conf: fix reloading on IN_CREATE

If the config file is a symlink, wayfire attempts to watch the parent
directory so that, if replaced via a subsequent `ln -sf`, it'll still
be notified. When notified, however, wayfire doesn't immediately
reload the config file. Rather, as this event may be triggered by any
file creation in the directory, we first make sure that the config file
really is the one being created anew.

Unfortunately, ln -sf is special: It'll create a randomly named file in
the directory before renameat(2)ing it to the proper location. So it
does generate an IN_CREATE event, but to know that the file was changed,
we need to additionally monitor for IN_MOVED_TO.

Fixes #2210.

src/default-config-backend.cpp

@@ -7,6 +7,7 @@
 #include <wayfire/plugin.hpp>
 #include <wayfire/core.hpp>
 
+#include <cstring>
 #include <sys/inotify.h>
 #include <filesystem>
 #include <unistd.h>
@@ -20,7 +21,7 @@ static int wd_cfg_dir, wd_cfg_file;
 
 static void add_watch(int fd)
 {
-    wd_cfg_dir  = inotify_add_watch(fd, config_dir.c_str(), IN_CREATE);
+    wd_cfg_dir  = inotify_add_watch(fd, config_dir.c_str(), IN_CREATE | IN_MOVED_TO);
     wd_cfg_file = inotify_add_watch(fd, config_file.c_str(), IN_CLOSE_WRITE);
 }
 
@@ -58,18 +59,24 @@ static int handle_config_updated(int fd, uint32_t mask, void *data)
          ptr += sizeof(inotify_event) + event->len)
     {
         event = reinterpret_cast<inotify_event*>(ptr);
-        // We reload in two main cases:
+        // We reload in two cases:
         //
-        // - Config file itself was modified
-        // - Config file was created inside parent directory
-        should_reload |=
-            (event->wd == wd_cfg_file) || (cfg_file_basename == event->name);
-
-        if ((event->name == cfg_file_basename) && (event->wd == wd_cfg_dir))
+        // 1. The config file itself was modified, or...
+        should_reload |= event->wd == wd_cfg_file;
+        // 2. The config file was moved nto or created inside the parent directory.
+        if (event->len > 0)
         {
-            inotify_rm_watch(fd, wd_cfg_file);
-            wd_cfg_file =
-                inotify_add_watch(fd, (config_dir + "/" + cfg_file_basename).c_str(), IN_CLOSE_WRITE);
+            // This is UB unless event->len > 0.
+            auto name_matches = cfg_file_basename == event->name;
+
+            if (name_matches)
+            {
+                inotify_rm_watch(fd, wd_cfg_file);
+                wd_cfg_file =
+                    inotify_add_watch(fd, (config_dir + "/" + cfg_file_basename).c_str(), IN_CLOSE_WRITE);
+            }
+
+            should_reload |= name_matches;
         }
     }