/ 
App-Test-Generator-0.18
/ App::Test::Generator

NAME

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

VERSION

Version 0.18

SYNOPSIS

From the command line:

fuzz-harness-generator -r t/conf/add.yml

extract-schemas bin/extract-schemas lib/Sample/Module.pm; fuzz-harness-generator -r schemas/greet.yaml

From Perl:

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

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

# Generate directly to a file
App::Test::Generator::generate('t/conf/add.yml', '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 that are 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 to run through prove.

It reads configuration files in any format, and optional YAML corpus files. All of the examples in this documenation are in YAML format, other formats may not work as they aren't so heavily tested. It then generates a Test::Most-based fuzzing harness 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

CONFIGURATION

The configuration file, for each set of tests to be produced, is a file containng a schema that can be read by Config::Abstraction.

SCHEMA

The schema is split into several sections.

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

When using named parameters

input:
  name:
    type: string
    optional: false
  age:
    type: integer
    optional: true

Supported basic types used by the fuzzer: string, integer, float, number, boolean, arrayref, hashref. See also Params::Validate::Strict. You can add more custom types using properties.

For routines with one unnamed parameter

input:
  type: string

For routines with more than one named parameter, use the position keyword.

module: Math::Simple::MinMax
fuction: max

input:
  left:
    type: number
    position: 0
  right:
    type: number
    position: 1

output:
  type: number

The keyword undef is used to indicate that the function takes no arguments.

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

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. The output can be set to the string 'undef' if the routine should return the undefined value:

---
module: Scalar::Util
function: blessed

input:
  type: string

output: undef

The keyword undef is used to indicate that the function returns nothing.

%config - optional hash of configuration.

The current supported variables are

  • test_nuls, inject NUL bytes into strings (default: 1)

  • test_undef, test with undefined value (default: 1)

  • test_empty, test with empty strings (default: 1)

  • test_non_ascii, test with strings that contain non ascii characaters (default: 1)

  • dedup, fuzzing can create duplicate tests, go some way to remove duplicates (default: 1)

  • properties, enable Test::LectroTest Property tests (default: 0)

%transforms - list of transformations from input sets to output sets

Transforms allow you to define how input data should be transformed into output data. This is useful for testing functions that convert between formats, normalize data, or apply business logic transformations on a set of data to different set of data. It takes a list of subsets of the input and output definitions, and verifies that data from each input subset is correctly transformed into data from the matching output subset.

Transform Validation Rules

For each transform:

1. Generate test cases using the transform's input schema
2. Call the function with those inputs
3. Validate the output matches the transform's output schema
4. If output has a specific 'value', check exact match
5. If output has constraints (min/max), validate within bounds

Example 1

---
module: builtin
function: abs

config:
  test_undef: no
  test_empty: no
  test_nuls: no
  test_non_ascii: no

input:
  number:
    type: number
    position: 0

output:
  type: number
  min: 0

transforms:
  positive:
    input:
      number:
        type: number
        position: 0
        min: 0
    output:
      type: number
      min: 0
  negative:
    input:
      number:
        type: number
        position: 0
        max: 0
    output:
      type: number
      min: 0
  error:
    input:
      undef
    output:
      _STATUS: DIES

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.

The keyword undef is used to indicate that the function returns nothing.

Example 2

---
module: Math::Utils
function: normalize_number

input:
  value:
    type: number
    position: 0

output:
  type: number

transforms:
  positive_stays_positive:
    input:
      value:
        type: number
        min: 0
        max: 1000
    output:
      type: number
      min: 0
      max: 1

  negative_becomes_zero:
    input:
      value:
        type: number
        max: 0
    output:
      type: number
      value: 0

  preserves_zero:
    input:
      value:
        type: number
        value: 0
    output:
      type: number
      value: 0

$module

The name of the module (optional).

Using the reserved word builtin means you're testing a Perl builtin function.

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

$function

The function/method to test.

This defaults to run.

%new

An optional hashref of args to pass to the module's constructor.

new:
  api_key: ABC123
  verbose: true

To ensure new() is called with no arguments, you still need to define new, thus:

module: MyModule
function: my_function

new:

%cases

An optional Perl static corpus, when the output is a simple string (expected => [ args... ]).

Maps the expected output string to the input and _STATUS

cases:
  ok:
    input: ping
    _STATUS: OK
  error:
    input: ""
    _STATUS: DIES

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

$seed

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

$iterations

An optional integer controlling how many fuzz iterations to perform (default 50).

%edge_cases

An optional hash mapping of extra values to inject.

# Two named parameters
edge_cases:
	name: [ '', 'a' x 1024, \"\x{263A}" ]
	age: [ -1, 0, 99999999 ]

# Takes a string input
edge_cases: [ 'foo', 'bar' ]

Values can be strings or numbers; strings will be properly quoted. Note that this only works with routines that take named parameters.

%type_edge_cases

An optional hash mapping types to arrayrefs of extra values to try for any field of that type:

type_edge_cases:
	string: [ '', ' ', "\t", "\n", "\0", 'long' x 1024, chr(0x1F600) ]
	number: [ 0, 1.0, -1.0, 1e308, -1e308, 1e-308, -1e-308, 'NaN', 'Infinity' ]
	integer: [ 0, 1, -1, 2**31-1, -(2**31), 2**63-1, -(2**63) ]

%edge_case_array

Specify edge case values for routines that accept a single unnamed parameter. This is specifically designed for simple functions that take one argument without a parameter name. These edge cases supplement the normal random string generation, ensuring specific problematic values are always tested. During fuzzing iterations, there's a 40% probability that a test case will use a value from edge_case_array instead of randomly generated data.

---
module: Text::Processor
function: sanitize

input:
  type: string
  min: 1
  max: 1000

edge_case_array:
  - "<script>alert('xss')</script>"
  - "'; DROP TABLE users; --"
  - "\0null\0byte"
  - "emoji😊test"
  - ""
  - " "

seed: 42
iterations: 50

Semantic Data Generators

For property-based testing with Test::LectroTest, you can use semantic generators to create realistic test data. Fuzz testing support for semantic entries is being developed.

input:
  email:
    type: string
    semantic: email

  user_id:
    type: string
    semantic: uuid

  phone:
    type: string
    semantic: phone_us

Available Semantic Types

  • email - Valid email addresses (user@domain.tld)

  • url - HTTP/HTTPS URLs

  • uuid - UUIDv4 identifiers

  • phone_us - US phone numbers (XXX-XXX-XXXX)

  • phone_e164 - International E.164 format (+XXXXXXXXXXXX)

  • ipv4 - IPv4 addresses (0.0.0.0 - 255.255.255.255)

  • ipv6 - IPv6 addresses

  • username - Alphanumeric usernames with _ and -

  • slug - URL slugs (lowercase-with-hyphens)

  • hex_color - Hex color codes (#RRGGBB)

  • iso_date - ISO 8601 dates (YYYY-MM-DD)

  • iso_datetime - ISO 8601 datetimes (YYYY-MM-DDTHH:MM:SSZ)

  • semver - Semantic version strings (major.minor.patch)

  • jwt - JWT-like tokens (base64url format)

  • json - Simple JSON objects

  • base64 - Base64-encoded strings

  • md5 - MD5 hashes (32 hex chars)

  • sha256 - SHA-256 hashes (64 hex chars)

EDGE CASE GENERATION

In addition to purely random fuzz cases, the harness generates deterministic edge cases for parameters that declare min, max 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, float

    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.

  • arrayref

    Uses references to arrays of with the number of elements one below, equal to, and one above the boundary.

  • 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 - arrayref of allowed values for a parameter

    This example is for a routine called input() that takes two arguments: status and level. status is a string that must have the value ok, error or pending. The level argument is an integer that must be one of 1, 5 or 111.

    ---
input:
  status:
    type: string
    memberof:
      - ok
      - error
      - pending
  level:
    type: integer
    memberof:
      - 1
      - 5
      - 111

    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 or croak, _STATUS = 'DIES'). This works for strings, integers, and numbers.

  • boolean - automatic boundary tests for boolean fields

    input:
  flag:
    type: boolean

    The generator will automatically create test cases for 0 and 1; true and false; off and on, and 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.

EXAMPLES

See the files in t/conf for examples.

Adding Scheduled fuzz Testing with GitHub Actions to Your Code

To automatically create and run tests on a regular basis on GitHub Actions, you need to create a configuration file for each method and subroutine that you're testing, and a GitHub Actions configuration file.

This example takes you through testing the online_render method of HTML::Genealogy::Map.

t/conf/online_render.yml

---

module: HTML::Genealogy::Map
function: onload_render

input:
  gedcom:
    type: object
    can: individuals
  geocoder:
    type: object
    can: geocode
  debug:
    type: boolean
    optional: true
  google_key:
    type: string
    optional: true
    min: 39
    max: 39
    matches: "^AIza[0-9A-Za-z_-]{35}$"

config:
  test_undef: 0

.github/actions/fuzz.t

---
name: Fuzz Testing

permissions:
  contents: read

on:
  push:
    branches: [main, master]
  pull_request:
    branches: [main, master]
  schedule:
    - cron: '29 5 14 * *'

jobs:
  generate-fuzz-tests:
    strategy:
      fail-fast: false
      matrix:
        os:
          - macos-latest
          - ubuntu-latest
          - windows-latest
        perl: ['5.42', '5.40', '5.38', '5.36', '5.34', '5.32', '5.30', '5.28', '5.22']

    runs-on: ${{ matrix.os }}
    name: Fuzz testing with perl ${{ matrix.perl }} on ${{ matrix.os }}

    steps:
      - uses: actions/checkout@v5

      - name: Set up Perl
        uses: shogo82148/actions-setup-perl@v1
        with:
          perl-version: ${{ matrix.perl }}

      - name: Install App::Test::Generator this module's dependencies
        run: |
          cpanm App::Test::Generator
          cpanm --installdeps .

      - name: Make Module
        run: |
          perl Makefile.PL
          make
        env:
          AUTOMATED_TESTING: 1
          NONINTERACTIVE_TESTING: 1

      - name: Generate fuzz tests
        run: |
          mkdir t/fuzz
          find t/conf -name '*.yml' | while read config; do
            test_name=$(basename "$config" .conf)
            fuzz-harness-generator "$config" > "t/fuzz/${test_name}_fuzz.t"
          done

      - name: Run generated fuzz tests
        run: |
          prove -lr t/fuzz/
        env:
          AUTOMATED_TESTING: 1
          NONINTERACTIVE_TESTING: 1

Fuzz Testing your CPAN Module

Running fuzz tests when you run make test in your CPAN module.

Create a directory <t/conf> which contains the schemas.

Then create this file as <t/fuzz.t>:

  #!/usr/bin/env perl

  use strict;
  use warnings;

  use FindBin qw($Bin);
  use IPC::Run3;
  use IPC::System::Simple qw(system);
  use Test::Needs 'App::Test::Generator';
  use Test::Most;

  my $dirname = "$Bin/conf";

  if((-d $dirname) && opendir(my $dh, $dirname)) {
	while (my $filename = readdir($dh)) {
		# Skip '.' and '..' entries and vi temporary files
		next if ($filename eq '.' || $filename eq '..') || ($filename =~ /\.swp$/);

		my $filepath = "$dirname/$filename";

		if(-f $filepath) {	# Check if it's a regular file
			my ($stdout, $stderr);
			run3 ['fuzz-harness-generator', '-r', $filepath], undef, \$stdout, \$stderr;

			ok($? == 0, 'Generated test script exits successfully');

			if($? == 0) {
				ok($stdout =~ /^Result: PASS/ms);
				if($stdout =~ /Files=1, Tests=(\d+)/ms) {
					diag("$1 tests run");
				}
			} else {
				diag("$filepath: STDOUT:\n$stdout");
				diag($stderr) if(length($stderr));
				diag("$filepath Failed");
				last;
			}
			diag($stderr) if(length($stderr));
		}
	}
	closedir($dh);
  }

  done_testing();

Property-Based Testing with Transforms

The generator can create property-based tests using Test::LectroTest when the properties configuration option is enabled. This provides more comprehensive testing by automatically generating thousands of test cases and verifying that mathematical properties hold across all inputs.

Basic Property-Based Transform Example

Here's a complete example testing the abs builtin function:

t/conf/abs.yml:

---
module: builtin
function: abs

config:
  test_undef: no
  test_empty: no
  test_nuls: no
  properties:
    enable: true
    trials: 1000

input:
  number:
    type: number
    position: 0

output:
  type: number
  min: 0

transforms:
  positive:
    input:
      number:
        type: number
        min: 0
    output:
      type: number
      min: 0

  negative:
    input:
      number:
        type: number
        max: 0
    output:
      type: number
      min: 0

This configuration:

  • Enables property-based testing with 1000 trials per property

  • Defines two transforms: one for positive numbers, one for negative

  • Automatically generates properties that verify abs() always returns non-negative numbers

Generate the test:

fuzz-harness-generator t/conf/abs.yml > t/abs_property.t

The generated test will include:

  • Traditional edge-case tests for boundary conditions

  • Random fuzzing with 50 iterations (or as configured)

  • Property-based tests that verify the transforms with 1000 trials each

What Properties Are Tested?

The generator automatically detects and tests these properties based on your transform specifications:

  • Range constraints - If output has min or max, verifies results stay within bounds

  • Type preservation - Ensures numeric inputs produce numeric outputs

  • Definedness - Verifies the function doesn't return undef unexpectedly

  • Specific values - If output specifies a value, checks exact equality

For the abs example above, the generated properties verify:

# For the "positive" transform:
- Given a positive number, abs() returns >= 0
- The result is a valid number
- The result is defined

# For the "negative" transform:
- Given a negative number, abs() returns >= 0
- The result is a valid number
- The result is defined

Advanced Example: String Normalization

Here's a more complex example testing a string normalization function:

t/conf/normalize.yml:

---
module: Text::Processor
function: normalize_whitespace

config:
  properties:
    enable: true
    trials: 500

input:
  text:
    type: string
    min: 0
    max: 1000
    position: 0

output:
  type: string
  min: 0
  max: 1000

transforms:
  empty_preserved:
    input:
      text:
        type: string
        value: ""
    output:
      type: string
      value: ""

  single_space:
    input:
      text:
        type: string
        min: 1
        matches: '^\S+(\s+\S+)*$'
    output:
      type: string
      matches: '^\S+( \S+)*$'

  length_bounded:
    input:
      text:
        type: string
        min: 1
        max: 100
    output:
      type: string
      min: 1
      max: 100

This tests that the normalization function:

  • Preserves empty strings (empty_preserved transform)

  • Collapses multiple spaces into single spaces (single_space transform)

  • Maintains length constraints (length_bounded transform)

Interpreting Property Test Results

When property-based tests run, you'll see output like:

ok 123 - negative property holds (1000 trials)
ok 124 - positive property holds (1000 trials)

If a property fails, Test::LectroTest will attempt to find the minimal failing case and display it:

not ok 123 - positive property holds (47 trials)
# Property failed
# Reason: counterexample found

This helps you quickly identify edge cases that your function doesn't handle correctly.

Configuration Options for Property-Based Testing

In the config section:

config:
  properties:
    enable: true     # Enable property-based testing (default: false)
    trials: 1000     # Number of test cases per property (default: 1000)

You can also disable traditional fuzzing and only use property-based tests:

config:
  properties:
    enable: true
    trials: 5000

iterations: 0  # Disable random fuzzing, use only property tests

When to Use Property-Based Testing

Property-based testing with transforms is particularly useful for:

  • Mathematical functions (abs, sqrt, min, max, etc.)

  • Data transformations (encoding, normalization, sanitization)

  • Parsers and formatters

  • Functions with clear input-output relationships

  • Code that should satisfy mathematical properties (commutativity, associativity, idempotence)

Requirements

Property-based testing requires Test::LectroTest to be installed:

cpanm Test::LectroTest

If not installed, the generated tests will automatically skip the property-based portion with a message.

Testing Email Validation

---
module: Email::Valid
function: rfc822

config:
  properties:
    enable: true
    trials: 200
  test_undef: no
  test_empty: no
  test_nuls: no

input:
  email:
    type: string
    semantic: email
    position: 0

output:
  type: boolean

transforms:
  valid_emails:
    input:
      email:
        type: string
        semantic: email
    output:
      type: boolean

This generates 200 realistic email addresses for testing, rather than random strings.

Combining Semantic with Regex

You can combine semantic generators with regex validation:

input:
  corporate_email:
    type: string
    semantic: email
    matches: '@company\.com$'

The semantic generator creates realistic emails, and the regex ensures they match your domain.

Custom Properties for Transforms

You can define additional properties that should hold for your transforms beyond the automatically detected ones.

Using Built-in Properties

transforms:
  positive:
    input:
      number:
        type: number
        min: 0
    output:
      type: number
      min: 0
    properties:
      - idempotent       # f(f(x)) == f(x)
      - non_negative     # result >= 0
      - positive         # result > 0

Available built-in properties:

  • idempotent - Function is idempotent: f(f(x)) == f(x)

  • non_negative - Result is always >= 0

  • positive - Result is always > 0

  • non_empty - String result is never empty

  • length_preserved - Output length equals input length

  • uppercase - Result is all uppercase

  • lowercase - Result is all lowercase

  • trimmed - No leading/trailing whitespace

  • sorted_ascending - Array is sorted ascending

  • sorted_descending - Array is sorted descending

  • unique_elements - Array has no duplicates

  • preserves_keys - Hash has same keys as input

Custom Property Code

Custom properties allows the definition additional invariants and relationships that should hold for their transforms, beyond what's auto-detected. For example:

  • Idempotence: f(f(x)) == f(x)

  • Commutativity: f(x, y) == f(y, x)

  • Associativity: f(f(x, y), z) == f(x, f(y, z))

  • Inverse relationships: decode(encode(x)) == x

  • Domain-specific invariants: Custom business logic

Define your own properties with custom Perl code:

transforms:
  normalize:
    input:
      text:
        type: string
    output:
      type: string
    properties:
      - name: single_spaces
        description: "No multiple consecutive spaces"
        code: $result !~ /  /

      - name: no_leading_space
        description: "No space at start"
        code: $result !~ /^\s/

      - name: reversible
        description: "Can be reversed back"
        code: length($result) == length($text)

The code has access to:

  • $result - The function's return value

  • Input variables - All input parameters (e.g., $text, $number)

  • The function itself - Can call it again for idempotence checks

Combining Auto-detected and Custom Properties

The generator automatically detects properties from your output spec, and adds your custom properties:

transforms:
  sanitize:
    input:
      html:
        type: string
    output:
      type: string
      min: 0              # Auto-detects: defined, min_length >= 0
      max: 10000
    properties:           # Additional custom checks:
      - name: no_scripts
        code: $result !~ /<script/i
      - name: no_iframes
        code: $result !~ /<iframe/i

GENERATED OUTPUT

The generated test:

  • Seeds RND (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

  • Runs Test::LectroTest tests

METHODS

generate($schema_file, $test_file)

Takes a schema file and produces a test file (or STDOUT).

_generate_transform_properties

Converts transform specifications into LectroTest property definitions.

_process_custom_properties

Processes custom property definitions from the schema.

_detect_transform_properties

Automatically detects testable properties from transform input/output specs.

_get_semantic_generators

Returns a hash of built-in semantic generators for common data types.

_get_builtin_properties

Returns a hash of built-in property templates that can be applied to transforms.

_schema_to_lectrotest_generator

Converts a schema field spec to a LectroTest generator string.

Helper functions for type detection

_render_properties

Renders property definitions into Perl code for the template.

NOTES

seed and iterations really should be within config.

SEE ALSO

AUTHOR

Nigel Horne, <njh at nigelhorne.com>

Portions of this module's initial design and documentation were created with the assistance of AI.