[jira] [Commented] (CASSANDRA-13747) Fix short read protection

["Aleksey Yeschenko (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-13747?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16117564#comment-16117564 ] 

Aleksey Yeschenko commented on CASSANDRA-13747:
-----------------------------------------------

Stack trace for the triggered assertion:

{code}
java.lang.AssertionError
	at org.apache.cassandra.service.DataResolver$ShortReadProtection$ShortReadRowProtection.moreContents(DataResolver.java:449)
	at org.apache.cassandra.service.DataResolver$ShortReadProtection$ShortReadRowProtection.moreContents(DataResolver.java:412)
	at org.apache.cassandra.db.transform.BaseIterator.tryGetMoreContents(BaseIterator.java:111)
	at org.apache.cassandra.db.transform.BaseIterator.hasMoreContents(BaseIterator.java:101)
	at org.apache.cassandra.db.transform.BaseRows.hasNext(BaseRows.java:155)
	at org.apache.cassandra.utils.MergeIterator$Candidate.advance(MergeIterator.java:369)
	at org.apache.cassandra.utils.MergeIterator$ManyToOne.advance(MergeIterator.java:189)
	at org.apache.cassandra.utils.MergeIterator$ManyToOne.computeNext(MergeIterator.java:158)
	at org.apache.cassandra.utils.AbstractIterator.hasNext(AbstractIterator.java:47)
	at org.apache.cassandra.db.rows.UnfilteredRowIterators$UnfilteredRowMergeIterator.computeNext(UnfilteredRowIterators.java:509)
	at org.apache.cassandra.db.rows.UnfilteredRowIterators$UnfilteredRowMergeIterator.computeNext(UnfilteredRowIterators.java:369)
	at org.apache.cassandra.utils.AbstractIterator.hasNext(AbstractIterator.java:47)
	at org.apache.cassandra.db.transform.BaseRows.hasNext(BaseRows.java:129)
	at org.apache.cassandra.db.transform.FilteredRows.isEmpty(FilteredRows.java:50)
	at org.apache.cassandra.db.transform.Filter.closeIfEmpty(Filter.java:73)
	at org.apache.cassandra.db.transform.Filter.applyToPartition(Filter.java:43)
	at org.apache.cassandra.db.transform.Filter.applyToPartition(Filter.java:26)
	at org.apache.cassandra.db.transform.BasePartitions.hasNext(BasePartitions.java:96)
	at org.apache.cassandra.service.StorageProxy$SingleRangeResponse.computeNext(StorageProxy.java:2200)
	at org.apache.cassandra.service.StorageProxy$SingleRangeResponse.computeNext(StorageProxy.java:2172)
	at org.apache.cassandra.utils.AbstractIterator.hasNext(AbstractIterator.java:47)
	at org.apache.cassandra.db.transform.BasePartitions.hasNext(BasePartitions.java:92)
	at org.apache.cassandra.service.StorageProxy$RangeCommandIterator.computeNext(StorageProxy.java:2243)
	at org.apache.cassandra.service.StorageProxy$RangeCommandIterator.computeNext(StorageProxy.java:2210)
	at org.apache.cassandra.utils.AbstractIterator.hasNext(AbstractIterator.java:47)
	at org.apache.cassandra.db.transform.BasePartitions.hasNext(BasePartitions.java:92)
	at org.apache.cassandra.cql3.statements.SelectStatement.process(SelectStatement.java:707)
{code}

> Fix short read protection
> -------------------------
>
>                 Key: CASSANDRA-13747
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13747
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Coordination
>            Reporter: Aleksey Yeschenko
>            Assignee: Aleksey Yeschenko
>             Fix For: 3.0.x, 3.11.x
>
>
> {{ShortReadRowProtection.moreContents()}} expects that by the time we get to that method, the global post-reconciliation counter was already applied to the current partition. However, sometimes it won’t get applied, and the global counter continues counting with {{rowInCurrentPartition}} value not reset from previous partition, which in the most obvious case would trigger the assertion we are observing - {{assert !postReconciliationCounter.isDoneForPartition();}}. In other cases it’s possible because of this lack of reset to query a node for too few extra rows, causing unnecessary SRP data requests.
> Why is the counter not always applied to the current partition?
> The merged {{PartitionIterator}} returned from {{DataResolver.resolve()}} has two transformations applied to it, in the following order:
> {{Filter}} - to purge non-live data from partitions, and to discard empty partitions altogether (except for Thrift)
> {{Counter}}, to count and stop iteration
> Problem is, {{Filter}} ’s {{applyToPartition()}} code that discards empty partitions ({{closeIfEmpty()}} method) would sometimes consume the iterator, triggering short read protection *before* {{Counter}} ’s {{applyToPartition()}} gets called and resets its {{rowInCurrentPartition}} sub-counter.
> We should not be consuming iterators until all transformations are applied to them. For transformations it means that they cannot consume iterators unless they are the last transformation on the stack.
> The linked branch fixes the problem by splitting {{Filter}} into two transformations. The original - {{Filter}} - that does filtering within partitions - and a separate {{EmptyPartitionsDiscarder}}, that discards empty partitions from {{PartitionIterators}}. Thus {{DataResolve.resolve()}}, when constructing its {{PartitionIterator}}, now does merge first, then applies {{Filter}}, then {{Counter}}, and only then, as its last (third) transformation - the {{EmptyPartitionsDiscarder}}. Being the last one applied, it’s legal for it to consume the iterator, and triggering {{moreContents()}} is now no longer a problem.
> Fixes: [3.0|https://github.com/iamaleksey/cassandra/commits/13747-3.0], [3.11|https://github.com/iamaleksey/cassandra/commits/13747-3.11], [4.0|https://github.com/iamaleksey/cassandra/commits/13747-4.0]. dtest [here|https://github.com/iamaleksey/cassandra-dtest/commits/13747].



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