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From "Brett Wooldridge (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4279) Statement cache deadlock
Date Thu, 28 Jun 2012 13:51:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13403101#comment-13403101

Brett Wooldridge commented on DERBY-4279:


None of this is documented obviously, but here is my best guess why that code is the way that
it is.  Trying to open the conglomerate acts as a check that the schema has not been modified
by another thread/transaction.  There may be other locks in place, as you mentioned, that
attempt to prevent DDL run running but my guess is there are still 'seams' where it can happen.
 Hence the opening of the conglomerate, and note also (especially) the comments in the try...catch
in GenericStatement.prepare() where the StandardException is caught and explicitly checked
for an error.  The comment is:

    // There is a chance that we didn't see the invalidation 
    // request from a DDL operation in another thread because 
    // the statement wasn't registered as a dependent until 
    // after the invalidation had been completed.  Assume that's 
    // what has happened if we see a conglomerate does not exist 
    // error...

Which implies to me that DDL might not always be blocked, and code was put in place to handle
that case.  Since DDL is rare in a running system, catching an exception might therefore be
much cheaper than some kind of read/write lock to try to prevent it.

The other possibility is that DDL and DML are completely protected/synchronized now (compared
to when that code was written), and some of that code is simply unnecessary.  I'm not in a
position to make that judgement.  Hence my attempt to retain as much as of the original behavior,
be it inefficient or not, to minimize side-effects.

I would be perfectly happy with the patch as submitted along with additional comments in the
code basically asking 'Is the still necessary?' with a reference to this bug.  Maybe at some
point in the future someone digging through the code will unravel the logic completely and
determine that 'yes, it is necessary' or 'no, it is not'.

> Statement cache deadlock
> ------------------------
>                 Key: DERBY-4279
>                 URL: https://issues.apache.org/jira/browse/DERBY-4279
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions:,,,,,,
>         Environment: Windows Vista, OS X 10.5+
>            Reporter: Jeff Stuckman
>              Labels: derby_triage10_5_2
>         Attachments: Derby4279.java, client_stacktrace_activation_closed.txt, no-lock-experiment.diff,
patch4279.txt, patch4279_2.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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