Add middleware to log errors

This proposes the split between logging and converting errors into
response.

The idea is to always add a debug message with the error that happened,
which is handy for both production and development.
In combination with an access log middleware we can log errors or
warnings, depending on the final response status code.

For production, we assume that a fingers crossed handler is used so that
only errors are sent to log - with enough information.

More info:

 - https://github.com/middlewares/access-log/blob/master/src/AccessLog.php
 - https://github.com/Seldaek/monolog/blob/master/src/Monolog/Handler/FingersCrossedHandler.php

Author: lcobucci

src/ErrorLoggingMiddleware.php

@@ -0,0 +1,40 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ErrorHandling;
+
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+use Psr\Log\LoggerInterface;
+use Throwable;
+
+final class ErrorLoggingMiddleware implements MiddlewareInterface
+{
+    /**
+     * @var LoggerInterface
+     */
+    private $logger;
+
+    public function __construct(LoggerInterface $logger)
+    {
+        $this->logger = $logger;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @throws Throwable
+     */
+    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
+    {
+        try {
+            return $handler->handle($request);
+        } catch (Throwable $error) {
+            $this->logger->debug('Error happened while processing request', ['exception' => $error]);
+
+            throw $error;
+        }
+    }
+}

tests/ErrorLoggingMiddlewareTest.php

@@ -0,0 +1,75 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ErrorHandling\Tests;
+
+use Lcobucci\ErrorHandling\ErrorLoggingMiddleware;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Server\RequestHandlerInterface;
+use Psr\Log\LoggerInterface;
+use RuntimeException;
+use Zend\Diactoros\Response;
+use Zend\Diactoros\ServerRequest;
+
+/**
+ * @coversDefaultClass \Lcobucci\ErrorHandling\ErrorLoggingMiddleware
+ */
+final class ErrorLoggingMiddlewareTest extends TestCase
+{
+    /**
+     * @var LoggerInterface&MockObject
+     */
+    private $logger;
+
+    /**
+     * @before
+     */
+    public function createLogger(): void
+    {
+        $this->logger = $this->createMock(LoggerInterface::class);
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     * @covers ::process
+     */
+    public function processShouldLogAllExceptionsOrErrorsThatHappenedDuringRequestHandling(): void
+    {
+        $error = new RuntimeException('Testing');
+
+        $this->logger->expects(self::once())
+                     ->method('debug')
+                     ->with('Error happened while processing request', ['exception' => $error]);
+
+        $handler = $this->createMock(RequestHandlerInterface::class);
+        $handler->method('handle')->willThrowException($error);
+
+        $middleware = new ErrorLoggingMiddleware($this->logger);
+
+        $this->expectExceptionObject($error);
+        $middleware->process(new ServerRequest(), $handler);
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     * @covers ::process
+     */
+    public function processShouldReturnResponseWhenEverythingIsAlright(): void
+    {
+        $this->logger->expects(self::never())->method('debug');
+
+        $response = new Response();
+
+        $handler = $this->createMock(RequestHandlerInterface::class);
+        $handler->method('handle')->willReturn($response);
+
+        $middleware = new ErrorLoggingMiddleware($this->logger);
+
+        self::assertSame($response, $middleware->process(new ServerRequest(), $handler));
+    }
+}