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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2861) Thread safety issue in TableDescriptor
Date Sat, 14 Jun 2008 23:56:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12605113#action_12605113
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Dag H. Wanvik commented on DERBY-2861:
--------------------------------------

It seems there is a race condition here in that referencedColumnMap
is being set to null *after* the test at the start of getObjectName but before
the usage.

By inserting a Thread.sleep(1000) at a line just before the access

>    if (referencedColumnMap.isSet(cd.getPosition())

I can reliably reproduce the error. I will try to find out what usage/access sets 
referencedColumnMap to null now ;)


> Thread safety issue in TableDescriptor
> --------------------------------------
>
>                 Key: DERBY-2861
>                 URL: https://issues.apache.org/jira/browse/DERBY-2861
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>         Environment: Tested on a dual-core 3GHz Pentium machine running Windows Vista
Business, using JDK 1.4.2_13 and Derby trunk revision 548822.
>            Reporter: Jeff Clary
>         Attachments: TestEmbeddedMultiThreading.java
>
>
> A NullPointerException occurs in org.apache.derby.iapi.sql.dictionary.TableDescriptor.getObjectName
when accessing the same object on many threads (each with its own connection).  The attached
test program starts N threads each creating and then dropping a separate view against the
same source view, repeated M times.  I can reproduce the problem with N=100 and M=100 on my
machine, but not every run.
> An instance member named referencedColumnMap is checked for null at the top of the getObjectName
method, but later when it is dereferenced it is null, because it was set to null by another
thread.  I am not sure what getObjectName is used for other than error reporting.  I have
considered a fix of just saving the non-null reference as a method variable, to avoid the
later NullPointerException.   But I don't know what unintended consequences this may have.

> When the test program does show the exception, the stack trace looks like this:
>  java.lang.NullPointerException
>    at org.apache.derby.iapi.sql.dictionary.TableDescriptor.getObjectName(TableDescriptor.java:758)
>    at org.apache.derby.impl.sql.depend.BasicDependencyManager.getPersistentProviderInfos(BasicDependencyManager.java:677)
>    at org.apache.derby.impl.sql.compile.CreateViewNode.bindViewDefinition(CreateViewNode.java:287)
>    at org.apache.derby.impl.sql.compile.CreateViewNode.bind(CreateViewNode.java:183)
>    at org.apache.derby.impl.sql.GenericStatement.prepMinion(GenericStatement.java:345)
>    at org.apache.derby.impl.sql.GenericStatement.prepare(GenericStatement.java:119)
>    at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.prepareInternalStatement(GenericLanguageConnectionContext.java:745)
>    at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:568)
>    at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:517)
>    at TestEmbeddedMultiThreading.executeStatement(TestEmbeddedMultiThreading.java:109)
>    at TestEmbeddedMultiThreading.access$100(TestEmbeddedMultiThreading.java:10)
>    at TestEmbeddedMultiThreading$ViewCreatorDropper.run(TestEmbeddedMultiThreading.java:173)
>    at java.lang.Thread.run(Thread.java:534)
>  

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