Better eunit failure output with unite and `assertEqual`

Problem: an eunit test was using ?assertMatch over a large datastructure and the output was truncated and impossible to debug. The failure was:

**error:{assertMatch_failed,[{module,riak_kv_qry},
                    {line,428},
                    {expression,"Res"},
                    {pattern,"{ ok , { _ , _ , ExpectedRows } }"},
                    {value,{ok,{[<<"Subquery">>,<<"Coverage Plan">>,
                                 <<"Range Scan Start"...>>,
                                 <<"Is Start Inc"...>>,<<"Range Sc"...>>,
                                 <<"Is E"...>>,<<...>>],
                                [sint64,varchar,varchar,boolean,varchar,
                                 boolean|...],
                                [[1,<<"dev1@127"...>>,<<"c = "...>>,false|...],
                                 [2,<<"dev1"…>>,<<...>>|...]]}}}]}

The test code was:

?assertMatch(
    {ok, {_, _, ExpectedRows}},
    Res),

To debug this we enabled the unite formatter in the rebar.config:

{eunit_opts, [no_tty, {report, {unite_compact, []}}]}.

And changed the test so that ?assertEqual could be used, this allows unite to format the failure as a diff between the expected and actual result.

{ok, {_, _, ActualRows}} = Res,
?assertEqual(ExpectedRows, ActualRows),

In the first line, the result is unpacked and just the part we want to run the assertion is bound to a variable. If this results in a badmatch then hopefully the error will be easy to debug, for example {error, _} will not match {ok, {_,_,_,ActualRows}} and it should be easy to compare the difference.

With the ?assertEqual and unite, we get this failure output:

 1) explain_query_test/0 (src/riak_kv_qry.erl:428)
   Assert equal failed! -Expected- +Actual+
   [[1,<<"[email protected]/0, [email protected]/1, [email protected]/1">>,<<"c = 'hola', b = 1">>,false,
     <<"c = 'hola', b = 1000">>,false,-<<"(((d = 15) OR ((e = true) AND (f = 'adios'))) AND (a = 319))">>-+<<"(((d = 15) OR ((e = true) AND (f = <<\"adios\">>))) AND (a = 319))">>+],
    [2,<<"[email protected]/0, [email protected]/1, [email protected]/1">>,<<"c = 'hola', b = 1000">>,false,
     <<"c = 'hola', b = 2000">>,false,-<<"(((d = 15) OR ((e = true) AND (f = 'adios'))) AND (a = 319))">>-+<<"(((d = 15) OR ((e = true) AND (f = <<\"adios\">>))) AND (a = 319))">>+]]

The expected and actual is shown inline, the expected in blue and the actual in yellow.

andytill/failure-diffs.md