Teach prim_file:set_cwd() to avoid entering non-translatable directories

We have decided that we don't want to deal with the compilations
of prim_file:get_cwd() returning a binary when the current
directory name cannot be translated losslessly to a list (i.e.
when the run-time system was started with +fnu and the current directory
name contains bytes that are not part of a valid UTF-8 sequence).

Therefore, if prim_file:set_cwd() is given a binary as the pathname,
we will need to check the binary to make sure it can be translated
to a list. We will introduce a new BIF, called prim_file:is_translatable/1,
which will check both filename encoding mode, and if it is one of
Unicode modes, the binary as well.

We don't need to do anything special if prim_file:set_cwd() is passed
a list.

1 files changed, 18 insertions, 5 deletions

diff --git a/erts/preloaded/src/prim_file.erl b/erts/preloaded/src/prim_file.erl
index 27e7640b7e..305abb8b0a 100644
--- a/erts/preloaded/src/prim_file.erl
+++ b/erts/preloaded/src/prim_file.erl
@@ -152,7 +152,8 @@
 
 -export([internal_name2native/1,
          internal_native2name/1,
-         internal_normalize_utf8/1]).
+         internal_normalize_utf8/1,
+	 is_translatable/1]).
 
 -type prim_file_name() :: string() | unicode:unicode_binary().
 -type prim_file_name_error() :: 'error' | 'ignore' | 'warning'.
@@ -173,6 +174,11 @@ internal_native2name(_) ->
 internal_normalize_utf8(_) ->
     erlang:nif_error(undefined).
 
+-spec is_translatable(prim_file_name()) -> boolean().
+
+is_translatable(_) ->
+    erlang:nif_error(undefined).
+
 %%% End of BIFs
 
 %%%-----------------------------------------------------------------
@@ -671,10 +677,17 @@ set_cwd(Dir) ->
 set_cwd(Port, Dir) when is_port(Port) ->
     set_cwd_int(Port, Dir).
 
-set_cwd_int(Port, Dir) ->
-    %% Dir is now either a string or an EXIT tuple.
-    %% An EXIT tuple will fail in the following catch.
-    drv_command(Port, [?FILE_CHDIR, pathname(Dir)]).
+set_cwd_int(Port, Dir) when is_binary(Dir) ->
+    case prim_file:is_translatable(Dir) of
+	false ->
+	    {error, no_translation};
+	true ->
+	    drv_command(Port, [?FILE_CHDIR, pathname(Dir)])
+    end;
+set_cwd_int(Port, Dir) when is_list(Dir) ->
+    drv_command(Port, [?FILE_CHDIR, pathname(Dir)]);
+set_cwd_int(_, _) ->
+    {error, badarg}.