What changes were proposed in this pull request?

SSE support for Knox.

Currently Knox is not working with SSE. In case of SSE the response header contains Content-Type = text/event-stream, in which case the server will send messages terminated by \n\n. Knox should send these messages to the client as they arrive. Currently Knox collects them all, and once the server closes the connection sends all of them concatenated to each other to the client.

Requirement:

Full support in Knox for Server Sent Events.
Definition of done:

Clients can connect to SSE endpoints in the backend via Knox.
Server Sent Events (https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events) are forwarded by Knox from the server to the client as soon as they are received.

How was this patch tested?

Unit tests.
Locally tested with a custom SSE service.
Any SSE service that is the follows the specs is supported in Knox.
Setup:
Asynchronous behaviour and therefore SSE support is not enabled by default. Modify the below config to ‘true’ in /conf/gateway-site.xml file.

<property>
   <name>gateway.servlet.async.supported</name>
   <value>true</value>
   <description>Enable/Disable async support.</description>
</property>

Change the existing topology by adding an SSE service to /conf/{topology}.xml

<service>
   <role>SSERVICE</role>
   <url>http://localhost:7435</url>
</service>

Add the SSE service to the <KNOX_HOME>/data/services folder.

/data/services/{myservice}/{version}/rewrite.xml

<rules>
   <rule dir="IN" name="SSERVICE/sservice/inbound" pattern="*://*:*/**/sservice/*">
       <rewrite template="{$serviceUrl[SSERVICE]}/sse"/>
   </rule>
</rules>

/data/services/{myservice}/{version}/service.xml (Make sure to add the SSEDispatch to the service)

<service role="SSERVICE" name="sservice" version="0.1">
   <routes>
       <route path="/sservice/**"></route>
   </routes>
   <dispatch classname="org.apache.knox.gateway.sse.SSEDispatch"/>
</service>

curl command to test the configured SSE service:
curl -k -i -u admin:admin-password https://localhost:8443/gateway/sandbox/sservice/sse

GET, POST, PUT, PATCH methods were tested.