Heuristics to set memory limit for mutants running with PHPUnit.

Since we know how much memory the initial test suite used,
and only if we know, we can enforce a memory limit upon all
mutation processes. Limit is set to be twice the known amount,
because if we know that a normal test suite used X megabytes,
if a mutants uses a lot more, this is a definite error.

Memory limit is introduced by altering a known temporary php.ini
to include a directive to enable the limit as the very last line.

src/Command/InfectionCommand.php

@@ -20,6 +20,7 @@
 use Infection\Mutant\Exception\MsiCalculationException;
 use Infection\Mutant\Generator\MutationsGenerator;
 use Infection\Mutant\MetricsCalculator;
+use Infection\Php\ConfigBuilder;
 use Infection\Process\Builder\ProcessBuilder;
 use Infection\Process\Listener\InitialTestsConsoleLoggerSubscriber;
 use Infection\Process\Listener\MutantCreatingConsoleLoggerSubscriber;
@@ -31,6 +32,7 @@
 use Infection\TestFramework\AbstractTestFrameworkAdapter;
 use Infection\TestFramework\Coverage\CodeCoverageData;
 use Infection\TestFramework\PhpSpec\PhpSpecExtraOptions;
+use Infection\TestFramework\PhpUnit\Adapter\PhpUnitAdapter;
 use Infection\TestFramework\PhpUnit\Coverage\CoverageXmlParser;
 use Infection\TestFramework\PhpUnit\PhpUnitExtraOptions;
 use Infection\TestFramework\TestFrameworkExtraOptions;
@@ -192,6 +194,10 @@ protected function execute(InputInterface $input, OutputInterface $output)
             return 1;
         }
 
+        if ($adapter instanceof PhpUnitAdapter) {
+            $this->applyMemoryLimitFromPhpUnitProcess($initialTestSuitProcess, $adapter);
+        }
+
         $codeCoverageData = $this->getCodeCoverageData($testFrameworkKey);
         $mutationsGenerator = new MutationsGenerator(
             $container->get('src.dirs'),
@@ -253,6 +259,38 @@ private function getOutputFormatter(): OutputFormatter
         throw new \InvalidArgumentException('Incorrect formatter. Possible values: "dot", "progress"');
     }
 
+    /**
+     * By default we let a mutant process use as much more memory as an initial test suite consumed.
+     *
+     * @var int
+     */
+    const DEFAULT_MEMORY_LIMIT_MULTIPLIER = 2;
+
+    private function applyMemoryLimitFromPhpUnitProcess(Process $process, PhpUnitAdapter $adapter)
+    {
+        $memoryLimit = $adapter->getMemoryUsed($process->getOutput());
+
+        if ($memoryLimit < 0) {
+            // Cannot detect memory used, not setting any limits
+            return;
+        }
+
+        /*
+         * Since we know how much memory the initial test suite used,
+         * and only if we know, we can enforce a memory limit upon all
+         * mutation processes. Limit is set to be twice the known amount,
+         * because if we know that a normal test suite used X megabytes,
+         * if a mutants uses a lot more, this is a definite error.
+         */
+        $memoryLimit *= self::DEFAULT_MEMORY_LIMIT_MULTIPLIER;
+
+        $tempConfigPath = (string) getenv(ConfigBuilder::ENV_TEMP_PHP_CONFIG_PATH);
+
+        if (!empty($tempConfigPath) && file_exists($tempConfigPath)) {
+            file_put_contents($tempConfigPath, PHP_EOL . sprintf('memory_limit = %dM', $memoryLimit), FILE_APPEND);
+        }
+    }
+
     private function registerSubscribers(
         MetricsCalculator $metricsCalculator,
         AbstractTestFrameworkAdapter $testFrameworkAdapter