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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Issue Comment Edited: (DERBY-2861) Thread safety issue in TableDescriptor
Date Sun, 15 Jun 2008 01:48:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12605117#action_12605117
] 

dagw edited comment on DERBY-2861 at 6/14/08 6:47 PM:
---------------------------------------------------------------

I got rid of that one by adding:

     synchronized(getDataDictionary().getDependencyManager()) {
          :

to setReferencedColumnMap. But this all seems problematic,
since the referenceColumnMap is used unsynchronized also from:

    FromBaseTable#getMatchingColumn

It seems the mechanism of this shared state (refrencedColumnMap) in the TableDescriptor which
is being manipulated by several threads is not sound and needs some rethinking.

Uploading the patch I used to get this far, experiment.diff.



      was (Author: dagw):
    I got rid of that one by adding:

     synchronized(getDataDictionary().getDependencyManager()) {
          :

to setReferencedColumnMap. But this all seems problematic,
since the referenceColumnMap is used unsynchronized also from:

    FromBaseTable#getMatchingColumn

It seems the mechanism of this shared state (refrencedColumnMap) in the TableDescriptor 
is not sound which is being manipulated by several threads needs some thinking.

Uploading the patch I used to get this far, experiment.diff.


  
> 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: experiment.diff, 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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