Wire up cache, file watcher, and Unix socket daemon

Adds a gen_server cache holding the rendered menu, an fs/inotify-based
watcher that debounces changes and invalidates it, a Unix domain
socket serving the cached binary, and the escript client Openbox
invokes (with an inline render fallback if the daemon isn't running).
Also documents local deployment (systemd user unit, Openbox menu.xml
wiring) in the README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/src/er_xdg_pipe_menu_socket.erl b/src/er_xdg_pipe_menu_socket.erl
new file mode 100644
index 0000000..1bcca91
--- /dev/null
+++ b/src/er_xdg_pipe_menu_socket.erl
@@ -0,0 +1,66 @@
+%%%-------------------------------------------------------------------
+%% @doc Serves the cached menu over a Unix domain socket.
+%%
+%% Connecting to the socket *is* the request: each accepted connection
+%% is handed the current `er_xdg_pipe_menu_cache:get_menu/0' binary and
+%% closed. There is no request payload to parse.
+%% @end
+%%%-------------------------------------------------------------------
+
+-module(er_xdg_pipe_menu_socket).
+
+-export([start_link/0, socket_path/0]).
+-export([init/1]).
+
+-spec start_link() -> {ok, pid()}.
+start_link() ->
+    proc_lib:start_link(?MODULE, init, [self()]).
+
+-spec socket_path() -> file:filename_all().
+socket_path() ->
+    case application:get_env(er_xdg_pipe_menu, socket_path) of
+        {ok, Path} -> Path;
+        undefined -> default_socket_path()
+    end.
+
+default_socket_path() ->
+    case env("XDG_RUNTIME_DIR") of
+        undefined -> filename:join("/tmp", "er_xdg_pipe_menu-" ++ os:getenv("USER", "unknown") ++ ".sock");
+        Dir -> filename:join(Dir, "er_xdg_pipe_menu.sock")
+    end.
+
+env(Name) ->
+    case os:getenv(Name) of
+        false -> undefined;
+        "" -> undefined;
+        Value -> Value
+    end.
+
+init(Parent) ->
+    Path = socket_path(),
+    %% Clean up a stale socket file left behind by a previous run that
+    %% didn't shut down cleanly; gen_tcp:listen fails with eaddrinuse
+    %% otherwise.
+    _ = file:delete(Path),
+    {ok, Listen} = gen_tcp:listen(0, [
+        {ifaddr, {local, Path}},
+        binary,
+        {active, false},
+        {packet, raw},
+        {reuseaddr, true}
+    ]),
+    proc_lib:init_ack(Parent, {ok, self()}),
+    accept_loop(Listen).
+
+accept_loop(Listen) ->
+    case gen_tcp:accept(Listen) of
+        {ok, Socket} ->
+            spawn(fun() -> serve(Socket) end),
+            accept_loop(Listen);
+        {error, Reason} ->
+            exit(Reason)
+    end.
+
+serve(Socket) ->
+    gen_tcp:send(Socket, er_xdg_pipe_menu_cache:get_menu()),
+    gen_tcp:close(Socket).