#line 1
package Sub::Uplevel;
use 5.006;
use strict;
# ABSTRACT: apparently run a function in a higher stack frame

our $VERSION = '0.25';

# Frame check global constant
our $CHECK_FRAMES;
BEGIN {
  $CHECK_FRAMES = !! $CHECK_FRAMES;
}
use constant CHECK_FRAMES => $CHECK_FRAMES;

# We must override *CORE::GLOBAL::caller if it hasn't already been
# overridden or else Perl won't see our local override later.

if ( not defined *CORE::GLOBAL::caller{CODE} ) {
  *CORE::GLOBAL::caller = \&_normal_caller;
}

# modules to force reload if ":aggressive" is specified
my @reload_list = qw/Exporter Exporter::Heavy/;

sub import {
  no strict 'refs'; ## no critic
  my ($class, @args) = @_;
  for my $tag ( @args, 'uplevel' ) {
    if ( $tag eq 'uplevel' ) {
      my $caller = caller(0);
      *{"$caller\::uplevel"} = \&uplevel;
    }
    elsif( $tag eq ':aggressive' ) {
      _force_reload( @reload_list );
    }
    else {
      die qq{"$tag" is not exported by the $class module\n}
    }
  }
  return;
}

sub _force_reload {
  no warnings 'redefine';
  local $^W = 0;
  for my $m ( @_ ) {
    $m =~ s{::}{/}g;
    $m .= ".pm";
    require $m if delete $INC{$m};
  }
}

#pod =head1 SYNOPSIS
#pod
#pod   use Sub::Uplevel;
#pod
#pod   sub foo {
#pod       print join " - ", caller;
#pod   }
#pod
#pod   sub bar {
#pod       uplevel 1, \&foo;
#pod   }
#pod
#pod   #line 11
#pod   bar();    # main - foo.plx - 11
#pod
#pod =head1 DESCRIPTION
#pod
#pod Like Tcl's uplevel() function, but not quite so dangerous.  The idea
#pod is just to fool caller().  All the really naughty bits of Tcl's
#pod uplevel() are avoided.
#pod
#pod B
#pod
#pod =over 4
#pod
#pod =item B
#pod
#pod   uplevel $num_frames, \&func, @args;
#pod
#pod Makes the given function think it's being executed $num_frames higher
#pod than the current stack level.  So when they use caller($frames) it
#pod will actually give caller($frames + $num_frames) for them.
#pod
#pod C is effectively C but
#pod you don't immediately exit the current subroutine.  So while you can't
#pod do this:
#pod
#pod     sub wrapper {
#pod         print "Before\n";
#pod         goto &some_func;
#pod         print "After\n";
#pod     }
#pod
#pod you can do this:
#pod
#pod     sub wrapper {
#pod         print "Before\n";
#pod         my @out = uplevel 1, &some_func;
#pod         print "After\n";
#pod         return @out;
#pod     }
#pod
#pod C has the ability to issue a warning if C<$num_frames> is more than
#pod the current call stack depth, although this warning is disabled and compiled
#pod out by default as the check is relatively expensive.
#pod
#pod To enable the check for debugging or testing, you should set the global
#pod C<$Sub::Uplevel::CHECK_FRAMES> to true before loading Sub::Uplevel for the
#pod first time as follows:
#pod
#pod     #!/usr/bin/perl
#pod
#pod     BEGIN {
#pod         $Sub::Uplevel::CHECK_FRAMES = 1;
#pod     }
#pod     use Sub::Uplevel;
#pod
#pod Setting or changing the global after the module has been loaded will have
#pod no effect.
#pod
#pod =cut

# @Up_Frames -- uplevel stack
# $Caller_Proxy -- whatever caller() override was in effect before uplevel
our (@Up_Frames, $Caller_Proxy);

sub _apparent_stack_height {
    my $height = 1; # start above this function
    while ( 1 ) {
        last if ! defined scalar $Caller_Proxy->($height);
        $height++;
    }
    return $height - 1; # subtract 1 for this function
}

sub uplevel {
    # Backwards compatible version of "no warnings 'redefine'"
    my $old_W = $^W;
    $^W = 0;

    # Update the caller proxy if the uplevel override isn't in effect
    local $Caller_Proxy = *CORE::GLOBAL::caller{CODE}
        if *CORE::GLOBAL::caller{CODE} != \&_uplevel_caller;
    local *CORE::GLOBAL::caller = \&_uplevel_caller;

    # Restore old warnings state
    $^W = $old_W;

    if ( CHECK_FRAMES and $_[0] >= _apparent_stack_height() ) {
      require Carp;
      Carp::carp("uplevel $_[0] is more than the caller stack");
    }

    local @Up_Frames = (shift, @Up_Frames );

    my $function = shift;
    return $function->(@_);
}

sub _normal_caller (;$) { ## no critic Prototypes
    my ($height) = @_;
    $height++;
    my @caller = CORE::caller($height);
    if ( CORE::caller() eq 'DB' ) {
        # Oops, redo picking up @DB::args
        package DB;
        @caller = CORE::caller($height);
    }

    return if ! @caller;                  # empty
    return $caller[0] if ! wantarray;     # scalar context
    return @_ ? @caller : @caller[0..2];  # extra info or regular
}

sub _uplevel_caller (;$) { ## no critic Prototypes
    my $height = $_[0] || 0;

    # shortcut if no uplevels have been called
    # always add +1 to CORE::caller (proxy caller function)
    # to skip this function's caller
    return $Caller_Proxy->( $height + 1 ) if ! @Up_Frames;

#pod =begin _private
#pod
#pod So it has to work like this:
#pod
#pod     Call stack               Actual     uplevel 1
#pod CORE::GLOBAL::caller
#pod Carp::short_error_loc           0
#pod Carp::shortmess_heavy           1           0
#pod Carp::croak                     2           1
#pod try_croak                       3           2
#pod uplevel                         4
#pod function_that_called_uplevel    5
#pod caller_we_want_to_see           6           3
#pod its_caller                      7           4
#pod
#pod So when caller(X) winds up below uplevel(), it only has to use
#pod CORE::caller(X+1) (to skip CORE::GLOBAL::caller).  But when caller(X)
#pod winds up no or above uplevel(), it's CORE::caller(X+1+uplevel+1).
#pod
#pod Which means I'm probably going to have to do something nasty like walk
#pod up the call stack on each caller() to see if I'm going to wind up
#pod before or after Sub::Uplevel::uplevel().
#pod
#pod =end _private
#pod
#pod =begin _dagolden
#pod
#pod I found the description above a bit confusing.  Instead, this is the logic
#pod that I found clearer when CORE::GLOBAL::caller is invoked and we have to
#pod walk up the call stack:
#pod
#pod * if searching up to the requested height in the real call stack doesn't find
#pod a call to uplevel, then we can return the result at that height in the
#pod call stack
#pod
#pod * if we find a call to uplevel, we need to keep searching upwards beyond the
#pod requested height at least by the amount of upleveling requested for that
#pod call to uplevel (from the Up_Frames stack set during the uplevel call)
#pod
#pod * additionally, we need to hide the uplevel subroutine call, too, so we search
#pod upwards one more level for each call to uplevel
#pod
#pod * when we've reached the top of the search, we want to return that frame
#pod in the call stack, i.e. the requested height plus any uplevel adjustments
#pod found during the search
#pod
#pod =end _dagolden
#pod
#pod =cut

    my $saw_uplevel = 0;
    my $adjust = 0;

    # walk up the call stack to fight the right package level to return;
    # look one higher than requested for each call to uplevel found
    # and adjust by the amount found in the Up_Frames stack for that call.
    # We *must* use CORE::caller here since we need the real stack not what
    # some other override says the stack looks like, just in case that other
    # override breaks things in some horrible way

    for ( my $up = 0; $up <= $height + $adjust; $up++ ) {
        my @caller = CORE::caller($up + 1);
        if( defined $caller[0] && $caller[0] eq __PACKAGE__ ) {
            # add one for each uplevel call seen
            # and look into the uplevel stack for the offset
            $adjust += 1 + $Up_Frames[$saw_uplevel];
            $saw_uplevel++;
        }
    }

    # For returning values, we pass through the call to the proxy caller
    # function, just at a higher stack level
    my @caller = $Caller_Proxy->($height + $adjust + 1);
    if ( CORE::caller() eq 'DB' ) {
        # Oops, redo picking up @DB::args
        package DB;
        @caller = $Sub::Uplevel::Caller_Proxy->($height + $adjust + 1);
    }

    return if ! @caller;                  # empty
    return $caller[0] if ! wantarray;     # scalar context
    return @_ ? @caller : @caller[0..2];  # extra info or regular
}

#pod =back
#pod
#pod =head1 EXAMPLE
#pod
#pod The main reason I wrote this module is so I could write wrappers
#pod around functions and they wouldn't be aware they've been wrapped.
#pod
#pod     use Sub::Uplevel;
#pod
#pod     my $original_foo = \&foo;
#pod
#pod     *foo = sub {
#pod         my @output = uplevel 1, $original_foo;
#pod         print "foo() returned:  @output";
#pod         return @output;
#pod     };
#pod
#pod If this code frightens you B
#pod
#pod
#pod =head1 BUGS and CAVEATS
#pod
#pod Well, the bad news is uplevel() is about 5 times slower than a normal
#pod function call.  XS implementation anyone?  It also slows down every invocation
#pod of caller(), regardless of whether uplevel() is in effect.
#pod
#pod Sub::Uplevel overrides CORE::GLOBAL::caller temporarily for the scope of
#pod each uplevel call.  It does its best to work with any previously existing
#pod CORE::GLOBAL::caller (both when Sub::Uplevel is first loaded and within
#pod each uplevel call) such as from Contextual::Return or Hook::LexWrap.
#pod
#pod However, if you are routinely using multiple modules that override
#pod CORE::GLOBAL::caller, you are probably asking for trouble.
#pod
#pod You B load Sub::Uplevel as early as possible within your program.  As
#pod with all CORE::GLOBAL overloading, the overload will not affect modules that
#pod have already been compiled prior to the overload.  One module that often is
#pod unavoidably loaded prior to Sub::Uplevel is Exporter.  To forcibly recompile
#pod Exporter (and Exporter::Heavy) after loading Sub::Uplevel, use it with the
#pod ":aggressive" tag:
#pod
#pod     use Sub::Uplevel qw/:aggressive/;
#pod
#pod The private function C may be passed a list of
#pod additional modules to reload if ":aggressive" is not aggressive enough.
#pod Reloading modules may break things, so only use this as a last resort.
#pod
#pod As of version 0.20, Sub::Uplevel requires Perl 5.6 or greater.
#pod
#pod =head1 HISTORY
#pod
#pod Those who do not learn from HISTORY are doomed to repeat it.
#pod
#pod The lesson here is simple:  Don't sit next to a Tcl programmer at the
#pod dinner table.
#pod
#pod =head1 THANKS
#pod
#pod Thanks to Brent Welch, Damian Conway and Robin Houston.
#pod
#pod See http://www.perl.com/perl/misc/Artistic.html
#pod
#pod =head1 SEE ALSO
#pod
#pod PadWalker (for the similar idea with lexicals), Hook::LexWrap,
#pod Tcl's uplevel() at http://www.scriptics.com/man/tcl8.4/TclCmd/uplevel.htm
#pod
#pod =cut

1;

__END__