Don't send response chunk for HEAD request

[mikkokar]

A StyxProxySpec "should respond to HEAD with bodiless response" is failing intermittently. The root cause is a combination of slightly incorrect usage of Netty combined with asynchronous behaviour of the e2e tests.

The test case fails because Netty HttpResponseEncoder leaks (to the network) an empty content chunk whenever Styx responds to a HTTP request. For example:

08:17:58.984 ProxySpecLog [scalatest-e2e-client-Client-Worker-0-Thread] DEBUG i.n.handler.logging.LoggingHandler - [id: 0xe3c37f78, L:/127.0.0.1:40064 - R:localhost/127.0.0.1:8080] READ: 189B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 54 54 50 2f 31 2e 31 20 32 30 30 20 4f 4b 0d |HTTP/1.1 200 OK.|
|00000010| 0a 53 65 72 76 65 72 3a 20 4a 65 74 74 79 28 36 |.Server: Jetty(6|
|00000020| 2e 31 2e 32 36 29 0d 0a 58 2d 53 74 79 78 2d 4f |.1.26)..X-Styx-O|
|00000030| 72 69 67 69 6e 2d 49 64 3a 20 67 65 6e 65 72 69 |rigin-Id: generi|
|00000040| 63 2d 61 70 70 2d 30 31 0d 0a 56 69 61 3a 20 31 |c-app-01..Via: 1|
|00000050| 2e 31 20 73 74 79 78 0d 0a 58 2d 53 74 79 78 2d |.1 styx..X-Styx-|
|00000060| 49 6e 66 6f 3a 20 6e 6f 4a 76 6d 52 6f 75 74 65 |Info: noJvmRoute|
|00000070| 53 65 74 3b 63 63 31 64 35 37 64 33 2d 36 33 31 |Set;cc1d57d3-631|
|00000080| 38 2d 31 31 65 38 2d 38 66 31 36 2d 35 32 35 34 |8-11e8-8f16-5254|
|00000090| 30 30 37 61 36 39 35 61 0d 0a 74 72 61 6e 73 66 |007a695a..transf|
|000000a0| 65 72 2d 65 6e 63 6f 64 69 6e 67 3a 20 63 68 75 |er-encoding: chu|
|000000b0| 6e 6b 65 64 0d 0a 0d 0a 30 0d 0a 0d 0a          |nked....0....   |
+--------+-------------------------------------------------+----------------+

The last 0.... in above log represents the leaked content chunk. The test case fails, specifically, when the leaked chunk arrives at the receiving end (a test HTTP client at StyxProxySpec) before a test client has started decoding the response message.

• The test pass when the test client had started decoding the HTTP response before the leaked 0... arrives. In this case the response is decoded correctly and the test case validation completes.

• The test fails when HTTP response decoding starts after the 0... had arrived. In this case the 0... is decoded together with the response and treated as a beginning of the next message. This causes a decoding failure due to invalid HTTP version format, and the decoder throws a Netty Channel Exception resulting in a test failure.

The 0... is leaked to the network because Styx had separate HttpResponseEncoder and HttpRequestDecoder codecs in the channel pipeline. The separate codecs are unaware of each other and therefore the response encoder cannot know it is responding to a HEAD request.

The fix is to use HttpServerCodec instead. It is aware of the HEAD response and prevents extra content chunks from being sent.