Abuse error_handler to get free metacalls in r3

This uses the very risky '$handle_undefined_function'/2 export from the
r3 and rebar_agent modules to allow meta-calls that can support plugins
and all other rebar3 extensions.

This is nasty but very tempting. Currently we only support:

- r3:do(Command)
- r3:do(Namespace, Command)

There is currently no way to pass arguments to the function such that we
can, for example, run cover analysis or tests on a subset of suites.

With the new abuse of '$handle_undefined_function'/2, we can detect the
unused commands (since they are not exported) and re-route them:

- r3:Command()
- r3:Command("--args=as a string")
- r3:Command(Namespace, "--args=as a string")

Of course, in doing so, we make it impossible to use the 'do' provider
(as in 'rebar3 do ct -c, cover') since the 'do' function is already
required for things to work.

Since the previous function had very strict guards, we can, without
conflict, add manual overrides that simulate the meta-calls fine.

Sample run:
https://gist.github.com/ferd/2c06d59c7083c146d25e4ee301de0073

1 files changed, 5 insertions, 0 deletions

diff --git a/src/r3.erl b/src/r3.erl
index d0d6c47..bbf9eea 100644
--- a/src/r3.erl
+++ b/src/r3.erl
@@ -2,6 +2,7 @@
 %%% calls from a shell.
 -module(r3).
 -export([do/1, do/2]).
+-export(['$handle_undefined_function'/2]).
 
 %% @doc alias for `rebar_agent:do/1'
 -spec do(atom()) -> ok | {error, term()}.
@@ -10,3 +11,7 @@ do(Command) -> rebar_agent:do(Command).
 %% @doc alias for `rebar_agent:do/2'
 -spec do(atom(), atom()) -> ok | {error, term()}.
 do(Namespace, Command) -> rebar_agent:do(Namespace, Command).
+
+%% @private defer to rebar_agent
+'$handle_undefined_function'(Cmd, Args) ->
+    rebar_agent:'$handle_undefined_function'(Cmd, Args).