Uncommon Things for Common Tests

As far as the Erlang testing goes, here are some snippets I’ve collected over the years with the great help of my teammates (shout out to the Regulators crew!).

First things first

At least if you’re using rebar everyone is so keen about, you can put your helper libs in the CT test directory. I usually make one called test_helpers and alias it with ?th macro.

Run a single Common Test case via Makefile

It greatly improves your workflow, especially if the whole local build takes up to 30 minutes. Here’s a make target ready to use:

	@mkdir -p ct_log
	@if [ -z "$(suite)" ] || [ -z "$(case)" ]; then \
		echo "Provide args. e.g. suite=moo case=boo"; exit 1; \
		else true; fi
	@ct_run -noshell -pa deps/*/ebin -pa apps/*/ebin -include apps/*/include \
		-include deps/*/include -sname test -logdir ct_log -ct_hooks cth_surefire \
		-sasl sasl_error_logger false \
		-dir $(dir $(shell find . -name $(suite)_SUITE.erl)) -suite $(suite)_SUITE -case $(case)

Of course this won’t work if you’re using CT groups. That can be tweaked with the -group switch.

Common fixtures directory

The CT-enforced convention is to have one data directory per suite. We were fed up with that; reusing fixtures is usually the bread and butter of medium/large systems.

set_fixt_dir(Test, Config) ->
    case code:which(Test) of
        Filename when is_list(Filename) ->
            CommonDir = filename:dirname(Filename) ++ "/fixtures/",
            [{common_data_dir, CommonDir}|Config]

load_fixt(Config, Filename) ->
    F = filename:join(?config(common_data_dir, Config), Filename),
    {ok, Fixture} = file:read_file(F),

So the way you call it in your suites is:

init_per_suite(Config) ->
    % fiddling with application:env and all the mocking tyranny...
    Config2 = ?th:set_fixt_dir(?MODULE, Config),


t_do_what_i_tell_you(Config) ->
    BinData = ?th:load_fixt(Config, "data.raw"),
    % ...

Exporting all the tests

Have you noticed the t_NAME convention for a test case I used in the previous paragraph? This comes in handy when you choose not to use groups.

all_tests(Module) ->
      fun({F, _}) ->
              Name = atom_to_list(F),
              case Name of
                  [$t,$_|_] -> {true, F};
                  _ -> false
      end, Module:module_info(exports)).

And then, in CT all/0 callback:

all() ->

This way you don’t need to needlessly repeat yourself in all/0 and -export/0. Just export the tests you wish to run, and the compiler will warn you about the unexported ones.

Debug calls

Let’s assume, for the purpose of the following example, that your system has become too hard to reason about, especially when it comes to testing async routines. First of all, dbg works here too.

start_debug_modules(Mods) when is_list(Mods) ->
    lists:foreach(fun(M) -> dbg:tp(M, '_', '_', []) end, Mods),
    dbg:p(all, c).

stop_debug_modules() ->

That’s all you need in your helpers library. Call that on setup/teardown with the modules of interest/hate as an input to get things moving.

Sleep is the cousin of death

When testing e2e async routines, people often use timer:sleep/1 and similar to “wait for an output”. This is wrong due to the fact that any kind of sleep is non deterministic. In other words it will behave incalculably on different test environments - example: the Bongo DB on your ssd omgmacbook is way faster than the one on the CI box. If you really need to wait for an output, please keep trying to wait instead. Consider the following functions in your helpers module:

keep_trying(Match, F, Sleep, Tries) ->
        case F() of
            Match      -> ok;
            Unexpected -> throw({unexpected, Unexpected})
        _:_=E ->
            case Tries-1 of
                0 -> error(E);
                N -> keep_trying(Match, F, Sleep, N)

keep_trying_receive(Match, Sleep, Tries) ->
    keep_trying(Match, fun() -> receive Msg -> Msg
                                after Sleep -> error
                0, Tries).

Comparing JSON dudes

Dure to different circumstance I have never tried maps. We often need to put some JSON somewhere and check if the output matches a JSON elsewhere. We have written a function that tries to deeply compare two lists of tuples at least as far as jsx is concerned.

assert_deep_love(Expected, Got) ->
    compare(lists:sort(Expected), lists:sort(Got)).

compare(X, X) ->
compare([], []) ->
compare(Obj1, []) ->
    {false, {obj1, Obj1}};
compare([], Obj2) ->
    {false, {obj2, Obj2}};
compare([{K, V}|Obj1], [{K, V}|Obj2]) ->
    compare(Obj1, Obj2);
compare([{K, V1}|Obj1] , [{K, V2}|Obj2]) ->
    case compare_values(V1, V2) of
        true ->
            compare(Obj1, Obj2);
        {false, E} ->
            {false, {key, K, E}}
compare([V1|L1], [V2|L2]) ->
    case compare_values(V1, V2) of
        true ->
            compare(L1, L2);
        {false, E} ->
            {false, {list, E}}

compare_values([{_,_}|_] = Obj1, [{_,_}|_] = Obj2) ->
    assert_deep_love(Obj1, Obj2);
compare_values(L1, L2)
  when is_list(L1) and is_list(L2) ->
    assert_deep_love(L1, L2);
compare_values(V1, V2) ->
    {false, { {expected, V1}, {got, V2} }}.

Put that in your helpers module and benefit from it.

That’s all folks

Ping me if you need me!