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  1. Subversion
  2. SVN-4251

named atomic does not work for revprop cache synchronisation

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Details

    • Bug
    • Status: Closed
    • Blocker
    • Resolution: Fixed
    • trunk
    • 1.8.0
    • libsvn_fs_fs
    • None

    Description

      APR's shared memory implementation isn't suitable for writing Subversion's
      synchronisation code for the revprop cache. The problem is that when creating
      named shared memory using apr_shm_create APR installs a pool cleanup handler
      that removes the file.
      
      Without a long-lived parent process the failure mode is
      
       - process 1 starts and creates a shared memory segment
       - process 2 starts and attaches to the same segment
       - process 1 exits and cleans up, process 2 still using the segment
       - process 3 starts and creates a new shared memory segment
      
      At this point processes 2 and 3 are using different shared memory segments.
      
      There is another smaller issue: the creation of the segment is protected by lock
      that excludes other processes but the cleanup is not protected. There is a
      window during cleanup when the shared memory has been deleted but the file still
      exists. At this point another process will not be able to create a new shared
      memory segment or attach to an existing segment.
      
      http://mail-archives.apache.org/mod_mbox/subversion-dev/201210.mbox/%3C87lieoxey9.fsf@stat.home.lan%3E
      
      http://svn.haxx.se/dev/archive-2012-10/0448.shtml
      

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            stefan2 Stefan Fuhrmann
            philipm Philip Martin
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              Updated:
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