Since this will be embedded within MediaWiki, changes are we can reuse existing MediaWiki code for this.

See OutputHandler::handleGzip().

This probably makes more sense as a subtask of T221737: REST API Infrastructure in MediaWiki now.

Gzipped responses (Content-Encoding: gzip) are already handled by virtue of it using existing MediaWiki infrastructure. Note if testing locally you may have to configure PHP to have output_buffering = off for MediaWiki.

There's no HTTP standard for gzipped request bodies, as far as I know. Is there a standard?

In T221167#5642481, @Anomie wrote:

There's no HTTP standard for gzipped request bodies, as far as I know. Is there a standard?

Apart of RFC2616 that states If the content-coding of an entity in a request message is not acceptable to the origin server, the server SHOULD respond with a status code of 415 (Unsupported Media Type). I'm not aware of any standard.

Also, it states Request and Response messages MAY transfer an entity if not otherwise restricted by the request method and Content-Encoding being listed as a option in entity-header

So, standard-wise, we are allowed to accept gzipped request bodies with content-encoding header. I'm not aware of any browsers that do it, but it seems like a cool feature to have for an API.

@Anomie when you say "already handled", do you mean that it happens automatically, without our handler classes having to call any specific code? Or do you mean there's already a service to use within MW that our handler classes need to call?

The handler classes don't have to do anything special. MediaWiki sets it up in wfWebStartSetup(), assuming PHP hasn't already started output buffering for some reason.

$ curl --compressed -v http://mywiki.local.wmftest.net/w/rest.php/user/Anomie/hello
*   Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to mywiki.local.wmftest.net (127.0.0.1) port 80 (#0)
> GET /w/rest.php/user/Anomie/hello HTTP/1.1
> Host: mywiki.local.wmftest.net
> User-Agent: curl/7.66.0
> Accept: */*
> Accept-Encoding: deflate, gzip, br
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Date: Fri, 08 Nov 2019 18:19:48 GMT
< Server: Apache/2.4.41 (Debian)
< X-Content-Type-Options: nosniff
< P3P: CP="This is not a P3P policy! See http://mywiki.local.wmftest.net/wiki/Special:CentralAutoLogin/P3P for more info."
< Content-Encoding: gzip
< Vary: Accept-Encoding
< Transfer-Encoding: chunked
< Content-Type: application/json
< 
* Connection #0 to host mywiki.local.wmftest.net left intact
{"message":"Hello, Anomie!"}

Note the response includes Content-Encoding: gzip.

Great. That looks good to me. I'll confirm locally and then close this ticket.