Adds documentation for the new ContentLengthMiddleware

Per zendframework/zend-expressive-helpers#45

doc/book/features/helpers/content-length.md

@@ -0,0 +1,79 @@
+# Content-Length Middleware
+
+In some cases, you may want to include an explicit `Content-Length` response
+header, without having to inject it manually. To facilitate this, we provide
+`Zend\Expressive\Helper\ContentLengthMiddleware`.
+
+> ### When to use this middleware
+>
+> In most cases, you do not need to provide an explicit Content-Length value
+> in your responses. While the HTTP/1.1 specification indicates the header
+> SHOULD be provided, most clients will not degrade to HTTP/1.0 if the header
+> is omitted.
+>
+> The one exception that has been reported is when working with
+> [New Relic](https://newrelic.com), which requires valid `Content-Length`
+> headers for some of its analytics; in such cases, enabling this middleware
+> will fix those situations.
+
+This middleware delegates the request, and operates on the returned response. It
+will return a new response with the `Content-Length` header injected under the
+following conditions:
+
+- No `Content-Length` header is already present AND
+- the body size is non-null.
+
+To register it in your application, you will need to do two things: register the
+middleware with the container, and register the middleware in either your
+application pipeline, or within routed middleware.
+
+To add it to your container, add the following configuration:
+
+```php
+// In a `config/autoload/*.global.php` file, or a `ConfigProvider` class:
+
+use Zend\Expressive\Helper;
+
+return [
+    'dependencies' => [
+        'invokables' => [
+            Helper\ContentLengthMiddleware::class => Helper\ContentLengthMiddleware::class,
+        ],
+    ],
+];
+```
+
+To register it as pipeline middleware to execute on any request:
+
+```php
+// In `config/pipeline.php`:
+
+use Zend\Expressive\Helper;
+
+$app->pipe(Helper\ContentLengthMiddleware::class);
+```
+
+To register it within a routed middleware pipeline:
+
+```php
+// In `config/routes.php`:
+
+use Zend\Expressive\Helper;
+
+$app->get('/download/tarball', [
+    Helper\ContentLengthMiddleware::class,
+    Download\Tarball::class,
+], 'download-tar');
+```
+
+## Caveats
+
+One caveat to note is that if you use this middleware, but also write directly
+to the output buffer (e.g., via a `var_dump`, or if `display_errors` is on and
+an uncaught error or exception occurs), the output will not appear as you
+expect. Generally in such situations, the contents of the output buffer will
+appear, up to the specified `Content-Length` value. This can lead to truncated
+error content and/or truncated application content.
+
+We recommend that if you use this feature, you also use a PHP error and/or
+exception handler that logs errors in order to prevent truncated output.

doc/book/features/helpers/intro.md

@@ -6,6 +6,8 @@ may integrate features or simply provide standalone benefits.
 
 Currently, these include:
 
+- [Body Parsing Middleware](body-parse.md)
+- [Content-Length Middleware](content-length.md)
 - [UrlHelper](url-helper.md)
 - [ServerUrlHelper](server-url-helper.md)
 

mkdocs.yml

@@ -40,6 +40,7 @@ pages:
             - UrlHelper: features/helpers/url-helper.md
             - ServerUrlHelper: features/helpers/server-url-helper.md
             - 'Body Parsing Middleware': features/helpers/body-parse.md
+            - 'Content-Length Middleware': features/helpers/content-length.md
         - Emitters: features/emitters.md
     - Cookbook:
         - 'Autowiring routes and pipeline middleware': cookbook/autowiring-routes-and-pipelines.md