[yaws_sup] Fix RSS startup deadlock by reordering child specs

src/yaws_sup.erl

@@ -64,7 +64,10 @@ child_specs() ->
              {yaws_ws_sup, start_link, []},
              transient, infinity, supervisor, [yaws_ws_sup]},
 
-    [YawsLog, YawsTrace, YawsServ, Sup, WSSup].
+    %% Sup (yaws_sup_restarts) must start before YawsServ because
+    %% yaws_server:init may call yaws_rss:open/2 during config loading,
+    %% and yaws_rss is a child of yaws_sup_restarts.
+    [YawsLog, YawsTrace, Sup, YawsServ, WSSup].
 
 %%----------------------------------------------------------------------
 %%----------------------------------------------------------------------

test/rss_startup_SUITE.erl

@@ -0,0 +1,142 @@
+%% Regression test for yaws_rss startup ordering.
+%%
+%% yaws_rss:open/2 is called during yaws_config:load (inside
+%% yaws_server:init/1) when the config file contains an RSS section.
+%% yaws_rss must be registered before yaws_server starts, otherwise
+%% yaws_rss:open/2 blocks for up to 10 seconds then crashes.
+%%
+%% The fix moves yaws_sup_restarts (which starts yaws_rss) before
+%% yaws_server in yaws_sup's child spec list.
+-module(rss_startup_SUITE).
+
+-include("testsuite.hrl").
+
+-compile(nowarn_export_all).
+-compile(export_all).
+
+all() ->
+    [
+     config_load_blocks_without_rss,
+     config_load_completes_with_rss,
+     sup_starts_rss_before_server
+    ].
+
+groups() ->
+    [].
+
+%%====================================================================
+init_per_suite(Config) ->
+    _ = application:load(yaws),
+    Dir = ?tempdir(?MODULE),
+    ok = filelib:ensure_dir(filename:join([Dir, "www", "dummy"])),
+    ok = filelib:ensure_dir(filename:join([Dir, "rss", "dummy"])),
+    ConfPath = filename:join(Dir, "yaws.conf"),
+    [{conf_path, ConfPath} | Config].
+
+end_per_suite(_Config) ->
+    _ = application:unload(yaws),
+    ok.
+
+init_per_group(_Group, Config) ->
+    Config.
+
+end_per_group(_Group, _Config) ->
+    ok.
+
+init_per_testcase(_Test, Config) ->
+    Config.
+
+end_per_testcase(_Test, _Config) ->
+    %% Clean up any processes we may have started.
+    catch gen_server:stop(yaws_rss),
+    ok.
+
+%%====================================================================
+
+%% When yaws_rss is not running, yaws_config:load blocks at the </rss>
+%% tag because yaws_rss:open calls wait_for_server, which polls 20
+%% times at 500ms intervals. This reproduces the deadlock through the
+%% real code path: yaws_server:init -> yaws_config:load -> yaws_rss:open.
+config_load_blocks_without_rss(Config) ->
+    ConfPath = ?config(conf_path, Config),
+    Env = #env{debug = false, conf = {file, ConfPath}},
+
+    %% Ensure yaws_rss is not running.
+    undefined = erlang:whereis(yaws_rss),
+
+    Self = self(),
+    Pid = spawn_link(fun() ->
+        Result = yaws_config:load(Env),
+        Self ! {config_load_result, Result}
+    end),
+
+    %% Give it 1.5s. If yaws_rss were registered, load would return
+    %% near-instantly. The wait_for_server loop runs at 500ms intervals,
+    %% so after 1.5s it should still be polling.
+    receive
+        {config_load_result, _} ->
+            ct:fail("yaws_config:load returned without yaws_rss running")
+    after 1500 ->
+        %% Still blocked — confirms the deadlock. Kill the helper to
+        %% avoid waiting the full 10 seconds.
+        unlink(Pid),
+        exit(Pid, kill),
+        ok
+    end.
+
+%% When yaws_rss is running, yaws_config:load gets past the RSS
+%% section without blocking. This is the fixed scenario:
+%% yaws_sup_restarts starts yaws_rss before yaws_server calls
+%% yaws_config:load.
+config_load_completes_with_rss(Config) ->
+    ConfPath = ?config(conf_path, Config),
+    Env = #env{debug = false, conf = {file, ConfPath}},
+
+    {ok, _} = yaws_rss:start_link(),
+
+    Self = self(),
+    Pid = spawn_link(fun() ->
+        Result = yaws_config:load(Env),
+        Self ! {config_load_result, Result}
+    end),
+
+    %% Config loading should get past yaws_rss:open within 3s. The
+    %% result (ok or error) doesn't matter — the point is it returned
+    %% instead of blocking in wait_for_server.
+    receive
+        {config_load_result, _} ->
+            ok
+    after 3000 ->
+        unlink(Pid),
+        exit(Pid, kill),
+        ct:fail("yaws_config:load blocked with yaws_rss running")
+    end.
+
+%% Verify that yaws_sup's child spec list starts yaws_sup_restarts
+%% (which contains yaws_rss) before yaws_server. This is the fix.
+sup_starts_rss_before_server(_Config) ->
+    ChildSpecs = yaws_sup:child_specs(),
+    Ids = [element(1, Spec) || Spec <- ChildSpecs],
+
+    RestartsPos = index_of(yaws_sup_restarts, Ids),
+    ServerPos   = index_of(yaws_server, Ids),
+
+    ?assertNotEqual(false, RestartsPos),
+    ?assertNotEqual(false, ServerPos),
+
+    %% yaws_sup_restarts must start before yaws_server.
+    ?assert(RestartsPos < ServerPos).
+
+%%====================================================================
+%% Helpers
+%%====================================================================
+
+index_of(Elem, List) ->
+    index_of(Elem, List, 1).
+
+index_of(_Elem, [], _N) ->
+    false;
+index_of(Elem, [Elem | _], N) ->
+    N;
+index_of(Elem, [_ | T], N) ->
+    index_of(Elem, T, N + 1).

test/rss_startup_SUITE_data/templates/yaws.conf

@@ -0,0 +1,11 @@
+logdir = $logdir$
+
+<server localhost>
+    port = 8099
+    listen = 127.0.0.1
+    docroot = $tempdir$/www
+    <rss>
+        rss_id = test_feed
+        rss_dir = $tempdir$/rss
+    </rss>
+</server>