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https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877761#action_12877761
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Knut Anders Hatlen commented on DERBY-4279:
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Hi Brett,
Thanks for contributing the patch. I think creating a new statement instead of
waiting sounds like a reasonable approach to solve this bug.
I have a couple of questions about the patch:
1) The synchronized block in GenericActivationHolder.execute() is expanded.
What's the reasoning behind this change?
2) In GenericStatement, I noticed that the old code that generates a new
statement if the session schema is references, sets foundInCache to false to
prevent the call to removeStatement() in catch blocks further down. Should we
do the same in the new code that checks the value of compilingStatement? I'm
not sure if calling removeStatement() on a statement that's not in the cache
does any harm, but I thought I'd mention it in any case.
3) Since the patch removes the call to wait() on the prepared statement (the
only call to wait() as far as I can tell), do you think it would be safe to
also remove the calls to notifyAll() in GenericStatement and
GenericPreparedStatement?
> Statement cache deadlock
> ------------------------
>
> Key: DERBY-4279
> URL: https://issues.apache.org/jira/browse/DERBY-4279
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.0.2.1, 10.1.3.1, 10.2.2.0, 10.3.3.0, 10.4.2.0,
> 10.5.1.1
> Environment: Windows Vista
> Reporter: Jeff Stuckman
> Assignee: Brett Wooldridge
> Attachments: Derby4279.java, patch4279.txt
>
>
> Due to a design flaw in the statement cache, a deadlock can occur if a
> prepared statement becomes out-of-date.
> I will illustrate this with the following example:
> The application is using the embedded Derby driver. The application has two
> threads, and each thread uses its own connection.
> There is a table named MYTABLE with column MYCOLUMN.
> 1. A thread prepares and executes the query SELECT MYCOLUMN FROM MYTABLE. The
> prepared statement is stored in the statement cache (see
> org.apache.derby.impl.sql.GenericStatement for this logic)
> 2. After some time, the prepared statement becomes invalid or out-of-date for
> some reason (see org.apache.derby.impl.sql.GenericPreparedStatement)
> 3. Thread 1 begins a transaction and executes LOCK TABLE MYTABLE IN EXCLUSIVE
> MODE
> 4. Thread 2 begins a transaction and executes SELECT MYCOLUMN FROM MYTABLE.
> The statement is in the statement cache but it is out-of-date. The thread
> begins to recompile the statement. To compile the statement, the thread needs
> a shared lock on MYTABLE. Thread 1 already has an exclusive lock on MYTABLE.
> Thread 2 waits.
> 5. Thread 1 executes SELECT MYCOLUMN FROM MYTABLE. The statement is in the
> statement cache but it is being compiled. Thread 1 waits on the statement's
> monitor.
> 6. We have a deadlock. Derby eventually detects a lock timeout, but the error
> message is not descriptive. The stacks at the time of the deadlock are:
> This deadlock is unique because it can still occur in a properly designed
> database. You are only safe if all of your transactions are very simple and
> cannot be interleaved in a sequence that causes the deadlock, or if your
> particular statements do not require a table lock to compile. (For the sake
> of simplicity, I used LOCK TABLE in my example, but any UPDATE statement
> would fit.)
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