[arch-general] udev rules 2 times exec script

[mpan]

   If the command shares any resources, and name “autobackup” suggests 
it does, it should include some mechanism that prevents concurrent 
execution if another instance is already running. Otherwise, even if a 
single match happens, multiple instances of the script may be executed. 
For USB devices that may be a tiny power interruption during insertion, 
causing it to be detected, removed, detected again:

insertion - - - removal - - - insertion - - - - - - -
   |-- script ---------------------------------------> \_ two instances
                                 |-- script ---------> /  of the code

It should be either of those:
   |-- script ------------------------------------------------->
                                 |--- BAIL OUT! -|
or
   |-- script --------------------------------- end|
                                 |-- WAIT .......... script --->

> SUBSYSTEM=="block", ACTION=="add", ATTRS{idVendor}=="058f",
> ATTRS{idProduct}=="6387", SYMLINK+="external%n",
> RUN+="/usr/local/bin/autobackup.sh"
> 
> This works well, buth exec 2 times the script...
> 
> Jun 12 13:53:09 archlaptop autobackup.sh: BEGIN SCRIPT
> Jun 12 13:53:13 archlaptop autobackup.sh: BEGIN SCRIPT
   Because there are two matches for that pattern:
  - //devices/platform/…/target0:0:0/0:0:0:0/block/sda
  - //devices/platform/…/target0:0:0/0:0:0:0/block/sda/sda1
Both of them have SUBSYSTEM=="block", ACTION=="add", 
ATTRS{idVendor}=="058f" and ATTRS{idProduct}=="6387".

> If add ENV{DEVTYPE}=="usb_device" and change the
> SUBSYSTEM=="usb_device" , when plug usb, udev does nothing ...
   Because there is nothing that matches SUBSYSTEM=="usb_device".

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