improve behaviour of rebar shell

attempt to emulate the behavior of
`erl -pa ebin -pa deps/*/ebin`

fix error messages and formatting issues of `rebar shell` by
shutting down and restarting the user subsystem in a mode more
hospitable to the shell than the simple user started when run
as an escript. emulate `error_logger` behaviour when the shell
is run via `erl`

add documentation of the shell command

limitations:

the erlang interrupt handler is not enabled when running as an
escript and there is no interface to re-enable it via erlang code.
this means `ctrl-c` will immediately exit the running process
unlike when running the shell via `erl`. `ctrl-g` is, however,
unaffected

the user subsystem is killed and restarted but not supervised. if
your code somehow relies on the user subsystem crashing and
restarting `rebar shell` may interfere with it's operation

THANKS

@@ -123,3 +123,4 @@ Dave Thomas
 Evgeniy Khramtsov
 YeJun Su
 Yuki Ito
+alisdair sullivan

src/rebar.erl

@@ -409,6 +409,9 @@ qc                                       Test QuickCheck properties
 
 xref                                     Run cross reference analysis
 
+shell                                    Start a shell similar to
+                                         'erl -pa ebin -pa deps/*/ebin'
+
 help                                     Show the program options
 version                                  Show version information
 ">>,

src/rebar_shell.erl

@@ -30,27 +30,40 @@
 
 -include("rebar.hrl").
 
--export([shell/2]).
+-export([shell/2, info/2]).
+
+%% NOTE:
+%% this is an attempt to replicate `erl -pa ./ebin -pa deps/*/ebin`. it is
+%% mostly successful but does stop and then restart the user io system to get
+%% around issues with rebar being an escript and starting in `noshell` mode.
+%% it also lacks the ctrl-c interrupt handler that `erl` features. ctrl-c will
+%% immediately kill the script. ctrl-g, however, works fine
 
 shell(_Config, _AppFile) ->
-    ?CONSOLE("NOTICE: Using experimental 'shell' command~n", []),
+    true = code:add_pathz(rebar_utils:ebin_dir()),
-    %% backwards way to say we only want this executed
+    %% terminate the current user
-    %% for the "top level" directory
+    ok = supervisor:terminate_child(kernel_sup, user),
-    case is_deps_dir(rebar_utils:get_cwd()) of
+    %% start a new shell (this also starts a new user under the correct group)
-        false ->
+    user_drv:start(),
-            true = code:add_pathz(rebar_utils:ebin_dir()),
+    %% enable error_logger's tty output
-            user_drv:start(),
+    ok = error_logger:swap_handler(tty),
-            %% this call never returns (until user quits shell)
+    %% disable the simple error_logger (which may have been added multiple
-            shell:server(false, false);
+    %% times). removes at most the error_logger added by init and the
-        true ->
+    %% error_logger added by the tty handler
-            ok
+    ok = remove_error_handler(3),
-    end,
+    %% this call never returns (until user quits shell)
-    ok.
+    timer:sleep(infinity).
 
-is_deps_dir(Dir) ->
+info(help, shell) ->
-    case lists:reverse(filename:split(Dir)) of
+    ?CONSOLE(
-        [_, "deps" | _] ->
+       "Start a shell with project and deps preloaded similar to~n"
-            true;
+       "'erl -pa ebin -pa deps/*/ebin'.~n",
-        _V ->
+       []).
-            false
+
-    end.
+remove_error_handler(0) ->
+    ?WARN("Unable to remove simple error_logger handler~n", []);
+remove_error_handler(N) ->
+    case gen_event:delete_handler(error_logger, error_logger, []) of
+        {error, module_not_found} -> ok;
+        {error_logger, _} -> remove_error_handler(N-1)
+    end.