[erlang] 什么是supervisor_bridge

在OTP的源代码中，我发现了一个陌生的behaviour，名为supervisor_bridge。它的作用是什么，和普通的supervisor又有什么区别呢？

于是我找到了supervisor_bridge.erl这个文件。

首先它是一个gen_server.

-module(supervisor_bridge).

-behaviour(gen_server).

-include("logger.hrl").

%% External exports
-export([start_link/2, start_link/3]).
%% Internal exports
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2]).
-export([code_change/3]).

这个behaviour只有两个回调，描述其如何启动，如何终止。

-callback init(Args :: term()) ->
    {ok, Pid :: pid(), State :: term()} | ignore | {error, Error :: term()}.
-callback terminate(Reason :: (shutdown | term()), State :: term()) ->
    Ignored :: term().

发现了这样一段注释，大意是这个模块的功能是当其作为supervisor时，启动和终止子进程都是调用特定的模块中自定义的函数，而不是系统默认的。

%%% This module is built to function as process code
%%% for a process sitting inbetween a real supervisor
%%% and a not start&recovery complient server/system
%%% The process inbetween simulates start&recovery
%%% behaviour of the server/system below.

启动时，会调用特定模块中的启动函数，并link到对应的进程。

init([Mod, StartArgs, Name0]) ->  
    process_flag(trap_exit, true),
    Name = supname(Name0, Mod),
    case Mod:init(StartArgs) of
    {ok, Pid, ChildState} when is_pid(Pid) ->
        link(Pid),
        report_progress(Pid, Mod, StartArgs, Name),
        {ok, #state{mod = Mod, pid = Pid,
            child_state = ChildState, name = Name}};
    ignore ->
        ignore;
    {error, Reason} ->
        {stop, Reason}
    end.

supname(self, Mod) -> {self(),Mod};
supname(N, _)      -> N.

子进程退出后，并不会重启，而是直接退出。

handle_info({'EXIT', Pid, Reason}, State) when State#state.pid =:= Pid ->
    case Reason of
    normal ->
        ok;
    shutdown ->
        ok;
    {shutdown, _Term} ->
        ok;
    _ ->
        report_error(child_terminated, Reason, State)
    end,
    {stop, Reason, State#state{pid = undefined}};

主动终结，调用对应模块中的终结函数来结束子进程。

terminate(_Reason, #state{pid = undefined}) ->
    ok;
terminate(Reason, State) ->
    terminate_pid(Reason, State).
    
...

%% This function is supposed to terminate the 'real' server.
terminate_pid(Reason, #state{mod = Mod, child_state = ChildState}) ->
    Mod:terminate(Reason, ChildState).

当使用supervisor_bridge的时候，对应的子进程是否真的启动或者终结了，还是在进行一些别的操作，都可以根据需求自定义。在外界看来，它们的行为和普通的supervisor、worker无异。