Add parsing middleware (#3)

src/ParsingMiddleware.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Amp\Http\Server\FormParser;
+
+use Amp\Http\Server\ErrorHandler;
+use Amp\Http\Server\Middleware;
+use Amp\Http\Server\Request;
+use Amp\Http\Server\RequestHandler;
+use Amp\Http\Server\Server;
+use Amp\Http\Server\ServerObserver;
+use Amp\Http\Status;
+use Amp\Promise;
+use Amp\Success;
+use function Amp\call;
+
+class ParsingMiddleware implements Middleware, ServerObserver
+{
+    /** @var BufferingParser */
+    private $parser;
+
+    /** @var ErrorHandler */
+    private $errorHandler;
+
+    public function __construct(int $fieldCountLimit = null)
+    {
+        $this->parser = new BufferingParser($fieldCountLimit);
+    }
+
+    public function handleRequest(Request $request, RequestHandler $requestHandler): Promise
+    {
+        return call(function () use ($request, $requestHandler) {
+            try {
+                $request->setAttribute(Form::class, yield $this->parser->parseForm($request));
+            } catch (ParseException $exception) {
+                return yield $this->errorHandler->handleError(Status::BAD_REQUEST, null, $request);
+            }
+
+            return yield $requestHandler->handleRequest($request);
+        });
+    }
+
+    public function onStart(Server $server): Promise
+    {
+        $this->errorHandler = $server->getErrorHandler();
+        return new Success;
+    }
+
+    public function onStop(Server $server): Promise
+    {
+        return new Success;
+    }
+}