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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4279) Statement cache deadlock
Date Thu, 24 Jun 2010 14:35:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12882166#action_12882166
] 

Knut Anders Hatlen commented on DERBY-4279:
-------------------------------------------

I haven't studied the details of this code yet, but the synchronization on getConnectionSynchronization()
is probably not enough to ensure safe access to the GenericPreparedStatement. getConnectionSynchronization()
is used to ensure that a single connection cannot be executing in two threads at the same
time, but one GenericPreparedStatement can be shared between multiple connections.

Assuming this is the same issue as DERBY-3823/DERBY-1635 and it is getActivationClass() that
returns null, it looks like getMetaData() must have been called when the statement was being
recompiled by another thread, and that thread had already called preparedStatement.setActivationClass(null)
in GenericPreparedStatement.prepMinion(). I'm not sure if more synchronization is what's needed
to fix this bug, or if the code just needs to handle null returned from getActivationClass(),
or perhaps both.

> 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
>             Fix For: 10.7.0.0
>
>         Attachments: Derby4279.java, patch4279.txt, stacktrace.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becomes out-of-date.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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