By treating the apps in the umbrella as dependencies to be managed and
built by rebar3, we can simplify the maintenance of the release, at
the cost of increased build times: using Mix as before, it could track
changed files better than using rebar. But the complexity and
possibility of discrepancies make it using rebar much more compelling.
This commit enables a minimal working build of EMQX release using
Mix. However, to properly start the release, several configuration
steps are still missing. A `mix_release.sh` script does a few hacks
to get the release built with Mix to start properly, by first assuming
that `make emqx` has been run prior to the release, ran once to
generate the `app.*.config` files, and then it copies that and some
other files to the expected places.
Also, `emqx_telemetry` hangs the start procedure because it thinks
it's in an official release and tries to make a request. We disable
it temporarily via config just to get a working build for now.
If the rpc in `emqx_dashboard_monitor_api:get_collect/1` fails, an
empty map is return. But the current function expects a 4-tuple,
which results in a 500 error returned on such occasions.
```
curl -su admin:public localhost:8888/api/v5/monitor/current | jq .
{
"code": "INTERNAL_ERROR",
"message": "error, function_clause, [{emqx_dashboard_monitor_api,format_current_metrics,[[#{},{0,0,0,0},{0,0,0,0}],{0,0,0,0}],[{file,\"emqx_dashboard_monitor_api.erl\"},{line,179}]},{emqx_dashboard_monitor_api,current_counters,2,[{file,\"emqx_dashboard_monitor_api.erl\"},{line,167}]},{minirest_handler,apply_callback,3,[{file,\"minirest_handler.erl\"},{line,112}]},{minirest_handler,init,2,[{file,\"minirest_handler.erl\"},{line,38}]},{cowboy_handler,execute,2,[{file,\"cowboy_handler.erl\"},{line,41}]},{cowboy_stream_h,execute,3,[{file,\"cowboy_stream_h.erl\"},{line,318}]},{cowboy_stream_h,request_process,3,[{file,\"cowboy_stream_h.erl\"},{line,302}]},{proc_lib,init_p_do_apply,3,[{file,\"proc_lib.erl\"},{line,226}]}]"
}
```
If an empty username or password are given during the initialization
of `emqx_dashboard_admin`, it'll return just the atom `ok`, which
breaks the expected `{ok, _}` in `emqx_dashboard_app:start/2`.
```
{'EXIT',
{{failed_to_start_app,emqx_dashboard,
{emqx_dashboard,
{bad_return,
{{emqx_dashboard_app,start,[normal,[]]},
{'EXIT',
{{badmatch,ok},
[{emqx_dashboard_app,start,2,
[{file,
"/home/thales/dev/emqx/emqx/apps/emqx_dashboard/src/emqx_dashboard_app.erl"},
{line,32}]},
{application_master,start_it_old,4,
[{file,"application_master.erl"},{line,293}]}]}}}}}}}}
```
