Reject non-HTTP schemes in StreamHandler (#2989)

* Handle non-HTTP schemes gracefully in StreamHandler

If an URI that does not use the HTTP stream wrapper is passed to the
StreamHandler then the magic `$http_response_header` variable will not be
filled, thus remaining `null`.

This ultimately results in a `TypeError`, because `null` is passed to
HeaderProcessor::parseHeaders(), which expects an `array`.

* Reject non-HTTP schemes in StreamHandler

Non-HTTP schemes are effectively not supported, because the HTTP response
headers will only be filled for the `http` and `https` stream wrappers. Also
Guzzle is an HTTP client after all.

Reject non-HTTP schemes early on to improve error messages and to prevent
possible exploits using odd stream wrappers in case an non-fully-trusted URL is
passed to Guzzle.

src/Handler/StreamHandler.php

@@ -266,6 +266,10 @@ private function createStream(RequestInterface $request, array $options)
             $methods = \array_flip(\get_class_methods(__CLASS__));
         }
 
+        if (!\in_array($request->getUri()->getScheme(), ['http', 'https'])) {
+            throw new RequestException(\sprintf("The scheme '%s' is not supported.", $request->getUri()->getScheme()), $request);
+        }
+
         // HTTP/1.1 streams using the PHP stream wrapper require a
         // Connection: close header
         if ($request->getProtocolVersion() == '1.1'
@@ -318,7 +322,7 @@ static function () use ($context, $params) {
         return $this->createResource(
             function () use ($uri, &$http_response_header, $contextResource, $context, $options, $request) {
                 $resource = @\fopen((string) $uri, 'r', false, $contextResource);
-                $this->lastHeaders = $http_response_header;
+                $this->lastHeaders = $http_response_header ?? [];
 
                 if (false === $resource) {
                     throw new ConnectException(sprintf('Connection refused for URI %s', $uri), $request, null, $context);

tests/Handler/StreamHandlerTest.php

@@ -738,4 +738,19 @@ public function testHandlesInvalidStatusCodeGracefully()
             ]
         )->wait();
     }
+
+    public function testRejectsNonHttpSchemes()
+    {
+        $handler = new StreamHandler();
+
+        $this->expectException(RequestException::class);
+        $this->expectExceptionMessage("The scheme 'file' is not supported.");
+
+        $handler(
+            new Request('GET', 'file:///etc/passwd'),
+            [
+                RequestOptions::STREAM => true,
+            ]
+        )->wait();
+    }
 }