[ZCM] [ZC] 1734/ 4 Comment "BTrees conflict resolution leads to index inconsistencies"

[Collector: Zope Bugs, Features, and Patches ...]

Issue #1734 Update (Comment) "BTrees conflict resolution leads to index inconsistencies"
 Status Accepted, Database/bug critical
To followup, visit:
  http://www.zope.org/Collectors/Zope/1734

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= Comment - Entry #4 by d.maurer on Mar 23, 2005 3:13 am


Uploaded:  "BTreesEmptyConflict.pat"
 - http://www.zope.org/Collectors/Zope/1734/BTreesEmptyConflict.pat/view
Patch attached
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= Edit - Entry #3 by tim_one on Mar 22, 2005 6:21 pm

 Changes: submitter email, revised version_info, new comment

Updated version info.
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= Assign - Entry #2 by tim_one on Mar 22, 2005 5:47 pm

 Status: Pending => Accepted

 Supporters added: tim_one

Brrrrrrr.  Thanks for the report, Dieter!  Assigned to myself.
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= Request - Entry #1 by d.maurer on Mar 22, 2005 2:47 pm

Zope indexes use code like this to remove document ids from
document lists:

	 def removeForwardIndexEntry(self, entry, documentId):
	     indexRow = self._index[entry]
	     indexRow.remove(documentId)
	     if not indexRow:
	         del self._index[entry]

If a concurrent transaction adds a document id to the same
"indexRow", then this document id can be lost because
it is inserted into a document list discarded by the other
transaction.

Under normal conditions, a "ConflictError" would result
because two transactions modify the same object
(the document list). However, the BTrees conflict resolution
prevents the "ConflictError" and allows the loss.


The result is an index inconsistency: the forward index ("_index")
entry and the backward one ("_unindex") not longer correspond.
Further operations lead to "unable to remove documentId
from document list" exceptions.


I suggest to tighten BTrees conflict resolution not to allow
concurrent insertions when a bucket was emptied.

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