[Playground] Start Google Cloud Datastore emulator from Go tests automatically

[TSultanov]

Start Google Datastore emulator from Go tests automatically

resolves #25588

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[damondouglas]

Usually in go it makes sense to evaluate one error at a time. I wonder if it makes sense to instead call Client.Close and emulator.Stop within the test code itself, or some shared teardown method used by all the tests.

[TSultanov]

I think it would just move the same problem but to a different place - any error during Client.Close() code will prevent a call to emulator.Stop().

[damondouglas]

It may make your code cleaner and easy to troubleshoot if you receive a ctx context.Context and use the pattern:

ctx, cancel := context.WithCancel(ctx)
defer cancel()

The test could perhaps do:

ctx, cancel := context.WithTimeout(...)

Would you like me to create an example? Please let me know.

[TSultanov]

If I understand you correctly, you basically are suggesting to use context cancellation to kill the emulator process, right?
I've tried this approach, but sadly it doesn't work that well as cancelling the context only terminates the first process in the child process tree (starting the datastore emulator creates at least three processes), and the actual emulator process gets reparented by the init. The current code works around this problem by creating a process group out of all emulator processes and sending the SIGTERM to the whole group.

[damondouglas]

Perhaps these errors could be collected in chan error and handled like:

for {
   select {
      case err := <- clientErr:
           // gracefully shut down other processes
           return err
     case err := <- datastoreDbErr:
           // gracefully shut down other processes
           return err
   }
}

[TSultanov]

We don't have any other processes here and I don't think there is much merit in complicating the error handling situation here with channels. Current code, while not exactly following Go conventions to the letter, is cleaner in my opinion.

[damondouglas]

@damccorm LGTM as MVP; needs clean up later