[Xapian-tickets] [Xapian] #675: Exception in [Chert/Glass]Cursor::find_entry()

[Xapian]

#675: Exception in [Chert/Glass]Cursor::find_entry()
---------------------------+-----------------------------
 Reporter:  Kronuz         |             Owner:  olly
     Type:  defect         |            Status:  assigned
 Priority:  high           |         Milestone:  1.2.21
Component:  Backend-Chert  |           Version:
 Severity:  normal         |        Resolution:
 Keywords:                 |        Blocked By:
 Blocking:                 |  Operating System:  All
---------------------------+-----------------------------
Changes (by olly):

 * milestone:  1.3.3 => 1.2.21


Comment:

 Before we actually use the data in the cursor, we check `B->cursor_version
 != version` and if it isn't, we call `rebuild()` which updates the cursor
 to match the table.

 The table's `cursor_version` gets incremented each time `add()` or `del()`
 is called if at least one cursor has been created since the last call (the
 reason for this condition is to avoid churning the counter unnecessarily
 and having it wrap round).

 Looking over the code involved, I see two bugs here though - we don't set
 `cursor_created_since_last_modification` in the cursor `rebuild()` method,
 but that flag is meant to really indicate if any cursor might exist with
 the current `cursor_version`, and after `rebuild()` a cursor now does.

 Also, we don't consider incrementing `cursor_version` after `cancel()` or
 `reopen()` (the latter being very relevant here).

 Fixing those, your testcase passes cleanly.  I've written a testcase based
 on it and committed the fixes + the testcase in
 [1900eca6d325e9185b9f9f5c75a4a4a545616d7f/git].  Testing I found that the
 fixes for `reopen()` and `rebuild()` are both required to fix the
 testcase.

 I think we're now good after those changes - although with chert the
 cursor still has a reference to a freed root block, the cursor will now
 get rebuilt before being used, so that doesn't actually matter as that dud
 reference will never get used.

 If this doesn't fix the problem in you full system, please comment with
 details.

 Marking for backporting for 1.2.21.

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Ticket URL: <http://trac.xapian.org/ticket/675#comment:9>
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