Issues mounting MTP on Kubuntu

I've had difficulty getting Kubuntu to mount a Samsung Galaxy S8 reliably. Using the Device Notifier gets as far as showing directory structure and thumbnails, but the MTP process dies if you try to read a file properly. Also, the phone asks the user whether it should be accessed via USB only after the first mount attempt. If you say “Allow”, it withdraws its current configuration (thereby invalidating the first mount), and re-offers it (thereby causing the Device Notifier to pop up again, and requiring the user to open another window). Perhaps it's a clash between USB and Android requirements: (say) the phone must respond to a mount-triggered USB request at once, but Android also has to wait for user authorization, and has no way to asynchronously inform the host of new files appearing on an existing mount? To get anywhere, I've had to abandon the Device Notifier, install jmtpfs on the host, and run it manually, and twice. I've also had to enable Developer Options on the phone(?!).

Now I'm trying to write an auto-mounting script for a headless machine, so that the latest photos and videos I've recorded on my devices can be automatically moved off the device simply by plugging it in. The files will later be dropped into an ingest process to make them ready for presentation over DLNA. I use this to watch for USB devices being plugged and unplugged:

$ inotifywait -m -r /dev/bus/usb -e CREATE -e DELETE
/dev/bus/usb/001 CREATE 010
/dev/bus/usb/001 DELETE 010
/dev/bus/usb/001 CREATE 011
/dev/bus/usb/001 DELETE 011

CREATE and DELETE events specify the bus number and device number (e.g., 001:010) when the phone is plugged in or unplugged. The output of lsusb -v -s 001:010 provides the vendor id and serial number of the device, and whether an MTP interface is provided, so events for non-MTP devices can be ignored.

On plugging in, the CREATE event is received. The phone lights up, but doesn't yet ask the user if the host has permission to access its files. I ensure a mount point exists, and run jmtpfs on it, specifying the device id:

mkdir -p "/var/mtp/$vendid-$serialno"
jmtpfs -device=001,010 "/var/mtp/$vendid-$serialno"

This triggers the phone to ask for authorization from the user. Although the response is still pending, the mount appears to succeed, so I proceed to scan the mount point for interesting files with find. All attempts to scan or access fail with I/O errors, so there's nothing to do but unmount with:

fusermount -u "/var/mtp/$vendid-$serialno"

Now I tap “Allow” on the phone, and I get a DELETE for 001:010, immediately followed by CREATE for 001:011. The device id has changed, but the vendor id and serial number are the same, so the same mount point is used, and mounts without error as before. This time, a scan of files succeeds, and unmounting can take place when they have been processed.

So, the trick seems to be:

• Expect failure and assume a retry will occur. (If it doesn't, it obviously wasn't that important.)
  
• Use the vendor id and serial number to avoid treating the retry as new device. (You don't actually need to remember that there was an earlier failure, just make sure that your action in the second cycle tries to do exactly what it tried to do before.)