[tahoe-lafs-trac-stream] [tahoe-lafs] #1670: KeyError in mutable read-modify-write

[tahoe-lafs]

#1670: KeyError in mutable read-modify-write
-------------------------+-------------------------------------------------
     Reporter:  vikarti  |      Owner:  zooko
         Type:  defect   |     Status:  assigned
     Priority:           |  Milestone:  1.11.0
  critical               |    Version:  1.9.1
    Component:  code-    |   Keywords:  mutable retrieve error tahoe-backup
  mutable                |  regression test-needed preservation blocks-
   Resolution:           |  release
Launchpad Bug:           |
-------------------------+-------------------------------------------------

Comment (by zooko):

 We investigated this during the Tahoe-LAFS Summit. It seems likely that
 directory-modification operations have been accidentally using the
 original version of the directory, even when there was a write-collision
 and a newer version of the directory was discovered. I didn't 100% confirm
 this, but I suspect that means that in those (rare‽) cases where there are
 write-collisions on a directory, the retrying code would blow away the
 other person's write, by re-applying the earlier version (plus the current
 modification). This would be a data loss bug and very much not the kind of
 thing we tolerate. ☹

 Now, there are ''no'' bug reports that I am aware of that could point to
 this having caused a directory modification to get thrown out in practice.
 There are several reports of this bug leading to an internal
 {{{KeyError}}}, but as far as I recall, nobody reported that a change they
 had made to their directory was subsequently discovered to have been lost.

 Nonetheless, the possibility of this happening seems to be present, from
 code inspection.

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1670#comment:45>
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