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https://issues.apache.org/jira/browse/CASSANDRA-20135?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17904189#comment-17904189
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Josh McKenzie edited comment on CASSANDRA-20135 at 12/9/24 3:27 PM:
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I've worked quite a bit recently with the {{StoppingTransformation}} in the
paging across tombstones code, but haven't really delved into
{{QueryCancellationChecker}} (CASSANDRA-17810 was an upstreaming of another
author's work and it's been 2 years so... brain cache purged =/).
If neither [~marcuse] or [~aleksey] have immediate context on that to determine
whether removal of that transformation application on the tail end is correct,
I can take a look at this maybe next week before I step away for holiday.
Edit: I'm getting more and more irritated w/JIRA comment formatting. /grumble
was (Author: jmckenzie):
I've worked quite a bit recently with the {{StoppingTransformation in the
paging across tombstones code, but haven't really delved into
{{QueryCancellationChecker (CASSANDRA-17810 was an upstreaming of another
author's work and it's been 2 years so... brain cache purged =/).}}}}
{{{{If neither [~marcuse] or [~aleksey] have immediate context on that to
determine whether removal of that transformation application on the tail end is
correct, I can take a look at this maybe next week before I step away for
holiday.}}}}
> Assertion errors on CheckForAbort / QueryCancellationChecker on multiple
> calls of applyToPartition
> --------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-20135
> URL: https://issues.apache.org/jira/browse/CASSANDRA-20135
> Project: Apache Cassandra
> Issue Type: Bug
> Components: Legacy/Core
> Reporter: Stefan Miklosovic
> Assignee: Stefan Miklosovic
> Priority: Normal
> Fix For: 4.0.x, 4.1.x, 5.0.x, 5.x
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> We see there are assertion errors thrown in 4.1 at least in
> StoppingTransformation like these:
> {code}
> java.lang.RuntimeException: java.lang.AssertionError
> at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:108)
> at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:45)
> at
> org.apache.cassandra.net.InboundMessageHandler$ProcessMessage.run(InboundMessageHandler.java:430)
> at
> org.apache.cassandra.concurrent.ExecutionFailure$1.run(ExecutionFailure.java:133)
> at org.apache.cassandra.concurrent.SEPWorker.run(SEPWorker.java:142)
> at
> io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
> at java.base/java.lang.Thread.run(Thread.java:829)
> Caused by: java.lang.AssertionError: null
> at
> org.apache.cassandra.db.transform.StoppingTransformation.attachTo(StoppingTransformation.java:72)
> at org.apache.cassandra.db.transform.BaseRows.add(BaseRows.java:104)
> at
> org.apache.cassandra.db.transform.UnfilteredRows.add(UnfilteredRows.java:49)
> at
> org.apache.cassandra.db.transform.Transformation.add(Transformation.java:198)
> at
> org.apache.cassandra.db.transform.Transformation.apply(Transformation.java:140)
> at
> org.apache.cassandra.db.ReadCommand$CheckForAbort.applyToPartition(ReadCommand.java:616)
> at
> org.apache.cassandra.db.ReadCommand$CheckForAbort.applyToPartition(ReadCommand.java:604)
> at
> org.apache.cassandra.db.transform.BasePartitions.hasNext(BasePartitions.java:97)
> at
> org.apache.cassandra.db.partitions.UnfilteredPartitionIterators$Serializer.serialize(UnfilteredPartitionIterators.java:303)
> at
> org.apache.cassandra.db.ReadResponse$LocalDataResponse.build(ReadResponse.java:201)
> at
> org.apache.cassandra.db.ReadResponse$LocalDataResponse.<init>(ReadResponse.java:186)
> at
> org.apache.cassandra.db.ReadResponse.createDataResponse(ReadResponse.java:48)
> at
> org.apache.cassandra.db.ReadCommand.createResponse(ReadCommand.java:337)
> at
> org.apache.cassandra.db.ReadCommandVerbHandler.doVerb(ReadCommandVerbHandler.java:63)
> at
> org.apache.cassandra.net.InboundSink.lambda$new$0(InboundSink.java:78)
> at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:97)
> ... 6 common frames omitted
> {code}
> This does not make sense at first sight and it is quite a rabbit hole to go
> through. If you follow the stacktrace, you see that
> {code}
> Caused by: java.lang.AssertionError: null
> at
> org.apache.cassandra.db.transform.StoppingTransformation.attachTo(StoppingTransformation.java:72)
> {code}
> but ... why? It means that this (1) was called twice because that is the only
> place where "this.rows" are ever updated in that class (and this.rows is
> private) which means that _something_ has to call this twice in a row. Once
> it sets it just fine and another time it goes to set it again but it fails as
> it is not null anymore. Hence, the question is why is that set twice?
> The reason is quite elaborative. "attachTo" which throws is ever called in
> BaseRows#add(Transformation) (2) and just on the next line it calls
> "super.add(transformation);" which adds that transformation at the end of a
> stack in Stack class which BaseRows extends.
> {code}
> void add(Transformation add)
> {
> if (length == stack.length)
> stack = resize(stack);
> stack[length++] = add;
> }
> {code}
> Next thing we see from the stacktrace is that CheckForAbort.applyToPartition
> is calling Transformation.apply (3) and what that ultimately does is that it
> will add itself, again, at the end of the stack (4).
> When we look at that stacktrace as a whole, what it does is that it is
> iterating over Unfiliteredpartition while building a local data response on a
> read and as it does so, it calls "BasePartitions.hasNext". Now we are getting
> to that ... (5). What "hasNext" is doing is that while this.next is null, it
> will take the stack and it loops in while by taking "next" from "input" and
> it applies all the transformations by calling "fs[i].applyToParition(next)".
> So, there is a stack of transformations and they are called just one after
> another until some result of "applyToPartition" returns null or we iterated
> over all transformations. The chain of transformations also include
> "CheckForAbort" transformation which we added here (6) so what happens is
> that when we call "applyToPartitions" for the first time on CheckForAbort, it
> will run just fine, but when that while loop / for loop in BasePartitions is
> called _again_ (e.g. we are calling "hasNext" upon iterating in
> UnfilteredPartitionIterators), then "applyToPartition" for "CheckForAbort"
> will be called again as well. But CheckForAbort is doing this (7).
> {code}
> protected UnfilteredRowIterator
> applyToPartition(UnfilteredRowIterator partition)
> {
> if (maybeAbort())
> {
> partition.close();
> return null;
> }
> return Transformation.apply(partition, this);
> }
> {code}
> Check the last line where it applies itself when it is not aborted:
> {code}
> Transformation.apply(partition, this)
> {code}
> The application of this stopping transformation to given partition means that
> it will add that transformation at the end of the stack as we already showed.
> Then, we will iterate over that stack again upon iterating in BasePartitions,
> which eventually calls "attachTo" for the second time, hence the assertion
> error.
> The stack might look like
> {code}
> stack[0] = transformation1
> stack[1] = transformation2
> stack[2] = CheckForAbort
> {code}
> then we call "fs[i].applyToParition(next)" which will modify the stack like
> this:
> {code}
> stack[0] = transformation1
> stack[1] = transformation2
> stack[2] = CheckForAbort
> stack[3] = CheckForAbort
> {code}
> Then we will loop over that again and if I am not mistaken, when we hit
> stack[2], it will call applyToPartition on that and it will do
> {code}
> stack[0] = transformation1
> stack[1] = transformation2
> stack[2] = CheckForAbort // this basically adds itself at the end again
> stack[3] = CheckForAbort
> stack[4] = CheckForAbort
> {code}
> but we actually never get this far because on adding itself to the stack, we
> will hit that assertion error.
> I also see that CheckForAbort was replaced by CASSANDRA-17810 (8) by
> QueryCancellationChecker but except some "cosmetic" changes, the logic
> remains the same. That stopping transformation is applying itself in
> applyToPartition so I think that this problem is present in 5.0+ too. No
> transformation is applying itself like that but this one.
> I am not completely sure what we should do about this but two ideas are
> obvious (with non-zero probability that both of them are wrong)
> 1) We apply _new instance_ of QueryCancellationChecker / CheckForAbort so we
> will never call attachTo on the _same_ instance (but then we would end up
> with a bunch of QueryCancellationChecker / CheckForAbort instances in the
> stack, brrr)
> 2) We will remove "assert" in attachTo in StoppingTransformation so we will
> enable this to be called twice so we will not throw at least ...
> (1)
> https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/transform/StoppingTransformation.java#L72
> (2)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/transform/BaseRows.java#L104
> (3)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/ReadCommand.java#L627
> (4)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/transform/Transformation.java#L198
> (5)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/transform/BasePartitions.java#L87-L109
> (6)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/ReadCommand.java#L436
> (7)
> https://github.com/apache/cassandra/blob/cassandra-4.1/src/java/org/apache/cassandra/db/ReadCommand.java#L619-L628
> (8)
> https://github.com/apache/cassandra/commit/f4b69ba0e82bb051e56a92d792142034d9f617f0#diff-554e7dff38b500f5eaed0b9b651c7098c3f8a1bd4f6aca12063eab352e685b9fR690
> cc [~jmckenzie] [~marcuse] [~aleksey]
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