Fix mango native proc crash

[eiri]

Overview

When soft limit is reached couch_proc_manager evicts idle processes by casting on them stop
message. Since mango_native_proc doesn't handle this message it results to its crash with
invalid_cast reason.

Testing recommendations

make eunit apps=mango or make eunit apps=mango suites=mango_native_proc for isolated test.

make mango-test for integration test.

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[tonysun83]

So iiuc, when the spawned process starts garbage collection, it'll exit with gc_start which would then cause our mango_native_proc to exit with an {invalid_cast, gc_start}

[eiri]

No, no, it's different.

We spawn and monitor a new process that sets a trace on the test process, because process can't trace itself. Then in the test process we call a garbage collection by executing handle_cast(garbage_collect, []) and asserting that we are getting proper noreply response. The tracer receives gc_start trace message, indicating that its target (the test process) started a garbage collection and exits with reason gc_start. Because we are monitoring it in our test process, we are receiving its exit reason and can assert that it was indeed for a garbage collection.

Interesting bit here is erlang:yield() - we need to give a scheduler a chance to run the tracer process or gc will be finished before the trace set.

Conceptually it's is not different from when we are mocking module with meck and then waiting for some of its functions to be called, but I had to use trace here because you can't mock garbage_collection. Plus, unlike mock, this is a real deal, we are checking actual, not simulated functionality.

[tonysun83]

@eiri : I completely forgot to check that we have handle_cast(garbage_collect, St) in the source. That's what confused me. This test was for that functionality. Got it!

[tonysun83]

The fix lgtm. The garbage collection test is a little tricky, just want to understand it a bit better.

[eiri]

Hey @tonysun83 , was my explanation sufficient? Do you want me to change anything in the tests?

[tonysun83]

+1