Fix infinite loops when a log handler triggers logging itself

src/Monolog/Logger.php

@@ -147,6 +147,11 @@ class Logger implements LoggerInterface, ResettableInterface
      */
     protected $exceptionHandler;
 
+    /**
+     * @var int Keeps track of depth to prevent infinite logging loops
+     */
+    private $logDepth = 0;
+
     /**
      * @psalm-param array<callable(array): array> $processors
      *
@@ -291,6 +296,15 @@ class Logger implements LoggerInterface, ResettableInterface
      */
     public function addRecord(int $level, string $message, array $context = []): bool
     {
+        $this->logDepth += 1;
+        if ($this->logDepth === 3) {
+            $this->warning('A possible infinite logging loop was detected and aborted. It appears some of your handler code is triggering logging, see the previous log record for a hint as to what may be the cause.');
+            return false;
+        } elseif ($this->logDepth >= 5) { // log depth 4 is let through so we can log the warning above
+            return false;
+        }
+
+        try {
             $record = null;
 
             foreach ($this->handlers as $handler) {
@@ -334,6 +348,9 @@ class Logger implements LoggerInterface, ResettableInterface
                     return true;
                 }
             }
+        } finally {
+            $this->logDepth--;
+        }
 
         return null !== $record;
     }

tests/Monolog/LoggerTest.php

@@ -11,6 +11,7 @@
 
 namespace Monolog;
 
+use Monolog\Handler\HandlerInterface;
 use Monolog\Processor\WebProcessor;
 use Monolog\Handler\TestHandler;
 
@@ -84,6 +85,28 @@ class LoggerTest extends \PHPUnit\Framework\TestCase
         $this->assertEquals('foo', $record['channel']);
     }
 
+    /**
+     * @covers Monolog\Logger::addRecord
+     */
+    public function testLogPreventsCircularLogging()
+    {
+        $logger = new Logger(__METHOD__);
+
+        $loggingHandler = new LoggingHandler($logger);
+        $testHandler = new TestHandler();
+
+        $logger->pushHandler($loggingHandler);
+        $logger->pushHandler($testHandler);
+
+        $logger->addRecord(Logger::ALERT, 'test');
+
+        $records = $testHandler->getRecords();
+        $this->assertCount(3, $records);
+        $this->assertSame('ALERT', $records[0]['level_name']);
+        $this->assertSame('DEBUG', $records[1]['level_name']);
+        $this->assertSame('WARNING', $records[2]['level_name']);
+    }
+
     /**
      * @covers Monolog\Logger::addRecord
      */
@@ -717,3 +740,36 @@ class LoggerTest extends \PHPUnit\Framework\TestCase
         $this->assertNotSame($uid2, $processorUid2->getUid());
     }
 }
+
+class LoggingHandler implements HandlerInterface
+{
+    /**
+     * @var Logger
+     */
+    private $logger;
+
+    public function __construct(Logger $logger)
+    {
+        $this->logger = $logger;
+    }
+
+    public function isHandling(array $record): bool
+    {
+        return true;
+    }
+
+    public function handle(array $record): bool
+    {
+        $this->logger->debug('Log triggered while logging');
+
+        return false;
+    }
+
+    public function handleBatch(array $records): void
+    {
+    }
+
+    public function close(): void
+    {
+    }
+}