Comment 35 for bug 1890791

Finally, after some hours of debugging and reading the logs I reached the reason of this bug. It's triggered when:
- KDirWatch uses inotify as a backend
- the watched directory contains a symlink as one of its parent. For example, /d is a symlink to /media/data, and our current directory in Dolphin is something like /d/1/2/3
- the directory is added and then removed:
  $ while true ; do mkdir abc ; sleep 1 ; rmdir abc ; sleep 1 ; done

The problem is that inotify_add_watch reuses the file descriptor if the directory is already watched. When the directory and the another path to it with a symlink (e. g. /d/1/2/3 and /media/data/1/2/3) are watched, inotify API still reuses the file descriptor. But KDirWatch intrinsically considers these path different, so uses different entries for them. The mapping between file descriptors and names is kept in QHash<int, Entry *> m_inotify_wd_to_entry, so the previous entry here is overwritten.

Moreover, when one of these path stops being watched, the file descriptor is freed and the map entry is removed, so the other path doesn't receive events.

I don't know how to fix this in a simple way (maybe use canonical paths inside KDirWatch?)

Also note that it's the only one scenario in which the file system updates are not delivered, there may be other ways to break it and the separate reasons why this happens. Any missing update in KDirWatch can be the reason.

As I stated in the previous message, KDirWatch can use other methods as default (FAM, inotify, QFSWatch and Stat). FAM seems to affect performance. Stat will poll the filesystem each 0.5 seconds, which is not the most efficient way. I don't know anything about QFSWatch and why it's not use as a default. Some comments in the code suggest that it lacks some functionality.

As I workaround, I set the environment variable:

KDIRWATCH_METHOD=QFSWatch