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https://issues.apache.org/jira/browse/DERBY-4193?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12703624#action_12703624
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Knut Anders Hatlen commented on DERBY-4193:
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I think the assert is harmless, in the sense that nothing bad should happen in
non-debug builds. It basically indicates that the last time we saved the
position of this scan, we didn't subsequently use it to reposition the scan, so
in this case I think it just means that we saved the position unnecessarily.
The code that fails tries to save the position of a max scan because it has to
wait in order to obtain the lock on the rightmost row in an index. In this case
it won't use the saved position when it wakes up, instead it performs a new
search from the root of the B-tree to find the rightmost row again (which is
possibly a different row now). In the repro, the max scan needs to wait a
second time to lock the rightmost row because a new row has become the
rightmost one, and it's still locked exclusively.
Since the locking of the last row is performed the same way as one would do
previous key locking (the comment above it actually says "backward scan initial
positioning will request a previous key lock for initial positioning"), I
believe that it would be correct to replace the call to lockScanRow() with a
call to lockNonScanPreviousRow(). Previous key locking will not save the
position when it needs to wait, so the assert failure will go away, and the
logic in BTreeMaxScan.positionAtStartPosition() will also be simpler as it
doesn't need to decrement/increment pos.current_slot before/after the lock call.
I think there may be similar problems in
BTreeScan.positionAtStartForForwardScan()
BTreeScan.positionAtStartForBackwardScan()
BTreeMaxScan.fetchMax()
> ASSERT FAILED Scan position already saved with multi-threaded
> insert/update/delete
> -----------------------------------------------------------------------------------
>
> Key: DERBY-4193
> URL: https://issues.apache.org/jira/browse/DERBY-4193
> Project: Derby
> Issue Type: Bug
> Components: Store
> Affects Versions: 10.5.1.1, 10.6.0.0
> Environment: Windows XP, IBM 1.6 SR6, Sun 1.6.0_01-b06
> Reporter: Kathey Marsden
> Attachments: ScanPosSaved.java, ScanPosSaved.java
>
>
> The attached program ScanPosSaved.java produces the error below, fairly
> quickly.
> The program has three threads, one doing inserts into a table with an
> identity column, one updating the row with the maximum id, one deleting the
> row with the maximum id.
> To reproduce, run >java ScanPosSaved and <ctrl> <c> out of the program after
> you get the error.
> I saw this 10.5 and trunk sane builds but did not see it on 10.4. With the
> insane build of 10.5.1.1 (RC2) I did not see any symptoms right away, so
> don't know how serious an issue this is for insane builds.
> org.apache.derby.shared.common.sanity.AssertFailure: ASSERT FAILED Scan
> position already saved
> at
> org.apache.derby.shared.common.sanity.SanityManager.ASSERT(SanityManager.java:120)
> at
> org.apache.derby.impl.store.access.btree.BTreeScan.savePositionAndReleasePage(BTreeScan.java:2148)
> at
> org.apache.derby.impl.store.access.btree.BTreeScan.savePositionAndReleasePage(BTreeScan.java:2212)
> at
> org.apache.derby.impl.store.access.btree.BTreeRowPosition.saveMeAndReleasePage(BTreeRowPosition.java:128)
> at
> org.apache.derby.impl.store.access.btree.index.B2IRowLocking3.lockRowOnPage(B2IRowLocking3.java:295)
> at
> org.apache.derby.impl.store.access.btree.index.B2IRowLocking3._lockScanRow(B2IRowLocking3.java:599)
> at
> org.apache.derby.impl.store.access.btree.index.B2IRowLockingRR.lockScanRow(B2IRowLockingRR.java:105)
> at
> org.apache.derby.impl.store.access.btree.BTreeMaxScan.positionAtStartPosition(BTreeMaxScan.java:347)
> at
> org.apache.derby.impl.store.access.btree.BTreeMaxScan.fetchMax(BTreeMaxScan.java:434)
> at
> org.apache.derby.impl.store.access.btree.index.B2I.fetchMaxOnBTree(B2I.java:739)
> at
> org.apache.derby.impl.store.access.RAMTransaction.fetchMaxOnBtree(RAMTransaction.java:1078)
> at
> org.apache.derby.impl.sql.execute.LastIndexKeyResultSet.openCore(LastIndexKeyResultSet.java:189)
> at
> org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.openCore(ProjectRestrictResultSet.java:168)
> at
> org.apache.derby.impl.sql.execute.ScalarAggregateResultSet.openCore(ScalarAggregateResultSet.java:133)
> at
> org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.openCore(ProjectRestrictResultSet.java:168)
> at
> org.apache.derby.impl.sql.execute.BasicNoPutResultSetImpl.open(BasicNoPutResultSetImpl.java:245)
> at
> org.apache.derby.impl.sql.GenericPreparedStatement.executeStmt(GenericPreparedStatement.java:416)
> at
> org.apache.derby.impl.sql.GenericPreparedStatement.execute(GenericPreparedStatement.java:297)
> at
> org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(EmbedStatement.java:1235)
> at
> org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:625)
> at
> org.apache.derby.impl.jdbc.EmbedStatement.executeQuery(EmbedStatement.java:152)
> at ScanPosSaved.updateOperation(ScanPosSaved.java:62)
> at ScanPosSaved$3.run(ScanPosSaved.java:17)
> I discovered this when trying to get a smaller repro for DERBY-4181, but I
> think it is a different issue, because it reproduces on multiple jvms and
> does not reproduce on 10.4. The assertion was added with DERBY-2991. Knut
> could you perhaps assess how serious this is?
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