[Bug] --mutate fails with "Cannot traverse an already closed generator" on Windows

[Rolliboy]

Pest Version

4.3.1 (also tested with 3.8.4)

PHP Version

8.3.16 (ZTS Visual C++ 2019 x64)

Operating System

Windows 10/11 (MSYS_NT-10.0-26200)

Description

Running --mutate fails with "Cannot traverse an already closed generator" error on Windows. The tests pass successfully, but mutation testing crashes when it tries to iterate over files.

Steps To Reproduce

1. Have a working Pest test suite on Windows
2. Add covers(SomeClass::class) to a test file
3. Run any of these commands:
   • ./vendor/bin/pest --mutate
   • ./vendor/bin/pest --mutate --path=src/Domain
   • ./vendor/bin/pest tests/SomeTest.php --mutate

Expected Behavior

Mutation testing should run and report mutants.

Actual Behavior

Tests:    48 passed (150 assertions)
Duration: 6.56s

Mutating application files...

   Exception

  Cannot traverse an already closed generator

  at vendor\pestphp\pest-plugin-mutate\src\Tester\MutationTestRunner.php:124
    120▕
    121▕         /** @var MutationGenerator $generator */
    122▕         $generator = Container::getInstance()->get(MutationGenerator::class);
    123▕
  ➜ 124▕         foreach ($files as $file) {

  1   vendor\pestphp\pest-plugin-mutate\src\Tester\MutationTestRunner.php:124
      AppendIterator::rewind()

  2   vendor\pestphp\pest-plugin-mutate\src\Plugins\Mutate.php:191
      Pest\Mutate\Tester\MutationTestRunner::run()

Notes

• Xdebug 3.5.0 is enabled
• Issue occurs with both Pest 3.x and 4.x
• Tests run correctly without --mutate
• Tried with --parallel, --no-parallel, --no-coverage - same result

[Rolliboy]

Hi everyone, I’ve done some debugging on my Windows machine and found why this is happening.
The Cause

The crash occurs in FileFinder::files() when the $paths array is empty (which happens if the section in phpunit.xml is missing or misconfigured).

On Windows, when Symfony\Component\Finder is initialized with empty directories or faces specific filesystem iterator states, it may return a Generator that is already closed or fails to rewind. Calling iterator_to_array($files) or iterating over it in MutationTestRunner.php then triggers the fatal error: Cannot traverse an already closed generator.
The Fix

The FileFinder should handle empty inputs gracefully before touching the Finder instance. I've verified that adding a guard clause prevents the crash and allows Pest to exit cleanly with a "No mutations created" message.

In src/Support/FileFinder.php, we can modify the files method as follows:
PHP

public static function files(array $paths, array $pathsToIgnore): array
{
    ['directories' => $dirs, 'files' => $files] = self::separateDirectoriesAndFiles($paths);

    // Guard clause: If no paths are provided, return early to avoid 
    // initializing a Finder that might fail on certain platforms.
    if (empty($dirs) && empty($files)) {
        return [];
    }

    $allPathsToIgnore = self::buildPathsToIgnore($pathsToIgnore, $dirs);

    $finder = Finder::create()
        ->in($dirs)
        ->name('*.php')
        ->append($files)
        ->files()
        ->filter(fn (SplFileInfo $file): bool => array_filter($allPathsToIgnore, fn (string $pathToIgnore): bool => preg_match($pathToIgnore, $file->getRealPath() ?: '') === 1) === []);

    return iterator_to_array($finder, false);
}

This ensures the plugin is more robust on Windows even when the mutation source is not explicitly defined.

[Rolliboy]

[SOLVED] see next comment | Follow-up Comment: Second issue after fixing the generator error

After resolving the generator issue (by modifying FileFinder.php), a new problem emerged:

Description

Mutation testing runs but creates 0 mutations even when:

• Test file has mutates(SomeClass::class)
• Coverage shows 100% on the target class
• The class contains mutable code (if statements, in_array, match expressions, etc.)

Steps To Reproduce

// tests/Unit/Domain/ValueObjects/ContrattoStateTest.php
mutates(ContrattoState::class);

describe('ContrattoState Enum', function () {
    // ... 48 passing tests with 100% coverage
});

./vendor/bin/pest tests/Unit/Domain/ValueObjects/ContrattoStateTest.php --mutate
# Output: "0 Mutations for 0 Files created"

./vendor/bin/pest --mutate --class="CAT\\Contratti\\Contratti\\Domain\\ValueObjects\\ContrattoState"
# Output: "0 Mutations for 0 Files created"

./vendor/bin/pest --mutate --everything
# Output: "0 Mutations for 0 Files created"

Expected Behavior

Mutations should be generated for the class methods containing boolean logic, comparisons, match expressions, etc.

[Rolliboy]

Pest Mutate on Windows: 0 Mutations (Solved)

Reference: GitHub Issue #1614

Environment

• OS: Windows 10/11 (Laragon)
• PHP: 8.3 with xDebug
• Pest: 4.3.1

Problem

Running mutation testing with Pest on Windows:

./vendor/bin/pest tests/Unit/Domain/ValueObjects/ContrattoStateTest.php --mutate

Tests pass but mutations are 0:

Tests:    48 passed (150 assertions)
Duration: 5.49s

Mutating application files...
0 Mutations for 0 Files created
Score:     0.00%

INFO  No mutations created.

Despite having:

• mutate() configuration in Pest.php
• Source files existing in the configured directory
• Tests with 100% coverage

Cause

The mutate('src/Contratti/Domain') function defined in Pest.php is not automatically applied when using the --mutate flag from CLI.

On Windows, Pest Mutate requires the source files path to be specified explicitly with the --path option.

Solution

Add --path to specify which source files to mutate:

# Single file
./vendor/bin/pest tests/Unit/Domain/ValueObjects/ContrattoStateTest.php --mutate --path="src/Contratti/Domain/ValueObjects/ContrattoState.php"

# Entire directory
./vendor/bin/pest tests/Unit/Domain --mutate --path="src/Contratti/Domain"

Result

Tests:    48 passed (150 assertions)
Duration: 5.49s

Mutating application files...
156 Mutations for 1 Files created

RUN ContrattoState.php
✓ Line 31: IdenticalToNotIdentical
✓ Line 40: RemoveArrayItem
...
Mutations: 156 tested
Score:     100.00%

NPM Scripts (package.json)

To simplify execution on Windows:

{
  "scripts": {
    "test": "vendor\\bin\\pest",
    "test:unit": "vendor\\bin\\pest tests/Unit",
    "mutate:domain": "vendor\\bin\\pest tests/Unit/Domain --mutate --path=src/Contratti/Domain",
    "mutate:vo": "vendor\\bin\\pest tests/Unit/Domain/ValueObjects --mutate --path=src/Contratti/Domain/ValueObjects",
    "mutate:entities": "vendor\\bin\\pest tests/Unit/Domain/Entities --mutate --path=src/Contratti/Domain/Entities"
  }
}

Note: On Windows use vendor\\bin\\pest (double backslash) instead of vendor/bin/pest.