kernel: Use persistent term to store os_cmd_shell value

This has two purposes:

1. Make it impossible to change the value through the application:set_env API.
2. Make the lookup slightly faster.

lib/kernel/src/os.erl

@@ -587,14 +587,16 @@ get_option(Opt, Options, Default) ->
         _ -> throw(badopt)
     end.
 
+-define(KERNEL_OS_CMD_SHELL_KEY, kernel_os_cmd_shell).
+
 mk_cmd({win32,_}, Cmd) ->
-    {ok, Shell} = application:get_env(kernel, os_cmd_shell),
+    Shell = persistent_term:get(?KERNEL_OS_CMD_SHELL_KEY),
     Command = lists:concat([Shell, " /c", Cmd]),
     {Command, [], [], <<>>};
 mk_cmd(_,Cmd) ->
     %% Have to send command in like this in order to make sh commands like
     %% cd and ulimit available.
-    {ok, Shell} = application:get_env(kernel, os_cmd_shell),
+    Shell = persistent_term:get(?KERNEL_OS_CMD_SHELL_KEY),
     {Shell ++ " -s unix:cmd", [out],
      %% We insert a new line after the command, in case the command
      %% contains a comment character.
@@ -615,13 +617,14 @@ mk_cmd(_,Cmd) ->
 
 -doc false.
 internal_init_cmd_shell() ->
-    case application:get_env(kernel, os_cmd_shell) of
-        undefined ->
-            application:set_env(kernel, os_cmd_shell,
-            internal_init_cmd_shell(os:type()));
-        _ ->
-            ok
-    end.
+    Shell =
+        case application:get_env(kernel, os_cmd_shell) of
+            undefined ->
+                internal_init_cmd_shell(os:type());
+            {ok, Val} ->
+                Val
+        end,
+    persistent_term:put(?KERNEL_OS_CMD_SHELL_KEY, Shell).
 internal_init_cmd_shell({win32,Wtype}) ->
     case {os:getenv("COMSPEC"),Wtype} of
         {false,windows} -> "command.com";

lib/kernel/test/os_SUITE.erl

@@ -469,21 +469,15 @@ error_info(Config) ->
         ],
     error_info_lib:test_error_info(os, L).
 
-os_cmd_shell(Config) ->
-    DataDir = proplists:get_value(data_dir, Config),
-    SysShell = filename:join(DataDir, "sys_shell"),
+%% Check that is *not* possible to change shell after startup
+os_cmd_shell(_Config) ->
 
-    {ok, OldShell} = application:get_env(kernel, os_cmd_shell),
-    try
-        application:set_env(kernel, os_cmd_shell, SysShell),
+    application:set_env(kernel, os_cmd_shell, "broken shell"),
 
-        %% os:cmd should not try to detect the shell location rather than use
-        %% the value from kernel:os_cmd_shell parameter
-        comp("sys_shell", os:cmd("ls"))
-    after
-        application:set_env(kernel, os_cmd_shell, OldShell)
-    end.
+    %% os:cmd should continue to work as normal
+    comp("hello", os:cmd("echo hello")).
 
+%% When started with os_cmd_shell set, we make sure that it is used.
 os_cmd_shell_peer(Config) ->
     DataDir = proplists:get_value(data_dir, Config),
     SysShell = "\"" ++ filename:join(DataDir, "sys_shell") ++ "\"",