💥 Encapsulate and rework all socket logic

This is a pretty huge change that will be important for being able to
handle nested or mutually recursive `dispatch()` and `audioMaster()`
calls. This sadly all had to be done in a single commit, so here's a
summary:

- `src/common/sockets.h:Sockets` contains all sockets on both the plugin
  and the Wine host side, and is used to both listen on and connect to
  the sockets.
- Sockets and other temporary files respect `$XDG_RUNTIME_DIR` instead
  of being dumped in `/tmp`.
- All sockets now have a unique endpoint in
  `/run/user/<uid>/yabridge-<plugin_name>-<random_id>/`. This is
  important for when we want to have multiple socket connections for
  handling `dispatch()` and `audioMaster()`.
- Because of the above, we no longer clean up the socket endpoint files
  after the connection gets established during initialization. Instead
  we'll remove the socket base directory when shutting down.

src/plugin/host-process.cpp

@@ -83,7 +83,7 @@ void HostProcess::async_log_pipe_lines(patched_async_pipe& pipe,
 IndividualHost::IndividualHost(boost::asio::io_context& io_context,
                                Logger& logger,
                                fs::path plugin_path,
-                               fs::path socket_endpoint)
+                               const Sockets& sockets)
     : HostProcess(io_context, logger),
       plugin_arch(find_vst_architecture(plugin_path)),
       host_path(find_vst_host(plugin_arch, false)),
@@ -93,7 +93,7 @@ IndividualHost::IndividualHost(boost::asio::io_context& io_context,
 #else
                        plugin_path,
 #endif
-                       socket_endpoint,
+                       sockets.base_dir,
                        bp::env = set_wineprefix(),
                        bp::std_out = stdout_pipe,
                        bp::std_err = stderr_pipe
@@ -127,17 +127,15 @@ void IndividualHost::terminate() {
     host.wait();
 }
 
-GroupHost::GroupHost(
-    boost::asio::io_context& io_context,
-    Logger& logger,
-    fs::path plugin_path,
-    fs::path socket_endpoint,
-    std::string group_name,
-    boost::asio::local::stream_protocol::socket& host_vst_dispatch)
+GroupHost::GroupHost(boost::asio::io_context& io_context,
+                     Logger& logger,
+                     fs::path plugin_path,
+                     Sockets& sockets,
+                     std::string group_name)
     : HostProcess(io_context, logger),
       plugin_arch(find_vst_architecture(plugin_path)),
       host_path(find_vst_host(plugin_arch, true)),
-      host_vst_dispatch(host_vst_dispatch) {
+      sockets(sockets) {
 #ifdef WITH_WINEDBG
     if (plugin_path.string().find(' ') != std::string::npos) {
         logger.log("Warning: winedbg does not support paths containing spaces");
@@ -167,6 +165,7 @@ GroupHost::GroupHost(
         wine_prefix = fs::path(host_env.at("HOME").to_string()) / ".wine";
     }
 
+    const fs::path endpoint_base_dir = sockets.base_dir;
     const fs::path group_socket_path =
         generate_group_endpoint(group_name, wine_prefix, plugin_arch);
     try {
@@ -175,9 +174,10 @@ GroupHost::GroupHost(
         boost::asio::local::stream_protocol::socket group_socket(io_context);
         group_socket.connect(group_socket_path.string());
 
-        write_object(group_socket,
-                     GroupRequest{.plugin_path = plugin_path.string(),
-                                  .socket_path = socket_endpoint.string()});
+        write_object(
+            group_socket,
+            GroupRequest{.plugin_path = plugin_path.string(),
+                         .endpoint_base_dir = endpoint_base_dir.string()});
         const auto response = read_object<GroupResponse>(group_socket);
 
         host_pid = response.pid;
@@ -199,7 +199,7 @@ GroupHost::GroupHost(
         // meantime.
         group_host_connect_handler = std::jthread([&, group_socket_path,
                                                    plugin_path,
-                                                   socket_endpoint]() {
+                                                   endpoint_base_dir]() {
             using namespace std::literals::chrono_literals;
 
             // TODO: Replace this polling with inotify
@@ -214,8 +214,9 @@ GroupHost::GroupHost(
 
                     write_object(
                         group_socket,
-                        GroupRequest{.plugin_path = plugin_path.string(),
-                                     .socket_path = socket_endpoint.string()});
+                        GroupRequest{
+                            .plugin_path = plugin_path.string(),
+                            .endpoint_base_dir = endpoint_base_dir.string()});
                     const auto response =
                         read_object<GroupResponse>(group_socket);
 
@@ -262,7 +263,7 @@ void GroupHost::terminate() {
     // There's no need to manually terminate group host processes as they will
     // shut down automatically after all plugins have exited. Manually closing
     // the dispatch socket will cause the associated plugin to exit.
-    host_vst_dispatch.shutdown(
+    sockets.host_vst_dispatch.shutdown(
         boost::asio::local::stream_protocol::socket::shutdown_both);
-    host_vst_dispatch.close();
+    sockets.host_vst_dispatch.close();
 }