From fffce9316ca03b13ff4617a7edd35b982f5bff49 Mon Sep 17 00:00:00 2001
From: Thales Macedo Garitezi <thalesmg@gmail.com>
Date: Thu, 1 Dec 2022 11:28:45 -0300
Subject: [PATCH] test(fix): avoid trying to connect to system_monitor db in
 tests

When `system_monitor`'s application is loaded, it sets a default
hostname:

```erlang
1> application:get_env(system_monitor, db_hostname).
undefined
2> application:load(system_monitor).
ok
3> application:get_env(system_monitor, db_hostname).
{ok,"localhost"}
```

And that makes tests crash when somehow it gets loaded.

By having a semantic `enable` field, we could avoid starting this
application by checking if the hostname is filled or not.

```
=ERROR REPORT==== 1-Dec-2022::13:57:20.855070 ===
** Generic server <0.7646.3> terminating
** Last message in was {command,epgsql_cmd_connect,
                                #{codecs => [],database => "postgres",
                                  host => [],
                                  password =>
                                      #Fun<epgsql_cmd_connect.0.29916615>,
                                  port => 5432,timeout => 5000,
                                  username => "system_monitor"}}
** When Server state == {state,undefined,undefined,<<>>,undefined,on_message,
                               undefined,
                               {[],[]},
                               undefined,undefined,undefined,undefined,[],
                               information_redacted,[],undefined,undefined,
                               undefined,undefined,undefined,#{}}
** Reason for termination ==
** {badarg,
       [{gen_tcp,connect_0,4,[{file,"gen_tcp.erl"},{line,207}]},
        {epgsql_cmd_connect,open_socket,2,
            [{file,
                 "/emqx/_build/default/lib/epgsql/src/commands/epgsql_cmd_connect.erl"},
             {line,94}]},
        {epgsql_cmd_connect,execute,2,
            [{file,
                 "/emqx/_build/default/lib/epgsql/src/commands/epgsql_cmd_connect.erl"},
             {line,54}]},
        {epgsql_sock,command_exec,4,
            [{file,"/emqx/_build/default/lib/epgsql/src/epgsql_sock.erl"},
             {line,338}]},
        {gen_server,try_handle_call,4,[{file,"gen_server.erl"},{line,721}]},
        {gen_server,handle_msg,6,[{file,"gen_server.erl"},{line,750}]},
        {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,226}]}]}
** Client system_monitor_pg stacktrace
** [{gen,do_call,4,[{file,"gen.erl"},{line,214}]},
    {gen_server,call,3,[{file,"gen_server.erl"},{line,243}]},
    {epgsql,call_connect,2,
            [{file,"/emqx/_build/default/lib/epgsql/src/epgsql.erl"},
             {line,203}]},
    {system_monitor_pg,connect,0,
                       [{file,"/emqx/_build/default/lib/system_monitor/src/system_monitor_pg.erl"},
                        {line,151}]},
    {system_monitor_pg,initialize,0,
                       [{file,"/emqx/_build/default/lib/system_monitor/src/system_monitor_pg.erl"},
                        {line,142}]},
    {system_monitor_pg,handle_info,2,
                       [{file,"/emqx/_build/default/lib/system_monitor/src/system_monitor_pg.erl"},
                        {line,92}]},
    {gen_server,try_dispatch,4,[{file,"gen_server.erl"},{line,695}]},
    {gen_server,handle_msg,6,[{file,"gen_server.erl"},{line,771}]},
    {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,226}]}]

=CRASH REPORT==== 1-Dec-2022::13:57:20.855697 ===
  crasher:
    initial call: epgsql_sock:init/1
    pid: <0.7646.3>
    registered_name: []
    exception exit: badarg
      in function  gen_tcp:connect_0/4 (gen_tcp.erl, line 207)
      in call from epgsql_cmd_connect:open_socket/2 (/emqx/_build/default/lib/epgsql/src/commands/epgsql_cmd_connect.erl, line 94)
      in call from epgsql_cmd_connect:execute/2 (/emqx/_build/default/lib/epgsql/src/commands/epgsql_cmd_connect.erl, line 54)
      in call from epgsql_sock:command_exec/4 (/emqx/_build/default/lib/epgsql/src/epgsql_sock.erl, line 338)
      in call from gen_server:try_handle_call/4 (gen_server.erl, line 721)
      in call from gen_server:handle_msg/6 (gen_server.erl, line 750)
    ancestors: [system_monitor_pg,system_monitor2_sup,system_monitor_sup,
                  <0.4297.3>]
    message_queue_len: 0
    messages: []
    links: [<0.4303.3>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 1598
    stack_size: 29
    reductions: 734
  neighbours:
```
---
 apps/emqx/test/emqx_common_test_helpers.erl | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/apps/emqx/test/emqx_common_test_helpers.erl b/apps/emqx/test/emqx_common_test_helpers.erl
index 87d9c1368..a56b2c3fa 100644
--- a/apps/emqx/test/emqx_common_test_helpers.erl
+++ b/apps/emqx/test/emqx_common_test_helpers.erl
@@ -161,7 +161,17 @@ boot_modules(Mods) ->
 
 -spec start_apps(Apps :: apps()) -> ok.
 start_apps(Apps) ->
-    start_apps(Apps, fun(_) -> ok end).
+    %% to avoid keeping the `db_hostname' that is set then loading
+    %% `system_monitor' application in `emqx_machine', and then it
+    %% crashing when trying to connect.
+    %% FIXME: add an `enable' option to sysmon_top and use that to
+    %% decide whether to start it or not.
+    DefaultHandler =
+        fun(_) ->
+            application:set_env(system_monitor, db_hostname, ""),
+            ok
+        end,
+    start_apps(Apps, DefaultHandler).
 
 -spec start_apps(Apps :: apps(), Handler :: special_config_handler()) -> ok.
 start_apps(Apps, SpecAppConfig) when is_function(SpecAppConfig) ->