[GitHub] eiri commented on a change in pull request #1067: Fix mango native proc crash

[GitBox <gi...@apache.org>]

eiri commented on a change in pull request #1067: Fix mango native proc crash
URL: https://github.com/apache/couchdb/pull/1067#discussion_r157477887
 
 

 ##########
 File path: src/mango/src/mango_native_proc.erl
 ##########
 @@ -363,4 +366,35 @@ validate_index_info(IndexInfo) ->
             [invalid_index | Results0]
         end
     end, [], IdxTypes),
-    lists:member(valid_index, Results).
\ No newline at end of file
+    lists:member(valid_index, Results).
+
+
+-ifdef(TEST).
+
+-include_lib("eunit/include/eunit.hrl").
+
+handle_garbage_collect_cast_test() ->
+    Pid = self(),
+    {_, TracerRef} = spawn_monitor(fun() ->
+        erlang:trace(Pid, true, [garbage_collection]),
+        receive {trace, Pid, gc_start, _} ->
+            erlang:trace(Pid, false, [garbage_collection]),
+            exit(gc_start)
+        end
+    end),
+    erlang:yield(),
+    ?assertEqual({noreply, []}, handle_cast(garbage_collect, [])),
+    receive
+        {'DOWN', TracerRef, _, _, Msg} -> ?assertEqual(gc_start, Msg)
 
 Review comment:
   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.
   

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