refactor(emqx_machine): move system_monitor to included_applications

system_monitor has to be found in application (or included_applications)
otherwise dialyzer fails with Unknown function

Adding to 'applications' would cause a deadlock because one
cannot call application:ensure_all_started in the application/2
callback.

apps/emqx_machine/priv/reboot_lists.eterm

@@ -24,8 +24,15 @@
             runtime_tools,
             redbug,
             xmerl,
+            %% has no application/2 callback
             {hocon, load},
-            telemetry
+            telemetry,
+            observer_cli,
+            covertool,
+            tools,
+            observer,
+            {system_monitor, load},
+            jq
         ],
     %% must always be of type `load'
     common_business_apps =>
@@ -33,11 +40,6 @@
             emqx,
             emqx_conf,
             esasl,
-            observer_cli,
-            tools,
-            covertool,
-            %% started by emqx_machine
-            system_monitor,
             emqx_utils,
             emqx_durable_storage,
             emqx_http_lib,
@@ -79,9 +81,7 @@
             emqx_plugins,
             emqx_opentelemetry,
             quicer,
-            bcrypt,
+            bcrypt
-            jq,
-            observer
         ],
     %% must always be of type `load'
     ee_business_apps =>

apps/emqx_machine/src/emqx_machine.app.src

@@ -6,7 +6,13 @@
     {vsn, "0.2.17"},
     {modules, []},
     {registered, []},
-    {applications, [kernel, stdlib, emqx_ctl, system_monitor, covertool]},
+    {applications, [kernel, stdlib, emqx_ctl, covertool]},
+    %% system_monitor is loaded but not booted,
+    %% emqx_machine.erl makes the decision when to start
+    %% the app after certain config injection.
+    %% it's a included_application because otherwise dialyzer
+    %% would report unknown functions
+    {included_applications, [system_monitor]},
     {mod, {emqx_machine_app, []}},
     {env, []},
     {licenses, ["Apache-2.0"]},

apps/emqx_machine/src/emqx_machine.erl

@@ -47,7 +47,7 @@ start() ->
             os:set_signal(sigterm, handle)
     end,
     ok = set_backtrace_depth(),
-    start_sysmon(),
+    ok = start_sysmon(),
     configure_shard_transports(),
     set_mnesia_extra_diagnostic_checks(),
     emqx_otel_app:configure_otel_deps(),

apps/emqx_machine/src/emqx_machine_boot.erl

@@ -36,9 +36,6 @@
 %% If any of these applications crash, the entire EMQX node shuts down:
 -define(BASIC_PERMANENT_APPS, [mria, ekka, esockd, emqx]).
 
-%% These apps should NOT be (re)started automatically:
--define(EXCLUDED_APPS, [system_monitor, observer_cli, jq]).
-
 %% These apps are optional, they may or may not be present in the
 %% release, depending on the build flags:
 -define(OPTIONAL_APPS, [bcrypt, observer]).
@@ -157,7 +154,7 @@ basic_reboot_apps() ->
 excluded_apps() ->
     %% Optional apps _should_ be (re)started automatically, but only
     %% when they are found in the release:
-    ?EXCLUDED_APPS ++ [App || App <- ?OPTIONAL_APPS, not is_app(App)].
+    [App || App <- ?OPTIONAL_APPS, not is_app(App)].
 
 is_app(Name) ->
     case application:load(Name) of