fix: Allow Elixir projects to read flags from a local file (#106)

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions

**Context**

While trying to configure our Elixir service to read feature flags from
a local file (for local development purposes), we ran into an issue
where the server SDK would throw an "unsupported extension" error, even
though the configured path pointed to a YAML file.

**Root cause**

Erlang uses charlists to represent strings, while Elixir uses binaries.
So when we pass a path to a YAML file to the `ldclient` config in our
Elixir project, the Erlang function that parses the file extension on
the SDK side receives a binary, which it does not support. Thus, the SDK
throws an error claiming that we're using an invalid file type.

**Describe the solution you've provided**

This PR adds a helper function that handles binary type strings. If the
`FilePath` is a binary, we simply convert it to a charlist.

---------

Co-authored-by: Luis Beligante <luis.beligante@thescore.com>

src/ldclient_config.erl

@@ -75,7 +75,7 @@
     use_ldd => boolean(),
     send_events => boolean(),
     file_datasource => boolean(),
-    file_paths => [string()],
+    file_paths => [string() | binary()],
     file_auto_update => boolean(),
     file_poll_interval => pos_integer(),
     file_allow_duplicate_keys => boolean(),

src/ldclient_instance.erl

@@ -36,7 +36,7 @@
     cache_ttl => integer(),
     send_events => boolean(),
     file_datasource => boolean(),
-    file_paths => [string()],
+    file_paths => [string() | binary()],
     file_auto_update => boolean(),
     file_poll_interval => pos_integer(),
     file_allow_duplicate_keys => boolean(),

src/ldclient_update_file_server.erl

@@ -15,7 +15,7 @@
 -export([code_change/3, handle_call/3, handle_cast/2, handle_info/2, terminate/2]).
 
 -type state() :: #{
-    file_paths := [string()],
+    file_paths := [string() | binary()],
     file_auto_update := boolean(),
     file_poll_interval := pos_integer(),
     file_allow_duplicate_keys := boolean(),
@@ -155,7 +155,7 @@ upsert_valid_flag_data(true, Tag, ParsedFlags, ParsedSegments) ->
 upsert_valid_flag_data(false, _Tag, _ParsedFlags, _ParsedSegments) ->
     error.
 
--spec read_file(FilePath :: string(), Extension :: string()) -> {ok | error, map()}.
+-spec read_file(FilePath :: string() | binary(), Extension :: string()) -> {ok | error, map()}.
 read_file(FilePath, ".yaml") ->
     Result = yamerl_constr:file(FilePath, [{detailed_constr, true}]),
     [Head | _] = ldclient_yaml_mapper:to_map_docs(Result, []),
@@ -167,7 +167,7 @@ read_file(FilePath, _Extension) ->
     error_logger:warning_msg("File had unrecognized file extension. Valid extensions are .yaml and .json. File: ~p", [FilePath]),
     {error, #{}}.
 
--spec try_read_file(FilePath :: string(), Extension :: string()) -> {ok | error, map()}.
+-spec try_read_file(FilePath :: string() | binary(), Extension :: string()) -> {ok | error, map()}.
 try_read_file(FilePath, Extension) ->
     try
         read_file(FilePath, Extension)
@@ -176,7 +176,7 @@ try_read_file(FilePath, Extension) ->
         {error, #{}}
     end.
 
--spec load_file(FilePath :: string(), Extension :: string(), State :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()) ->
+-spec load_file(FilePath :: string() | binary(), Extension :: string(), State :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()) ->
     {ok | error, NewState :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()}.
 load_file(FilePath, Extension, #{file_info := CurFileInfo} = State, LoadedFlags, LoadedSegments, FlagCount) ->
     ModifiedTime = filelib:last_modified(FilePath),
@@ -196,7 +196,7 @@ process_decoded_file(ok, Decoded, State, LoadedFlags, LoadedSegments, FlagCount)
 process_decoded_file(error, _Decoded, State, LoadedFlags, LoadedSegments, FlagCount) ->
     {error, State, LoadedFlags, LoadedSegments, FlagCount}.
 
--spec check_modified(FilesToCheck :: [string()], Modified :: boolean(), State :: state()) ->
+-spec check_modified(FilesToCheck :: [string() | binary()], Modified :: boolean(), State :: state()) ->
     {Modified :: boolean(), State :: state()}.
 check_modified([], Modified, State) ->
     {Modified, State};
@@ -207,29 +207,29 @@ check_modified([FileToCheck | RestOfFiles], Modified, State) ->
     NewModified = Modified or (ExistingModifiedTime =/= ModifiedTime),
     check_modified(RestOfFiles, NewModified, State).
 
--spec load_files_if_modified(Files :: [string()], State :: state()) -> {ok | error, State :: state()}.
+-spec load_files_if_modified(Files :: [string() | binary()], State :: state()) -> {ok | error, State :: state()}.
 load_files_if_modified(Files, State) ->
     case check_modified(Files, false, State) of
         {true, UpdatedState} -> load_files(Files, UpdatedState);
         {false, UpdatedState} -> {ok, UpdatedState}
     end.
 
--spec load_regular_file(FilePath :: string(), IsRegularFile :: boolean(), State :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()) ->
+-spec load_regular_file(FilePath :: string() | binary(), IsRegularFile :: boolean(), State :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()) ->
     {ok | error, NewState :: state(), LoadedFlags :: map(), LoadedSegments :: map(), FlagCount :: non_neg_integer()}.
 load_regular_file(FileToLoad, true, State, LoadedFlags, LoadedSegments, FlagCount) ->
     load_file(FileToLoad, filename:extension(FileToLoad), State, LoadedFlags, LoadedSegments, FlagCount);
 load_regular_file(_FileToLoad, false, State, LoadedFlags, LoadedSegments, FlagCount) ->
     {error, State, LoadedFlags, LoadedSegments, FlagCount}.
 
--spec load_files(Files :: [string()], State :: state()) -> {ok | error, State :: state()}.
+-spec load_files(Files :: [string() | binary()], State :: state()) -> {ok | error, State :: state()}.
 load_files(Files, State) ->
     load_files(Files, State, #{}, #{}, 0, ok).
 
--spec load_files(Files :: [string()], State :: state(), CombinedFlags :: map(), CombinedSegments :: map(), FlagCount :: non_neg_integer(), Status :: ok | error) ->
+-spec load_files(Files :: [string() | binary()], State :: state(), CombinedFlags :: map(), CombinedSegments :: map(), FlagCount :: non_neg_integer(), Status :: ok | error) ->
     {ok | error, State :: state()}.
 load_files([FileToLoad | RestOfFiles], State, LoadedFlags, LoadedSegments, FlagCount, ok) ->
     {NewStatus, NewState, CombinedFlags, CombinedSegments, UpdatedCount} =
-        load_regular_file(FileToLoad, filelib:is_regular(FileToLoad), State, LoadedFlags, LoadedSegments, FlagCount),
+        load_regular_file(handle_file_path_type(FileToLoad), filelib:is_regular(handle_file_path_type(FileToLoad)), State, LoadedFlags, LoadedSegments, FlagCount),
     load_files(RestOfFiles, NewState, CombinedFlags, CombinedSegments, UpdatedCount, NewStatus);
 load_files([], #{file_allow_duplicate_keys := AllowDuplicateKeys, storage_tag := Tag} = State, LoadedFlags, LoadedSegments, FlagCount, ok) ->
     Valid = (FlagCount == length(maps:keys(LoadedFlags))) or AllowDuplicateKeys,
@@ -238,3 +238,9 @@ load_files([], #{file_allow_duplicate_keys := AllowDuplicateKeys, storage_tag :=
 load_files(_Files, State, _LoadedFlags, _LoadedSegments, _FlagCount, error) ->
     %% If there is an error, then do not process any more files.
     {error, State}.
+
+-spec handle_file_path_type(FilePath :: string() | binary()) -> string().
+handle_file_path_type(FilePath) when is_binary(FilePath) ->
+    binary_to_list(FilePath);
+handle_file_path_type(FilePath) ->
+    FilePath.

test/ldclient_file_SUITE.erl

@@ -18,6 +18,7 @@
     check_file_simple_flags_yaml/1,
     check_file_all_properties_yaml/1,
     check_multiple_data_files/1,
+    check_binary_file_path/1,
     check_file_watcher/1,
     check_with_missing_file/1,
     check_with_only_missing_file/1,
@@ -35,6 +36,7 @@ all() ->
         check_file_simple_flags_yaml,
         check_file_all_properties_yaml,
         check_multiple_data_files,
+        check_binary_file_path,
         check_file_watcher,
         check_with_missing_file,
         check_with_only_missing_file,
@@ -91,6 +93,14 @@ init_per_suite(Config) ->
     },
     ldclient:start_instance("", simple_flags_yaml, OptionsSimpleFlagsYaml),
 
+    OptionsBinaryPathYaml = #{
+        datasource => file,
+        send_events => false,
+        file_paths => [list_to_binary(DataFileSimpleFlagsYaml)],
+        feature_store => ldclient_storage_map
+    },
+    ldclient:start_instance("", binary_path_yaml, OptionsBinaryPathYaml),
+
     DataFileAllPropertiesYaml = code:priv_dir(ldclient) ++ "/flags-all-properties.yaml",
     OptionsAllPropertiesYaml = #{
         datasource => file,
@@ -208,6 +218,11 @@ check_multiple_data_files(_) ->
     {{0, true, fallthrough}, _} = ldclient_eval:flag_key_for_context(all_properties_yaml, <<"my-boolean-flag-key">>, #{key => <<"user123">>, kind => <<"user">>}, "foo"),
     {{0, 3, fallthrough}, _} = ldclient_eval:flag_key_for_context(all_properties_yaml, <<"my-integer-flag-key">>, #{key => <<"user123">>, kind => <<"user">>}, "foo").
 
+check_binary_file_path(_) ->
+    {{0, <<"value-1">>, fallthrough}, _} = ldclient_eval:flag_key_for_context(binary_path_yaml, <<"my-string-flag-key">>, #{key => <<"user123">>, kind => <<"user">>}, "foo"),
+    {{0, true, fallthrough}, _} = ldclient_eval:flag_key_for_context(binary_path_yaml, <<"my-boolean-flag-key">>, #{key => <<"user123">>, kind => <<"user">>}, "foo"),
+    {{0, 3, fallthrough}, _} = ldclient_eval:flag_key_for_context(binary_path_yaml, <<"my-integer-flag-key">>, #{key => <<"user123">>, kind => <<"user">>}, "foo").
+
 check_file_watcher(_) ->
     {{0, true, fallthrough}, _} = ldclient_eval:flag_key_for_context(watch_files, <<"test-flag">>, #{key => <<"user123">>, kind => <<"user">>}, "foo"),
     %The timestamp for the modified time is in seconds. So we wait a moment to get a new timestamp compared to creation.