How to configure httpc profiles with rebar3?

How can I set configuration options for httpc's profiles when using rebar3?

Here is the only example being via erl -config inets.config that looks like this:

[{inets, 
[{services,[{httpc,[{profile, server1}]},
            {httpc, [{profile, server2}]}]}]
}].

I tried adopting it to my rebar3 project structure.

Code

Project was created with rebar3, with standard OTP layout:

rebar3 new release myapp

Here is my myapp/config/sys.config:

[
  { myapp, []},
  {inets, [{services, [{httpc, [{profile, myapp}]}]}]}
].

rebar.config:

{erl_opts, [debug_info]}.
{deps, []}.

{relx, [{release, { myapp, "0.1.0" },
         [myapp,
          sasl]},

        {sys_config, "./config/sys.config"},
        {vm_args, "./config/vm.args"},

        {dev_mode, true},
        {include_erts, false},

        {extended_start_script, true}]
}.

{profiles, [{prod, [{relx, [{dev_mode, false},
                            {include_erts, true}]}]
            }]
}.

Here is my myapp.app.src file for completeness:

{application, myapp,
 [{description, "An OTP application"},
  {vsn, "0.1.0"},
  {registered, []},
  {mod, { myapp_app, []}},
  {applications,
   [kernel,
    stdlib
   ]},
  {env,[]},
  {modules, []},

  {maintainers, []},
  {licenses, []},
  {links, []}
 ]}.

Requests

Here is a request I'm trying to make from rebar`s shell:

$ ./rebar3 shell
1> ===> Booted myapp
1> ===> Booted sasl
...
1> httpc:request( "http://reddit.com", myapp).
** exception exit: {noproc,
                    {gen_server,call,
                     [httpc_myapp,
                      {request,
                       {request,undefined,<0.88.0>,0,http,
                        {"reddit.com",80},
                        "/",[],get,
                        {http_request_h,undefined,"keep-alive",undefined,
                         undefined,undefined,undefined,undefined,undefined,
                         undefined,...},
                        {[],[]},
                        {http_options,"HTTP/1.1",infinity,true,
                         {essl,[]},
                         undefined,false,infinity,...},
                        "http://reddit.com",[],none,[],1478280329839,
                        undefined,undefined,false}},
                      infinity]}}
     in function  gen_server:call/3 (gen_server.erl, line 212)
     in call from httpc:handle_request/9 (httpc.erl, line 574)

Here is the request without a profile, to check that inets actually works:

2> httpc:request( "http://reddit.com").

=PROGRESS REPORT==== 4-Nov-2016::13:25:51 ===
          supervisor: {local,inet_gethost_native_sup}
             started: [{pid,<0.107.0>},{mfa,{inet_gethost_native,init,[[]]}}]

=PROGRESS REPORT==== 4-Nov-2016::13:25:51 ===
          supervisor: {local,kernel_safe_sup}
             started: [{pid,<0.106.0>},
                       {id,inet_gethost_native_sup},
                       {mfargs,{inet_gethost_native,start_link,[]}},
                       {restart_type,temporary},
                       {shutdown,1000},
                       {child_type,worker}]
{ok,{{"HTTP/1.1",200,"OK"},...

Solution

rebar3 itself use inets http clients so when It starts your application in shell, inets is already started and configured. One workaround would be stop inets before your application starts, as It's suggested by rebar3 developer (copied below). Another one would be boot your release in console mode:

./_build/default/rel/myapp/bin/myapp console

Beside that there is another problem with your project. You have not told you want inets being started for you. You should have this line in myapp.src:

{applications, [kernel, stdlib, inets]}

Or you can list inets in rebar.config release section, to tell relx this app should be included in release and started on boot.

{relx, [{release, { myapp, "0.1.0" }, [inets, myapp, sasl]} ]}

Stop Inets from Loading on rebar3 shell startup

Here is the copy of the full answer by Fred Hebert from the rebar3 mailing list:

We do need inets for package fetching and will likely not turn it off automatically for all use cases as this could compromise general usage of the rebar3 agent in case where the user application does not use inets, but still asks to fetch packages in a subsequent run. The workaround I would suggest would be to use a hook script for it. Hook scripts run before we boot user applications and are regular escripts:
#!/usr/bin/env escript 
main(_) -> application:stop(inets). 
You can then hook the script in with:
{shell, [{script_file, "path/to/file"}]} 
in rebar3.config, or with
rebar3 shell --script_file test/check_env.escript