Perl Hacks

Hack #69. Simulate Hostile Environments in Your Tests

Test devastating failures with aplomb.

When you publish a CPAN module that depends on other modules, you list the prerequisite modules in your Makefile.PL or Build.PL script.

Using Build.PL:

my $builder = Module::Build->new(
  # ... other Build.PL options ...
  requires =>
  {
      'Test::More'       => 0,
      'CGI'              => 2.0,
  }
);

Using Makefile.PL:

WriteMakefile(
    # ... other Makefile.PL options ...
    'PREREQ_PM' =>
    {
        'Test::More'     => 0,
        'CGI'            => 2.0,
    }
);

However, there are a few ways that this standard prerequisite checking can be insufficient. First, you may have optional prerequisites. For instance, your module will use Foo::Bar if it happens to be installed, but should fail gracefully when Foo::Bar is absent.

Second, if the behavior of a module changed between two versions, you may still want to support both versions. For example, CGI changed how it handles PATH_INFO in version 3.11. Your CGI::Super_Path_Info module probably wants to be compatible with both CGI version 3.11 and also with earlier (and later) versions.

Finally, occasionally a user will install your module by hand to bypass the prerequisite check, hoping to use an older version of Foo::Bar than the one you require. Sometimes your module works fine (maybe with some feature limitations), but your test suite breaks because your tests assumed the presence of a new feature.

For each of these cases. you can make your module and tests more robust. For example, you can skip tests that are incompatible ...