[Bug 1266763] Re: Race condition between status and backup

[Brian Murray]

** Description changed:

+ N.B. This should not be released until after deja-dup bug 1281066.
+ 
  SRU justification : Race condition exist when two instances of duplicity run in the same
  cache directory (.cache/duplicity)
  
  Impact : Potential corruption of the local cache
  
  Fix : Add a lockfile in the local cache & prevent execution of a second instance in the
  case of the presence of the lockfile
  
  Test Case :
  1) Run one instance of duplicity :
   $ sudo mkdir /tmp/backup
   $ sudo duplicity --exclude=/proc --exclude=/sys --exclude=/tmp / file:///tmp/backup
  
  While this command is running execute the following in a separate console :
   $ sudo duplicity collection-status file:///tmp/backup
  
  With the new locking mechanism you will see the following :
  $ sudo duplicity collection-status file:///tmp/backup
  Another instance is already running with this archive directory
  If you are sure that this is the  only instance running you may delete
  the following lockfile and run the command again :
     /home/ubuntu/.cache/duplicity/3fe07cc0f71075f95f411fb55ec60120/lockfile.lock
  
  Regression : In the case of spurrious interruption of duplicity, the lockfile
  will remain in .cache/duplicity which can prevent future use of duplicity. The cache
  directory will have to be cleaned as outlined in the error message
  
  Original description of the problem :
  
  When runnining "duply X status" while running "duply X backup" (sorry, I
  don't know which duplicity commands are created by duply) due to a race-
  condition the code of 'sync_archive' might happend to append newly
  created meta-data files to 'local_spurious' and subsequently delete
  them. This delete seems to have been the reason that triggered bug
  1216921.
  
  The race condition is that the backup command constantly creates meta-
  data files while the status command queries the list of local and remote
  files independently over a larger time span. This means that a local
  file might already been remote but the status command did not see it a
  few seconds ago.

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https://bugs.launchpad.net/bugs/1266763

Title:
  Race condition between status and backup

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