[Testing] Lock the fixture directory while a test uses its temp file

.gitignore

@@ -11,6 +11,7 @@ composer.lock
 .phpunit.result.cache
 .phpunit.cache
 .fastunit-cache
+.fixture.lock
 
 # scoped & downgraded version
 php-scoper.phar

src/Testing/PHPUnit/AbstractRectorTestCase.php

@@ -36,6 +36,11 @@ abstract class AbstractRectorTestCase extends AbstractLazyTestCase implements Re
 
     private ?string $inputFilePath = null;
 
+    /**
+     * @var resource|null
+     */
+    private $fixtureDirectoryLock;
+
     /**
      * @var array<string, true>
      */
@@ -133,6 +138,8 @@ protected function tearDown(): void
             FileSystem::delete($this->inputFilePath);
         }
 
+        $this->unlockFixtureDirectory();
+
         // a next test that does not set its own paths would still locate the deleted file
         $this->dynamicSourceLocatorProvider->reset();
     }
@@ -177,6 +184,10 @@ protected function doTestFile(string $fixtureFilePath, bool $includeFixtureDirec
             throw new ShouldNotHappenException('Fixture file and input file cannot be the same: ' . $fixtureFilePath);
         }
 
+        // test classes that share a fixture directory write the same temp files,
+        // so in parallel processes one could delete the other's file
+        $this->lockFixtureDirectory(dirname($inputFilePath));
+
         // write temp file
         FileSystem::write($inputFilePath, $inputFileContents, null);
 
@@ -282,6 +293,31 @@ private function processFilePath(string $filePath, bool $includeFixtureDirectory
         return new RectorTestResult($changedFileContents, $processResult);
     }
 
+    private function lockFixtureDirectory(string $directory): void
+    {
+        // a test can call doTestFile() more than once; a second lock from this process would wait for the first
+        $this->unlockFixtureDirectory();
+
+        $fixtureDirectoryLock = fopen($directory . '/.fixture.lock', 'c');
+        if ($fixtureDirectoryLock === false) {
+            throw new ShouldNotHappenException('Cannot open the lock file in ' . $directory);
+        }
+
+        flock($fixtureDirectoryLock, LOCK_EX);
+        $this->fixtureDirectoryLock = $fixtureDirectoryLock;
+    }
+
+    private function unlockFixtureDirectory(): void
+    {
+        if (! is_resource($this->fixtureDirectoryLock)) {
+            return;
+        }
+
+        flock($this->fixtureDirectoryLock, LOCK_UN);
+        fclose($this->fixtureDirectoryLock);
+        $this->fixtureDirectoryLock = null;
+    }
+
     private function createInputFilePath(string $fixtureFilePath): string
     {
         $inputFileDirectory = dirname($fixtureFilePath);