fix: handle already_started when starting instance (#111)

**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

**Related issues**


https://github.com/launchdarkly/erlang-server-sdk/blob/242986ec2dd4e42cf603ef2005de1fd96444aa42/src/ldclient_instance.erl#L61

**Describe the solution you've provided**

N/A

**Describe alternatives you've considered**

N/A

**Additional context**

Add any other context about the pull request here.

src/ldclient_instance.erl

@@ -58,7 +58,6 @@
 -spec start(Tag :: atom(), SdkKey :: string(), Options :: options()) ->
     ok | {error, already_started, term()}.
 start(Tag, SdkKey, Options) ->
-    % TODO check if Tag already exists and return already_started error
     % Parse options into settings
     Settings = ldclient_config:parse_options(SdkKey, Options),
     ok = ldclient_config:register(Tag, Settings),
@@ -67,14 +66,25 @@ start(Tag, SdkKey, Options) ->
     UpdateSupName = get_ref_from_tag(instance_stream, Tag),
     UpdateWorkerModule = get_update_processor(Settings),
     EventsSupName = get_ref_from_tag(instance_events, Tag),
-    {ok, _} = supervisor:start_child(ldclient_sup, ldclient_instance_sup:child_spec(SupName, [SupName, UpdateSupName, UpdateWorkerModule, EventsSupName, Tag])),
-    % Start storage backend
-    true = ldclient_update_processor_state:create_storage_initialized_state(Tag, false),
-    FeatureStore = maps:get(feature_store, Settings),
-    ok = FeatureStore:init(SupName, Tag, []),
-    % Start stream client
-    true = ldclient_update_processor_state:create_initialized_state(Tag, false),
-    start_updater(UpdateSupName, UpdateWorkerModule, Tag).
+    case
+        supervisor:start_child(
+            ldclient_sup,
+            ldclient_instance_sup:child_spec(SupName, [
+                SupName, UpdateSupName, UpdateWorkerModule, EventsSupName, Tag
+            ])
+        )
+    of
+        {ok, _} ->
+            % Start storage backend
+            true = ldclient_update_processor_state:create_storage_initialized_state(Tag, false),
+            FeatureStore = maps:get(feature_store, Settings),
+            ok = FeatureStore:init(SupName, Tag, []),
+            % Start stream client
+            true = ldclient_update_processor_state:create_initialized_state(Tag, false),
+            start_updater(UpdateSupName, UpdateWorkerModule, Tag);
+        {error, {already_started, Pid}} ->
+            {error, already_started, Pid}
+    end.
 
 %% @doc Stop a client instance
 %%
@@ -148,22 +158,25 @@ start_updater(UpdateSupName, UpdateWorkerModule, Tag) ->
 %% @private
 %%
 %% @end
--spec get_update_processor(Settings :: #{ stream := boolean(),
-                                          offline := boolean(),
-                                          use_ldd := boolean(),
-                                          file_datasource := boolean(),
-                                          datasource := atom(),
-                                          _ => _
-                                        }) -> atom().
-get_update_processor(#{ offline := true }) ->
+-spec get_update_processor(
+    Settings :: #{
+        stream := boolean(),
+        offline := boolean(),
+        use_ldd := boolean(),
+        file_datasource := boolean(),
+        datasource := atom(),
+        _ => _
+    }
+) -> atom().
+get_update_processor(#{offline := true}) ->
     ldclient_update_null_server;
-get_update_processor(#{ use_ldd := true }) ->
+get_update_processor(#{use_ldd := true}) ->
     ldclient_update_null_server;
-get_update_processor(#{ datasource := DataSource }) when DataSource /= undefined ->
+get_update_processor(#{datasource := DataSource}) when DataSource /= undefined ->
     list_to_atom("ldclient_update_" ++ atom_to_list(DataSource) ++ "_server");
-get_update_processor(#{ file_datasource := true }) ->
+get_update_processor(#{file_datasource := true}) ->
     ldclient_update_file_server;
-get_update_processor(#{ stream := false }) ->
+get_update_processor(#{stream := false}) ->
     ldclient_update_poll_server;
-get_update_processor(#{ stream := true }) ->
+get_update_processor(#{stream := true}) ->
     ldclient_update_stream_server.

test/ldclient_start_SUITE.erl

@@ -0,0 +1,43 @@
+-module(ldclient_start_SUITE).
+
+-include_lib("common_test/include/ct.hrl").
+
+-compile(export_all).
+
+%%====================================================================
+%% ct functions
+%%====================================================================
+all() ->
+    [
+        test_already_running_client
+    ].
+
+init_per_suite(Config) ->
+    {ok, _} = application:ensure_all_started(ldclient),
+    Config.
+
+end_per_suite(_) ->
+    ok = application:stop(ldclient).
+
+init_per_testcase(_, Config) ->
+    Options = #{
+        send_events => false,
+        feature_store => ldclient_storage_map,
+        datasource => testdata
+    },
+    ldclient:start_instance("", Options),
+    Config.
+
+end_per_testcase(_, _Config) ->
+    ldclient:stop_all_instances().
+
+%%====================================================================
+%% Tests
+%%====================================================================
+
+test_already_running_client(_) ->
+    {error, already_started, _} = ldclient:start_instance("", #{
+        send_events => false,
+        feature_store => ldclient_storage_map,
+        datasource => testdata
+    }).