From f48340039f5e0f4e571dd26371576e1facec8d5b Mon Sep 17 00:00:00 2001
From: Fred Hebert <mononcqc@ferd.ca>
Date: Fri, 3 Jul 2015 15:18:28 +0000
Subject: Run agent as current process & hibernate

This tries to reduce memory usage when running `rebar3 shell` by running
the agent in the current process (and avoiding to copy state
cross-boundaries), and using frequent hibernation after each run to
force a full GC and compaction of the current process.
---
 src/rebar_agent.erl     |  4 ++--
 src/rebar_prv_shell.erl | 12 +++++++++---
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/rebar_agent.erl b/src/rebar_agent.erl
index 24ac626..dc45dcf 100644
--- a/src/rebar_agent.erl
+++ b/src/rebar_agent.erl
@@ -26,11 +26,11 @@ init(State) ->
 handle_call({cmd, Command}, _From, State=#state{state=RState, cwd=Cwd}) ->
     MidState = maybe_show_warning(State),
     {Res, NewRState} = run(default, Command, RState, Cwd),
-    {reply, Res, MidState#state{state=NewRState}};
+    {reply, Res, MidState#state{state=NewRState}, hibernate};
 handle_call({cmd, Namespace, Command}, _From, State = #state{state=RState, cwd=Cwd}) ->
     MidState = maybe_show_warning(State),
     {Res, NewRState} = run(Namespace, Command, RState, Cwd),
-    {reply, Res, MidState#state{state=NewRState}};
+    {reply, Res, MidState#state{state=NewRState}, hibernate};
 handle_call(_Call, _From, State) ->
     {noreply, State}.
 
diff --git a/src/rebar_prv_shell.erl b/src/rebar_prv_shell.erl
index 84ad723..3c6369a 100644
--- a/src/rebar_prv_shell.erl
+++ b/src/rebar_prv_shell.erl
@@ -90,9 +90,10 @@ shell(State) ->
     %% their application masters never gets the new group leader (held in
     %% their internal state)
     maybe_boot_apps(State),
-    rebar_agent:start_link(State),
-    %% this call never returns (until user quits shell)
-    timer:sleep(infinity).
+    simulate_proc_lib(),
+    true = register(rebar_agent, self()),
+    {ok, GenState} = rebar_agent:init(State),
+    gen_server:enter_loop(rebar_agent, [], GenState, {local, rebar_agent}, hibernate).
 
 info() ->
     "Start a shell with project and deps preloaded similar to~n'erl -pa ebin -pa deps/*/ebin'.~n".
@@ -145,6 +146,11 @@ maybe_boot_apps(State) ->
             boot_apps(Apps)
     end.
 
+simulate_proc_lib() ->
+    FakeParent = spawn_link(fun() -> timer:sleep(infinity) end),
+    put('$ancestors', [FakeParent]),
+    put('$initial_call', {rebar_agent, init, 1}).
+
 setup_name(State) ->
     {Opts, _} = rebar_state:command_parsed_args(State),
     case {proplists:get_value(name, Opts), proplists:get_value(sname, Opts)} of
-- 
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