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Author image Curtis "Ovid" Poe
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NAME

CodeGen::Protection - Safely rewrite parts of generated code

VERSION

version 0.05

SYNOPSIS

    use CodeGen::Protection qw(:all);

    # Creating a new document:

    my $perl = create_protected_code(
        type           => 'Perl',
        protected_code => $sample,
    );

    # Or rewriting:

    my $rewritten = rewrite_code(
        type           => 'Perl',
        existing_code  => $perl,
        protected_code => $rewritten_code,
    );

DESCRIPTION

If you might find the Tutorial to be useful.

Code that writes code can be a powerful tool, especially when you need to generate lots of boilerplate. However, when a developer takes the generated code, they can easily rewrite that code in a way that no longer works, or make good changes that get wiped out if the code is regenerated. DBIx::Class::Schema::Loader protects against this by marking blocks of code with start and end comments and an MD5 checksum. If you change any of the code between those comments, regenerating your schema will fail.

This module takes this idea and generalizes it. It allows you to do a safe partial rewrite of documents. At the present time, we support Perl and HTML.

In short, we wrap your "protected" (protected_code) code in start and end comments, with checksums for the code:

    #<<< CodeGen::Protection::Format::Perl 0.01. Do not touch any code between this and the end comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660
    
    # protected code goes here

    #>>> CodeGen::Protection::Format::Perl 0.01. Do not touch any code between this and the start comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660

Or:

    <!-- CodeGen::Protection::Format::HTML 0.01. Do not touch any code between this and the end comment. Checksum: c286b9b2577e085df857227eae996c40 -->
    
        <ol>
          <li>This is a list</li>
          <li>This is the second entry.</li>
        </ol>
    
    <!-- CodeGen::Protection::Format::HTML 0.01. Do not touch any code between this and the start comment. Checksum: c286b9b2577e085df857227eae996c40 -->

If calling the rewrite_code function, this module removes the code between the existing_code's start and end markers and replaces it with the protected_code. If the code between the start and end markers has been altered, it will no longer match the checksums and rewriting the code will fail.

TYPES

As of this writing, we can protect Perl and HTML:

    my $rewritten = rewrite_code(
        type           => 'Perl',
        existing_code  => $perl,
        protected_code => $protected_code,
    );

    my $rewritten = rewrite_code(
        type           => 'HTML',
        existing_code  => $HTML,
        protected_code => $protected_code,
    );

See CodeGen::Protection::Role to learn how to create your own types to protect.

FUNCTIONS

Functions are exportable on-demand, or both can be exported via :all.

    use CodeGen::Protection qw(rewrite_code);
    use CodeGen::Protection qw(:all);

create_protected_code

    my $protected_code = create_protected_code(
        type           => 'Perl',
        protected_code => $text_of_code,
    );

Takes the code in $text_of_code and adds start and end markers to it.

rewrite_code

    my $protected_code = create_protected_code(
        type           => 'Perl',
        protected_code => $protected_code,
        existing_code  => $existing_code,
    );

Replaces the code in the protected block of $existing_code with the code from $protected_code.

ARGUMENTS

Both create_protected_code and rewrite_code take the same arguments, except that rewrite_code does not allow the protected_code argument.

  • protected_code

    This is a required string containing any new Perl code to be built with this tool.

  • existing_code

    This is an optional string containing Perl code already built with this tool. If provided, this code must have the start and end markers generated by this tool so that the rewriter knows the section of code to replace with the injected code.

  • name

    Optional name for the code. This is only used in error messages if you're generating a lot of code and an error occurs and you'd like to see the name in the error.

  • tidy

    If true, will attempt to tidy the protected_code block (the rest of the code is ignored). For Perl, if the value of perltidy is the number 1 (one), then a generic pass of Perl::Tidy will be done on the code. If the value is true and anything other than one, this is assumed to be the path to a .perltidyrc file and that will be used to tidy the code (or croak() if the .perltidyrc file cannot be found).

  • overwrite

    Optional boolean, default false. In "Rewrite mode", if the checksum in the start and end markers doesn't match the code within them, someone has manually altered that code and we do not automatically overwrite it (in fact, we croak()). Setting overwrite to true will cause it to be overwritten.

MODES

There are two modes: "Creation" and "Rewrite."

Creation Mode

    my $protected_code = create_protected_code(
        protected_code => $text,
    );

This will wrap the new text in start and end tags that "protect" the document if you rewrite it:

    my $perl = <<'END';
    sub sum {
        my $total = 0;
        $total += $_ foreach @_;
        return $total;
    }
    END
    my $protected_code = create_protected_code( protected_code => $perl );

Result:

    #<<< CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the end comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660

    sub sum {
        my $total = 0;
        $total += $_ foreach @_;
        return $total;
    }

    #>>> CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the start comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660

You can then take the marked up document and insert it into another Perl document and use the rewrite mode to safely rewrite the code between the start and end markers. The rest of the document will be ignored.

Note that leading and trailing comments start with #<<< and #>>> respectively. Those are special comments which tell Perl::Tidy to ignore what ever is between them. Thus, you can safely tidy code written with this.

The start and end checksums are the same and are the checksum of the text between the comments. Leading and trailing lines which are all whitespace are removed and one leading and one trailing newline will be added.

Rewrite Mode

Given a document created with the "Creating" mode, you can then take the marked up document and insert it into another Perl document and use the rewrite mode to safely rewrite the code between the start and end markers. The rest of the document will be ignored.

    my $rewrite = rewrite_code(
        existing_code  => $existing_code,
        protected_code => $protected_code,
    );

In the above, assuming that $existing_code is a rewritable document, the $protected_code will replace the rewritable section of the $existing_code, leaving the rest unchanged.

However, if $protected_code is also a rewritable document, then the rewritable portion of the $protected_code will be extract and used to replace the rewritable portion of the $existing_code.

So for the code shown in the "Creation mode" section, you could add more code like this:

    package My::Package;

    use strict;
    use warnings;

    sub average {
        return sum(@_)/@_;
    }

    #<<< CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the end comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660

    sub sum {
        my $total = 0;
        $total += $_ foreach @_;
        return $total;
    }

    #>>> CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the start comment. Checksum: fa97a021bd70bf3b9fa3e52f203f2660
    
    1;

However, later on I might realize that the sum function will happily try to sum things which are not numbers, so I want to fix that. I'll slurp the My::Package code into the $existing_code variable and then:

    my $perl = <<'END';
    use Scalar::Util 'looks_like_number';

    sub sum {
        my $total = 0;
        foreach my $number (@_) {
            unless (looks_like_number($number)) {
                die "'$number' doesn't look like a numbeer!";
            }
            $total += $number;
        }
        return $total;
    }
    END
    my $rewrite = rewrite_code( existing_code => $existing_code, protected_code => $perl );

And that will result in:

    package My::Package;
    
    use strict;
    use warnings;
    
    sub average {
        return sum(@_)/@_;
    }
    
    #<<< CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the end comment. Checksum: d135a051f158ee19fbd68af5466fb1ae
    
    use Scalar::Util 'looks_like_number';
    
    sub sum {
        my $total = 0;
        foreach my $number (@_) {
            unless (looks_like_number($number)) {
                die "'$number' doesn't look like a numbeer!";
            }
            $total += $number;
        }
        return $total;
    }
    
    #>>> CodeGen::Protection::Format::Perl 0.03. Do not touch any code between this and the start comment. Checksum: d135a051f158ee19fbd68af5466fb1ae
    
    1;

You can see that the code between the start and end checksum comments and been rewritten, while the rest of the code remains unchanged.

ACKNOWLEDGEMENTS

We would like to thank All Around the World

Thanks to Matt Trout (mst) for the inspiration from the schema loader. for sponsoring this work.

AUTHOR

Curtis "Ovid" Poe <ovid@allaroundtheworld.fr>

COPYRIGHT AND LICENSE

This software is copyright (c) 2021 by Curtis "Ovid" Poe.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.