--- controller.c.orig	2018-02-01 14:11:57.000000000 -0800
+++ controller.c	2021-10-05 15:04:37.682975000 -0700
@@ -188,6 +188,24 @@
         return -1;
     }
 
+    /*
+     * Don't allow the attrib or modify flags for a directory.
+     * It results in a file descriptor being opened for every single file
+     * in the directory, which can be insane if, for example, the directory
+     * is a maildir folder with hundreds of thousands of messages in it.
+     * Unfortunately the KDE directory watcher in the kcoreaddons framework
+     * library always sets IN_ATTRIB and IN_MODIFY which means this will
+     * in fact happen with KMail.
+     * (The Qt filesystem watcher also uses IN_ATTRIB, which is why this
+     * hack is being applied here, to kill two birds with one stone.)
+     * Note that this is only a workaround: really the correct thing to do
+     * is to fix the design to eliminate this side effect, but I'm not
+     * sure how to do that. For now at least, this makes KMail behave.
+     */
+
+    if (S_ISDIR(st.st_mode))
+        mask &= ~(IN_ATTRIB|IN_MODIFY);
+
     worker_cmd_add (&cmd, name, mask);
     return worker_exec (fd, &cmd);
 }