Perl ogd - ordered global destruction
Switch branches/tags
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Permalink
Failed to load latest commit information.
lib
t
CHANGELOG
MANIFEST
META.json
META.yml
Makefile.PL
README.md
TODO
VERSION

README.md

NAME

ogd - ordered global destruction

SYNOPSIS

perl -mogd yourscript.pl # recommended

export PERL5OPT=-mogd
perl yourscript.pl

use ogd;
ogd->register( @object ); # for objects from XSUBs only

VERSION

This documentation describes version 0.04.

DESCRIPTION

This module adds ordered destruction of objects stored in global variables in LIFO order during global destruction.

Ordered global destruction is only applicable to objects with DESTROY methods stored in non-lexical (i.e. global or our) variables (even if they are in file scope). Lexical our variables behave like global objects in this regard.

Note that global destruction should be avoided at all. Rather use my variables for all objects and file handles to get proper timely destruction and avoid weird sideeffects or even crashes with already destroyed objects being referenced in DESTROY blocks.

Perl destroys all lexical my and state objects before the first END block is called, at the end of the scope of each block where they are defined. Then the END block is executed, and then the remaining package objects and global IO objects are destructed, i.e. their DESTROY methods are called.

With a DEBUGGING perl you can use the -DD command-line flag to see the order of global destruction.

THE PROBLEM

If you store objects in global variables, and those objects contain references to other objects stored in global variables, then you cannot be sure of the order in which these objects are destroyed when executing of Perl is stopped (by reaching the end of the script, or by an exit()).

To get the proper behaviour it is better to use lexical my variables. But sometimes this is not possible, e.g. when you're using AutoLoader.

The random way these objects are destroyed, can sometimes be a problem. This pragma is intended to replace this random behaviour by a deterministic behaviour.

THEORY OF OPERATION

The ogd pragma install its own version of the bless() system function. This version keeps a list of weakened references to each and every object created during the execution of Perl. A cleanup run is done every 1024 objects that have been created, to reduce memory usage of this list of weakened references.

When execution of Perl stops and END code blocks are starting to get called, an internal subroutine is added as the very last END code block to be executed. This is when the B module is loaded to achieve this feat.

Once all other END code blocks have been executed, the internal subroutine loops through all still valid weakened references in LIFO (Last In, First Out) order and executes the DESTROY method on them. In case the DESTROY method would like to differentiate between a "real" object destruction, or a forced one, the parameter "1" is given to the DESTROY method. While looping through the list of objects, a list of packages in which still valid objects were available, is built.

When DESTROY has been called on all objects, the internal sub loops through all the packages it has seen and installs an empty DESTROY subroutine in those packages.

The internal sub then relinquishes control back to Perl, which will then call DESTROY on all the objects it still thinks are valid (in more or less random order). Since the DESTROY methods have all been replaced by empty stubs, this is effectively a noop.

CLASS METHODS

register

ogd->register( @object ); # only for blessed objects created in XSUBs

Not all blessed objects in Perl are necessarily created with "bless": they can also be created in XSUBs and thereby bypass the registration mechanism that ogd installs for "bless". For those cases, it is possible to register objects created in such a manner by calling the "register" class function. Any object passed to it will be registered.

REQUIRED MODULES

B (any)
Scalar::Util (with the XS version of List::Util)

ORDER OF LOADING

Since the ogd pragma installs its own version of the bless() system function and it can not work without that special version of bless (unless you wish to register your objects yourself). This means that the ogd pragma needs to be loaded before any modules that you want the special functionality of ogd to be applied to.

This can be achieved by loading the module from the command line (with the -m or -M option), or by adding loading of the ogd pragma in the PERL5OPT environment variable.

DEBUGGING

In order to facilitate debugging and testing of ogd, the OGD_DEBUG environment variable can be set to a numeric value before loading the ogd pragma for the first time. Currently, only the value 1 is supported. If set, the following messages will be sent to STDERR:

  • object registration

    As soon as one or more objects are registered, a line starting with "+", followed by the number of objects registered, followed by a newline, will be sent to STDERR. Since this usually happens when the bless() function is executed, you will usually see this as:

      +1
    

    on STDERR.

  • list cleanup

    If a list cleanup is done (by default, every 1024 object registrations), and destroyed objects have been removed, a line starting with "-", followed by the original number of elements in the list, followed by "->", the number of objects left after cleanup, and a newline. You would e.g. see this as:

      -1024->564
    

    on STDERR.

  • END block executed

    As soon as the END block of ogd itself is executed, a "*" followed by a newline is sent to STDERR:

      *
    
  • objects destroyed

    As soon as all of the valid objects registered have been called with the DESTROY method, a "!" followed by the number of objects handled, will be sent to STDERR. E.g.:

      !234
    
  • packages patched

    All of the packages of which the DESTROY method has been replaced by an empty stub, followed by the number of objects forcibly destroyed of that class between parentheses, will be sent to STDERR prefixed with "x". For instance:

      *Foo(123) Bar(234) Baz(13)
    

CLEANUP

In order to reduce the memory requirements of ogd, a regular cleanup is performed on the list of registered objects (which may contain reference to already destroyed objects). By default, this happens every 1024 object registrations, but this can be changed by setting the environment variable OGD_CLEANUP to a numeric value before loading ogd the first time. The value represents the power of 2 at which a cleanup will be performed: by default this is 10 (as 2**10 = 1024), but any other positive integer value is allowed (allowing for more or lesser aggressive cleanup checks).

TODO

Maybe an after and before class method should be added to manipulate the order in which objects will be destroyed at global destruction?

Examples should be added.

Check in which perl version deterministic order of global destruction was added and ogd is not needed anymore.

AUTHOR

Elizabeth Mattijsen, liz@dijkmat.nl.

Current maintainer: Reini Urban rurban@cpan.org

Please report bugs to the RT ticket queue.

ACKNOWLEDGEMENTS

Mark Jason Dominus for the initial impetus. Yitzchak Scott-Thoennes for the suggestion of using the B module. Inspired by similar work on Thread::Bless.

COPYRIGHT

Copyright (c) 2004, 2012 Elizabeth Mattijsen liz@dijkmat.nl. Copyright (c) 2014 Reini Urban rurban@cpanel.net. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

Thread::Bless, "PERL_DESTRUCT_LEVEL" in perlhacktips