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== Migrating from Cowboy 2.1 to 2.2

Cowboy 2.2 focused on adding features required for writing
gRPC servers and on completing test suites for the core
HTTP RFCs, fixing many bugs along the way.

=== Features added

* Add support for sending trailers at the end of response bodies.
  Trailers are additional header fields that may be sent after the
  body to add more information to the response. Their usage is
  required in gRPC servers. They are optional and may be discarded
  in other scenarios (for example if the request goes through an
  HTTP/1.0 proxy, as HTTP/1.0 does not support trailers).

* The `max_skip_body_length` option was added to `cowboy_http`.
  It controls how much of a request body Cowboy is willing to skip
  when the handler did not touch it. If the remaining body size is
  too large Cowboy instead closes the connection. It defaults to 1MB.

* The CONNECT and TRACE methods are now rejected as they are
  currently not implemented and must be handled differently than
  other methods. They will be implemented in a future release.

=== New functions

* The function `stream_trailers/2` has been added. It terminates
  a stream and adds trailer fields at the end of the response. A
  corresponding stream handler command `{trailers, Trailers}`
  has also been added.

=== Bugs fixed

* Test suites for the core HTTP RFCs RFC7230, RFC7231 and RFC7540
  have been completed. Many of the bugs listed here were fixed as
  a result of this work.

* Many HTTP/2 edge cases when clients are misbehaving have been
  corrected. This includes many cases where the request is malformed
  (for example when a pseudo-header is present twice).

* When the HTTP/2 SETTINGS_INITIAL_WINDOW_SIZE value changes,
  Cowboy now properly updates the flow control windows.

* HTTP/2 could mistakenly log stray messages that actually were
  expected. This is no longer the case.

* We no longer send a GOAWAY frame when the HTTP/2 preface is invalid.

* Some values in the Req object of pushed requests were in the
  wrong type. They are now the expected binary instead of iolist.

* A response body was sometimes sent in response to HEAD requests
  when using HTTP/2. The body is now ignored.

* The `max_headers` option for `cowboy_http` was not always respected
  depending on the contents of the buffer. The limit is now strict.

* When an early error occurred on the HTTP/1.1 request line, the
  partial Req given to stream handlers was missing the `ref` and
  `peer` information. This has been corrected.

* Absolute URIs with a userinfo component, or without an authority
  component, are now properly rejected for HTTP/1.0 and HTTP/1.1.

* Whitespace is no longer allowed in header lines before the colon.

* 408 responses to HTTP/1.1 requests now properly include a
  connection: close header indicating that we are going to
  close the connection. This header will also be sent for
  other early errors resulting in the closing of the connection.

* When both the transfer-encoding and content-length headers are
  sent in an HTTP/1.1 request, the transfer-encoding now takes
  precedence over the content-length header and the latter is
  removed from the Req object.

* A 400 response is now returned when the transfer-encoding
  header is invalid or contains any transfer-coding other
  than chunked.

* Invalid chunk sizes are now rejected immediately.

* Chunk extensions are now limited to 129 characters. They are
  not used in practice and are still ignored by Cowboy. The limit
  is not configurable.

* The final chunk was mistakenly sent in responses to HEAD
  requests. This is now corrected.

* `OPTIONS *` requests were broken in Cowboy 2.0. They are now
  working again. Both the routing and `cowboy_req:uri/1,2` have
  been corrected.

* 204 responses no longer include a content-length header.

* A packet could be lost when switching to Websocket or any
  other protocol via the `switch_protocol` command. This is
  now fixed.

* A 426 response will now be sent when a handler requires
  the client to upgrade to Websocket and the request did not
  include the required headers.

* Both experimental stream handlers `cowboy_metrics_h` and
  `cowboy_tracer_h` received a number of fixes and improvements.