Resolves a deadlock scenario applying constraint #9110
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The scenario, which takes place on database instance applying constraint creation as an external transaction, looks like this: - Transaction T1 creates the constraint index and population P starts - Transaction T2 which activates the constraint starts applying and now has a read lock on the counts store - Check point triggers, wants to rotate counts store and so acquires its write lock. It will have to block, but doing so will also blocks further read lock requests - T2 moves on to activate the constraint. Doing so means first waiting for the index to come online - P moves on to flip after population, something which includes initializing some sample data in counts store for this index. Will block on the counts store read lock, completing the deadlock The solution in this commit breaks the deadlock by having T2 unlock its read lock on the counts store before activating the constraint, i.e. as soon as it notices that it is a schema transaction. Schema transactions will not update the counts store anyway.
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| public final OtherThreadRule<Void> t3 = new OtherThreadRule<>( "T3" ); | ||
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| @Test | ||
| public void shouldNotDeadlock() throws Exception |
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we need to add a timeout to this test just in case it will hang
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| @Test | ||
| public void shouldNotDeadlock() throws Exception | ||
| { |
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also we need to add a bit of assertions to the test, now we do not have any
| @@ -68,4 +74,15 @@ public boolean visitRelationshipCountsCommand( Command.RelationshipCountsCommand | |||
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| return false; | |||
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| @Override | |||
| public boolean visitSchemaRuleCommand( SchemaRuleCommand command ) throws IOException | |||
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also as we just talk, would be nice to have guarding assertion that will fail in case when this assumption will brake
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Yup, this has been added
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ping @tinwelint |
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@MishaDemianenko updated |
MishaDemianenko
approved these changes
Apr 11, 2017
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The scenario, which takes place on database instance applying constraint
creation as an external transaction, looks like this:
and now has a read lock on the counts store
its write lock. It will have to block, but doing so will also blocks
further read lock requests
the index to come online
some sample data in counts store for this index. Will block on the counts
store read lock, completing the deadlock
The solution in this commit breaks the deadlock by having T2 unlock its
read lock on the counts store before activating the constraint, i.e.
as soon as it notices that it is a schema transaction. Schema transactions
will not update the counts store anyway.