Export access to original function in passthrough mode

[slfritchie]

Hi, sorry I didn't cook up a pull request the usual way. This is a really small change, but it's tremendously useful. When a module is defined in passthrough mode, when doing the following:

meck:new(foo, [passthrough]),
meck:expect(foo, bar, fun(X) case random:uniform(100) of
    N when N < 50 ->
        %% somehow call the original foo:bar/1 function but with
        %% the same X or perhaps a modified X.
    _ ->
        %% Do something completely novel, like we usually
        %% do with Meck.
    end).

Using this new function, we could put the following into the first case clause:

meck:orig_apply(foo, bar, [X]);

or perhaps:

meck:orig_apply(foo, bar, [modify_somehow(X)]);

The small patch is:

diff --git a/src/meck.erl b/src/meck.erl
index 1669c32..6f8a577 100644
--- a/src/meck.erl
+++ b/src/meck.erl
@@ -40,6 +40,7 @@
 -export([called/4]).
 -export([num_calls/3]).
 -export([num_calls/4]).
+-export([orig_apply/3]).

 %% Callback exports
 -export([init/1]).
@@ -455,6 +456,9 @@ proc_name(Name) -> list_to_atom(atom_to_list(Name) ++ "_meck").

 original_name(Name) -> list_to_atom(atom_to_list(Name) ++ "_meck_original").

+orig_apply(Name, Func, Args) ->
+    erlang:apply(original_name(Name), Func, Args).
+
 wait_for_exit(Mod) ->
     MonitorRef = erlang:monitor(process, proc_name(Mod)),
     receive {'DOWN', MonitorRef, _Type, _Object, _Info} -> ok end.

Thanks for considering. I'm happily using this new function for nefarious testing purposes. The rest of Meck is quite useful, thanks for all of your work.

[eproxus]

Thanks! The passthrough/1 function that already exists already allows you to call the original function. However, it had one limitation that it exited the expect-function when it was called. This is now changed in c336ce1a63997c14a27776d019afe715e0ae24e1 (based on the solution in your patch) where you can also use the return value inside the expect-function:

meck:expect(m, f, fun(N) -> M = meck:passthrough([N+1]), M / 2 end).

(This also closes the old #2 issue on the same topic).