Use prelaunch context first and fallback to app env. variable

[Why]
We use the application environment as some kind of "cache" for the
configuration parameters and some system environment variables. However,
because we don't use `sys.config`, the application environment could be
reset to its default/empty state if we were to use Erlang releases.

[How]
For values that may come from the system environment, we use the
prelaunch context instead.

V2: If the value is unavailable from the context, we fall back to the
    application environment variable. We still need this for the CLI and
    some testsuites.

Author: dumbbell

deps/rabbit/src/rabbit.erl

@@ -1175,8 +1175,13 @@ get_default_data_param(Param) ->
 %% </ul>
 
 data_dir() ->
-    {ok, DataDir} = application:get_env(rabbit, data_dir),
-    DataDir.
+    case rabbit_prelaunch:get_context() of
+        #{data_dir := DataDir} ->
+            DataDir;
+        _ ->
+            {ok, DataDir} = application:get_env(rabbit, data_dir),
+            DataDir
+    end.
 
 %%---------------------------------------------------------------------------
 %% logging

deps/rabbit/src/rabbit_feature_flags.erl

@@ -1164,9 +1164,14 @@ delete_enabled_feature_flags_list_file() ->
 %% @returns the path to the file.
 
 enabled_feature_flags_list_file() ->
-    case application:get_env(rabbit, feature_flags_file) of
-        {ok, Val} -> Val;
-        undefined -> throw(feature_flags_file_not_set)
+    case rabbit_prelaunch:get_context() of
+        #{feature_flags_file := File} ->
+            File;
+        _ ->
+            case application:get_env(rabbit, feature_flags_file) of
+                {ok, Val} -> Val;
+                undefined -> throw(feature_flags_file_not_set)
+            end
     end.
 
 copy_feature_states_after_reset(RemoteNode) ->

deps/rabbit/src/rabbit_ff_controller.erl

@@ -670,8 +670,10 @@ get_forced_feature_flag_names_from_env() ->
 %% @private
 
 get_forced_feature_flag_names_from_config() ->
-    Value = application:get_env(
-              rabbit, forced_feature_flags_on_init, undefined),
+    Value = maps:get(
+              forced_feature_flags_on_init,
+              rabbit_prelaunch:get_context(),
+              undefined),
     case Value of
         undefined ->
             Value;

deps/rabbit/src/rabbit_plugins.erl

@@ -32,7 +32,7 @@ ensure(FileJustChanged) ->
     end.
 
 ensure1(FileJustChanged0) ->
-    {ok, OurFile0} = application:get_env(rabbit, enabled_plugins_file),
+    OurFile0 = enabled_plugins_file(),
     FileJustChanged = filename:nativename(FileJustChanged0),
     OurFile = filename:nativename(OurFile0),
     case OurFile of
@@ -72,39 +72,51 @@ ensure1(FileJustChanged0) ->
 
 -spec plugins_expand_dir() -> file:filename().
 plugins_expand_dir() ->
-    case application:get_env(rabbit, plugins_expand_dir) of
-        {ok, ExpandDir} ->
+    case rabbit_prelaunch:get_context() of
+        #{plugins_expand_dir := ExpandDir} ->
             ExpandDir;
         _ ->
-            filename:join([rabbit:data_dir(), "plugins_expand_dir"])
+            case application:get_env(rabbit, plugins_expand_dir) of
+                {ok, ExpandDir} ->
+                    ExpandDir;
+                undefined ->
+                    filename:join([rabbit:data_dir(), "plugins_expand_dir"])
+            end
     end.
 
 -spec plugins_dir() -> file:filename().
 plugins_dir() ->
-    case application:get_env(rabbit, plugins_dir) of
-        {ok, PluginsDistDir} ->
+    case rabbit_prelaunch:get_context() of
+        #{plugins_path := PluginsDistDir} ->
             PluginsDistDir;
         _ ->
-            filename:join([rabbit:data_dir(), "plugins_dir_stub"])
+            case application:get_env(rabbit, plugins_dir) of
+                {ok, PluginsDistDir} ->
+                    PluginsDistDir;
+                undefined ->
+                    filename:join([rabbit:data_dir(), "plugins_dir_stub"])
+            end
     end.
 
 -spec enabled_plugins_file() -> file:filename().
 enabled_plugins_file() ->
-     case application:get_env(rabbit, enabled_plugins_file) of
-        {ok, Val} ->
-            Val;
+    case rabbit_prelaunch:get_context() of
+        #{enabled_plugins_file := File} ->
+            File;
         _ ->
-            filename:join([rabbit:data_dir(), "enabled_plugins"])
+            case application:get_env(rabbit, enabled_plugins_file) of
+                {ok, File} ->
+                    File;
+                undefined ->
+                    filename:join([rabbit:data_dir(), "enabled_plugins"])
+
+            end
     end.
 
 -spec enabled_plugins() -> [atom()].
 enabled_plugins() ->
-    case application:get_env(rabbit, enabled_plugins_file) of
-        {ok, EnabledFile} ->
-            read_enabled(EnabledFile);
-        _ ->
-            []
-    end.
+    EnabledFile = enabled_plugins_file(),
+    read_enabled(EnabledFile).
 
 %% @doc Prepares the file system and installs all enabled plugins.