Subscription refactor

[smyrick]

📝 Description

Refactor of the websocket logic to keep sending the keep alive messages until the client calls connection terminate.

Clients can open a connection with GQL_CONNECTION_INIT but never call GQL_START. The keep alive messages from the server should still be sent in that case, which also means we don't need to save the websocket session. We can just stop the session only on GQL_CONNECTION_TERMINATE

This also cleans up the keep alive connections after GQL_STOP is sent by the client or the server sends GQL_CONNECTION_ERROR

https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md

🔗 Related Issues

• fix subscriptions by not removing the ak subscription when one of the… #510
• Subscription stops working doing automated tests #499

[codecov-io]

Codecov Report

Merging #512 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #512      +/-   ##
============================================
+ Coverage      97.9%   97.98%   +0.08%     
  Complexity      335      335              
============================================
  Files           106      106              
  Lines          1290     1292       +2     
  Branches        207      206       -1     
============================================
+ Hits           1263     1266       +3     
  Misses            7        7              
+ Partials         20       19       -1

Impacted Files	Coverage Δ	Complexity Δ
...ing/execution/ApolloSubscriptionProtocolHandler.kt	96.61% <100%> (+1.87%)	12 <0> (ø)	⬇️

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