NAME

App::Test::Generator - Generate fuzz and corpus-driven test harnesses

SYNOPSIS

From the command line:

fuzz-harness-generator t/conf/add.conf > t/add_fuzz.t

From Perl:

use App::Test::Generator qw(generate);

# Generate to STDOUT
App::Test::Generator::generate("t/conf/add.conf");

# Generate directly to a file
App::Test::Generator::generate("t/conf/add.conf", "t/add_fuzz.t");

OVERVIEW

This module takes a formal input/output specification for a routine or method and automatically generates test cases. In effect, it allows you to easily add comprehensive black-box tests in addition to the more common white-box tests typically written for CPAN modules and other subroutines.

The generated tests combine:

• Random fuzzing based on input types
• Deterministic edge cases for min/max constraints
• Static corpus tests defined in Perl or YAML

This approach strengthens your test suite by probing both expected and unexpected inputs, helping you to catch boundary errors, invalid data handling, and regressions without manually writing every case.

DESCRIPTION

This module implements the logic behind fuzz-harness-generator. It parses configuration files (fuzz and/or corpus YAML), and produces a ready-to-run .t test script using Test::Most.

It reads configuration files (Perl .conf with our variables, and optional YAML corpus files), and generates a Test::Most-based fuzzing harness in t/fuzz.t.

Generates t/fuzz.t combining:

• Randomized fuzzing of inputs (with edge cases)
• Optional static corpus tests from Perl %cases or YAML file (yaml_cases key)
• Functional or OO mode (via $new)
• Reproducible runs via $seed and configurable iterations via $iterations

EDGE CASE GENERATION

In addition to purely random fuzz cases, the harness generates deterministic edge cases for parameters that declare min, max, len, or len in their schema definitions.

For each constraint, three edge cases are added:

• Just inside the allowable range

  This case should succeed, since it lies strictly within the bounds.

• Exactly on the boundary

  This case should succeed, since it meets the constraint exactly.

• Just outside the boundary

  This case is annotated with _STATUS = 'DIES' in the corpus and should cause the harness to fail validation or croak.

Supported constraint types:

• number, integer

  Uses numeric values one below, equal to, and one above the boundary.

• string

  Uses strings of lengths one below, equal to, and one above the boundary (minimum length = len, maximum length = len).

• arrayref

  Uses references to arrays of lengths one below, equal to, and one above the boundary (minimum length = len, maximum length = len).

• hashref

  Uses hashes with key counts one below, equal to, and one above the boundary (min = minimum number of keys, max = maximum number of keys).

• memberof - optional arrayref of allowed values for a parameter:

    our %input = (
        status => { type => 'string', memberof => [ 'ok', 'error', 'pending' ] },
        level  => { type => 'integer', memberof => [ 1, 2, 3 ] },
    );
  

  The generator will automatically create test cases for each allowed value (inside the member list), and at least one value outside the list (which should die, `_STATUS => 'DIES'`). This works for strings, integers, and numbers.

• boolean - automatic boundary tests for boolean fields

    our %input = (
        flag => { type => 'boolean' },
    );
  

  The generator will automatically create test cases for `0` and `1`, and optionally invalid values that should trigger `_STATUS => 'DIES'`.

These edge cases are inserted automatically, in addition to the random fuzzing inputs, so each run will reliably probe boundary conditions without relying solely on randomness.

CONFIGURATION

The configuration file is a Perl file that should set variables with our. Example: the generator expects your config to use our %input, our $function, etc.

Recognized items:

• %input - input params with keys => type/optional specs:

        our %input = (
                name => { type => 'string', optional => 0 },
                age => { type => 'integer', optional => 1 },
        );
  

  Supported basic types used by the fuzzer: string, integer, number, boolean, arrayref, hashref. (You can add more types; they will default to undef unless extended.)

• %output - output param types for Return::Set checking:

        our %output = (
                type => 'string'
        );
  

  If the output hash contains the key _STATUS, and if that key is set to DIES, the routine should die with the given arguments; otherwise, it should live. If it's set to WARNS, the routine should warn with the given arguments

• $module - module name (optional).

  If omitted, the generator will guess from the config filename: My-Widget.conf -> My::Widget.

• $function - function/method to test (defaults to run).

• $new - optional hashref of args to pass to the module's constructor (object mode):

        our $new = { api_key => 'ABC123', verbose => 1 };
  

  To ensure new is called with no arguments, you still need to defined new, thus:

    our $new = '';
  

• %cases - optional Perl static corpus (expected => [ args... ]):

    our %cases = (
      'ok'   => [ 'ping' ],
      'error'=> [ '' ],
    );
  

• $yaml_cases - optional path to a YAML file with the same shape as %cases.

• $seed - optional integer. When provided, the generated t/fuzz.t will call srand($seed) so fuzz runs are reproducible.

• $iterations - optional integer controlling how many fuzz iterations to perform (default 50).

• %edge_cases - optional hash mapping parameter names to arrayrefs of extra values to inject:

        our %edge_cases = (
                name => [ '', 'a' x 1024, \"\x{263A}" ],
                age  => [ -1, 0, 99999999 ],
        );
  

  (Values can be strings or numbers; strings will be properly quoted.)

• %type_edge_cases - optional hash mapping types to arrayrefs of extra values to try for any field of that type:

        our %type_edge_cases = (
                string => [ '', ' ', "\n", "\\0", "long" x 50 ],
                integer => [ -1, 0, 1, 2**31 - 1 ],
        );
  

EXAMPLES

Math::Simple::add()

Functional fuzz + Perl corpus + seed:

our $module = 'Math::Simple';
our $function = 'add';
our %input = ( a => { type => 'integer' }, b => { type => 'integer' } );
our %output = ( type => 'integer' );
our %cases = (
  '3'     => [1, 2],
  '0'     => [0, 0],
  '-1'    => [-2, 1],
  '_STATUS:DIES'  => [ 'a', 'b' ],     # non-numeric args should die
  '_STATUS:WARNS' => [ undef, undef ], # undef args should warn
);
our $seed = 12345;
our $iterations = 100;

Adding YAML file to generate tests

OO fuzz + YAML corpus + edge cases:

    our %input = ( query => { type => 'string' } );
    our %output = ( type => 'string' );
    our $function = 'search';
    our $new = { api_key => 'ABC123' };
    our $yaml_cases = 't/corpus.yml';
    our %edge_cases = ( query => [ '', '    ', '<script>' ] );
    our %type_edge_cases = ( string => [ \"\\0", "\x{FFFD}" ] );
    our $seed = 999;

YAML Corpus Example (t/corpus.yml)

A YAML mapping of expected -> args array:

    "success":
      - "Alice"
      - 30
    "failure":
      - "Bob"

Example with arrayref + hashref

our %input = (
  tags   => { type => 'arrayref', optional => 1 },
  config => { type => 'hashref' },
);
our %output = ( type => 'hashref' );

Example with memberof

our %input = (
    status => { type => 'string', memberof => [ 'ok', 'error', 'pending' ] },
);
our %output = ( type => 'string' );

This will generate fuzz cases for 'ok', 'error', 'pending', and one invalid string that should die.

OUTPUT

By default, writes t/fuzz.t. The generated test:

• Seeds RNG (if configured) for reproducible fuzz runs
• Uses edge cases (per-field and per-type) with configurable probability
• Runs $iterations fuzz cases plus appended edge-case runs
• Validates inputs with Params::Get / Params::Validate::Strict
• Validates outputs with Return::Set
• Runs static is(... ) corpus tests from Perl and/or YAML corpus

NOTES

- The conf file must use our declarations so variables are visible to the generator via require. - Use srand($seed) replay to reproduce failing cases. When you get a failure, re-run generator with the same $seed to reproduce.

SEE ALSO

Test::Most, Params::Get, Params::Validate::Strict, Return::Set, YAML::XS

AUTHOR

Nigel Horne, <njh at nigelhorne.com>

Portions of this module's design and documentation were created with the assistance of ChatGPT (GPT-5), with final curation and authorship by Nigel Horne.