From 69e009e3e1ad899a4609ff327a08512c86dba374 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@plataformatec.com.br>
Date: Thu, 22 Jun 2017 19:54:26 +0200
Subject: Add {continue, Term} and handle_continue/2 to gen_server

If the gen_server process needs to perform an action immediately
after initialization or to break the execution of a callback into
multiple steps, it can return {continue, term()} in place of the
time-out or hibernation value, which will immediately invoke the
handle_continue/2 callback with the given term.

This provides a more accessible approach to after initialization
compared to proc_lib+enter_loop that is also guaranteed to be
safe.

It also allows callbacks that need to do lengthy or stateful work
to checkpoint the state throughout multiple iterations. This can
be useful, for example, when implementing behaviours on top of
gen_server.
---
 lib/stdlib/src/gen_server.erl | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

(limited to 'lib/stdlib/src')

diff --git a/lib/stdlib/src/gen_server.erl b/lib/stdlib/src/gen_server.erl
index a3d53efd0d..48ef1c987a 100644
--- a/lib/stdlib/src/gen_server.erl
+++ b/lib/stdlib/src/gen_server.erl
@@ -116,23 +116,27 @@
 %%%=========================================================================
 
 -callback init(Args :: term()) ->
-    {ok, State :: term()} | {ok, State :: term(), timeout() | hibernate} |
+    {ok, State :: term()} | {ok, State :: term(), timeout() | hibernate | {continue, term()}} |
     {stop, Reason :: term()} | ignore.
 -callback handle_call(Request :: term(), From :: {pid(), Tag :: term()},
                       State :: term()) ->
     {reply, Reply :: term(), NewState :: term()} |
-    {reply, Reply :: term(), NewState :: term(), timeout() | hibernate} |
+    {reply, Reply :: term(), NewState :: term(), timeout() | hibernate | {continue, term()}} |
     {noreply, NewState :: term()} |
-    {noreply, NewState :: term(), timeout() | hibernate} |
+    {noreply, NewState :: term(), timeout() | hibernate | {continue, term()}} |
     {stop, Reason :: term(), Reply :: term(), NewState :: term()} |
     {stop, Reason :: term(), NewState :: term()}.
 -callback handle_cast(Request :: term(), State :: term()) ->
     {noreply, NewState :: term()} |
-    {noreply, NewState :: term(), timeout() | hibernate} |
+    {noreply, NewState :: term(), timeout() | hibernate | {continue, term()}} |
     {stop, Reason :: term(), NewState :: term()}.
 -callback handle_info(Info :: timeout | term(), State :: term()) ->
     {noreply, NewState :: term()} |
-    {noreply, NewState :: term(), timeout() | hibernate} |
+    {noreply, NewState :: term(), timeout() | hibernate | {continue, term()}} |
+    {stop, Reason :: term(), NewState :: term()}.
+-callback handle_continue(Info :: term(), State :: term()) ->
+    {noreply, NewState :: term()} |
+    {noreply, NewState :: term(), timeout() | hibernate | {continue, term()}} |
     {stop, Reason :: term(), NewState :: term()}.
 -callback terminate(Reason :: (normal | shutdown | {shutdown, term()} |
                                term()),
@@ -149,7 +153,7 @@
       Status :: term().
 
 -optional_callbacks(
-    [handle_info/2, terminate/2, code_change/3, format_status/2]).
+    [handle_info/2, handle_continue/2, terminate/2, code_change/3, format_status/2]).
 
 %%%  -----------------------------------------------------------------
 %%% Starts a generic server.
@@ -309,7 +313,7 @@ enter_loop(Mod, Options, State, ServerName, Timeout) ->
     Name = gen:get_proc_name(ServerName),
     Parent = gen:get_parent(),
     Debug = gen:debug_options(Name, Options),
-	HibernateAfterTimeout = gen:hibernate_after(Options),
+    HibernateAfterTimeout = gen:hibernate_after(Options),
     loop(Parent, Name, State, Mod, Timeout, HibernateAfterTimeout, Debug).
 
 %%%========================================================================
@@ -374,6 +378,19 @@ init_it(Mod, Args) ->
 %%% ---------------------------------------------------
 %%% The MAIN loop.
 %%% ---------------------------------------------------
+
+loop(Parent, Name, State, Mod, {continue, Continue} = Msg, HibernateAfterTimeout, Debug) ->
+    Reply = try_dispatch(Mod, handle_continue, Continue, State),
+    case Debug of
+	[] ->
+	    handle_common_reply(Reply, Parent, Name, undefined, Msg, Mod,
+				HibernateAfterTimeout, State);
+	_ ->
+	    Debug1 = sys:handle_debug(Debug, fun print_event/3, Name, Msg),
+	    handle_common_reply(Reply, Parent, Name, undefined, Msg, Mod,
+				HibernateAfterTimeout, State, Debug1)
+    end;
+
 loop(Parent, Name, State, Mod, hibernate, HibernateAfterTimeout, Debug) ->
     proc_lib:hibernate(?MODULE,wake_hib,[Parent, Name, State, Mod, HibernateAfterTimeout, Debug]);
 
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