[MINOR] Fix in Python API GatewayServerListener

[e-strauss]

two things:

• the current inner class DMLGateWayListener implements function from the GatewayServerListener interface, which are never invoked by the GatewayServer (since the GatewayServer, which also implements GatewayServerListener, does not implement these methods
• DMLGateWayListener previously called, Sys.exit(), which I think is not correct, since it breaks the proper shutdown of the GatewayServer

I added a new unit case, which checks the functionality of the DMLGateWayListener.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.59%. Comparing base (cfbe190) to head (c177509).
Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2243      +/-   ##
============================================
+ Coverage     72.47%   72.59%   +0.12%     
- Complexity    45466    45573     +107     
============================================
  Files          1469     1469              
  Lines        170893   171132     +239     
  Branches      33325    33377      +52     
============================================
+ Hits         123851   124236     +385     
+ Misses        37632    37490     -142     
+ Partials       9410     9406       -4     

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[Baunsgaard]

Hi @e-strauss ,

We added the shutdown calls to enforce that the Java process would terminate in case the Python API terminated. It does violate the default shutdown process since any live connections would be forcefully terminated. Because only one Python process is guaranteed to be using this listener, the design was valid.

But I do see your points, and I like the PR. I just want to verify the Python API startup and shutdown time of the change before merging.

[e-strauss]

@Baunsgaard ok gotcha. But from what I saw during my tests, is that the 4 methods that I removed, were unreachable, since the server didn't forwarded the events to the listener. the only signal that was forwarded was the shutdown signal