Handle all uncaught Exceptions and rejected promises with error page with status 500 (Internal Server Error)

composer.json

@@ -10,11 +10,6 @@
             "email": "christian@clue.engineering"
         }
     ],
-    "autoload": {
-        "psr-4": {
-            "Frugal\\": "src/"
-        }
-    },
     "require": {
         "php": ">=7.1",
         "nikic/fast-route": "^1.3",
@@ -23,5 +18,15 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^9.5 || ^7.5"
+    },
+    "autoload": {
+        "psr-4": {
+            "Frugal\\": "src/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Frugal\\Tests\\": "tests/"
+        }
     }
 }

examples/index.php

@@ -129,5 +129,12 @@
     );
 });
 
+$app->get('/error', function () {
+    throw new RuntimeException('Error');
+});
+$app->get('/error/null', function () {
+    return null;
+});
+
 $app->run();
 $loop->run();

src/App.php

@@ -300,7 +300,11 @@ private function sendResponse(ServerRequestInterface $request, ResponseInterface
     /**
      * @param ServerRequestInterface $request
      * @param Dispatcher $dispatcher
-     * @return ResponseInterface|PromiseInterface<ResponseInterface>
+     * @return ResponseInterface|PromiseInterface<ResponseInterface,void>
+     *     Returns a response or a Promise which eventually fulfills with a
+     *     response. This method never throws or resolves a rejected promise.
+     *     If the request can not be routed or the handler fails, it will be
+     *     turned into a valid error response before returning.
      */
     private function handleRequest(ServerRequestInterface $request, Dispatcher $dispatcher)
     {
@@ -326,7 +330,30 @@ private function handleRequest(ServerRequestInterface $request, Dispatcher $disp
                     $request = $request->withAttribute($key, rawurldecode($value));
                 }
 
-                return $handler($request);
+                try {
+                    $response = $handler($request);
+                } catch (\Throwable $e) {
+                    return $this->errorHandlerException($e, $handler);
+                }
+
+                if ($response instanceof ResponseInterface) {
+                    return $response;
+                } elseif ($response instanceof PromiseInterface) {
+                    return $response->then(function ($response) use ($handler) {
+                        if (!$response instanceof ResponseInterface) {
+                            return $this->errorHandlerResponse($response, $handler);
+                        }
+                        return $response;
+                    }, function ($e) use ($handler) {
+                        if ($e instanceof \Throwable) {
+                            return $this->errorHandlerException($e, $handler);
+                        } else {
+                            return $this->errorHandlerResponse(\React\Promise\reject($e), $handler);
+                        }
+                    });
+                } else {
+                    return $this->errorHandlerResponse($response, $handler);
+                }
         }
     } // @codeCoverageIgnore
 
@@ -403,4 +430,55 @@ private function errorMethodNotAllowed(ServerRequestInterface $request): Respons
             implode(', ', $request->getAttribute('allowed'))
         )->withHeader('Allowed', implode(', ', $request->getAttribute('allowed')));
     }
+
+    private function errorHandlerException(\Throwable $e, callable $handler): ResponseInterface
+    {
+        $where = ' (<code title="See ' . $e->getFile() . ' line ' . $e->getLine() . '">' . \basename($e->getFile()) . ':' . $e->getLine() . '</code>)';
+
+        return $this->error(
+            500,
+            'Uncaught <code>' . \get_class($e) . '</code> from ' . \lcfirst($this->describeHandler($handler, false)) . $where . ': ' . $e->getMessage()
+        );
+    }
+
+    private function errorHandlerResponse($value, callable $handler): ResponseInterface
+    {
+        return $this->error(
+            500,
+            $this->describeHandler($handler, true) . ' returned invalid value (<code>' . $this->describeType($value) . '</code>)'
+        );
+    }
+
+    private function describeType($value): string
+    {
+        if ($value === null) {
+            return 'null';
+        } elseif (\is_scalar($value) && !\is_string($value)) {
+            return \var_export($value, true);
+        }
+        return \is_object($value) ? \get_class($value) : \gettype($value);
+    }
+
+    private function describeHandler(callable $handler, bool $linkSourceFile): string
+    {
+        if (\is_object($handler) && !$handler instanceof \Closure) {
+            $ref = new \ReflectionMethod($handler, '__invoke');
+            $name = '<code>' . \get_class($handler) . '</code>';
+        } elseif (\is_string($handler)) {
+            $ref = new \ReflectionFunction($handler);
+            $name = '<code>' . $handler . '()</code>';
+        } elseif (\is_array($handler)) {
+            $ref = new \ReflectionMethod($handler[0], $handler[1]);
+            $name = '<code>' . (\is_string($handler[0]) ? $handler[0] : \get_class($handler[0])) . '::' . $handler[1] . '()</code>';
+        } else {
+            $ref = new \ReflectionFunction($handler);
+            $name = 'Request handler';
+        }
+
+        if ($linkSourceFile && !$ref->isInternal()) {
+            $name .= ' (<code title="See ' . $ref->getFileName() .' line ' . $ref->getStartLine() .'">' . \basename($ref->getFileName()) . ':' . $ref->getStartLine() . '</code>)';
+        }
+
+        return $name;
+    }
 }