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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2861) Thread safety issue in TableDescriptor
Date Sun, 15 Jun 2008 00:30:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12605115#action_12605115
] 

Dag H. Wanvik commented on DERBY-2861:
--------------------------------------

I instrumented the  code that sets referencedColumnMap to collect the thread and the thread's
stacktrace when it sets it to null,  and printed that as well as the current thread and the
stacktrace 
when the error occurs:

I got these two stack traces that show how two threads get in each others way here.


--->thread 1: Thread[Thread-10,5,main]
** org.apache.derby.iapi.sql.dictionary.TableDescriptor.setReferencedColumnMap(TableDescriptor.java:381)
** org.apache.derby.impl.sql.depend.BasicDependencyManager.clearColumnInfoInProviders(BasicDependencyManager.java:707)
** org.apache.derby.impl.sql.compile.CreateViewNode.bindViewDefinition(CreateViewNode.java:290)
** org.apache.derby.impl.sql.compile.CreateViewNode.bindStatement(CreateViewNode.java:181)
** org.apache.derby.impl.sql.GenericStatement.prepMinion(GenericStatement.java:314)
** org.apache.derby.impl.sql.GenericStatement.prepare(GenericStatement.java:88)
** org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.prepareInternalStatement(GenericLanguageConnectionContext.java:794)
** org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:606)
** org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:555)
** TestEmbeddedMultiThreading.executeStatement(TestEmbeddedMultiThreading.java:124)
** TestEmbeddedMultiThreading.access$100(TestEmbeddedMultiThreading.java:25)
** TestEmbeddedMultiThreading$ViewCreatorDropper.run(TestEmbeddedMultiThreading.java:188)
** java.lang.Thread.run(Thread.java:619)

--->thread 2: Thread[Thread-5,5,main]
java.lang.Exception: Stack trace
	at java.lang.Thread.dumpStack(Thread.java:1206)
	at org.apache.derby.iapi.sql.dictionary.TableDescriptor.getObjectName(TableDescriptor.java:796)
	at org.apache.derby.impl.sql.depend.BasicDependencyManager.getPersistentProviderInfos(BasicDependencyManager.java:681)
	at org.apache.derby.impl.sql.compile.CreateViewNode.bindViewDefinition(CreateViewNode.java:287)
	at org.apache.derby.impl.sql.compile.CreateViewNode.bindStatement(CreateViewNode.java:181)
	at org.apache.derby.impl.sql.GenericStatement.prepMinion(GenericStatement.java:314)
	at org.apache.derby.impl.sql.GenericStatement.prepare(GenericStatement.java:88)
	at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.prepareInternalStatement(GenericLanguageConnectionContext.java:794)
	at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:606)
	at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:555)
	at TestEmbeddedMultiThreading.executeStatement(TestEmbeddedMultiThreading.java:124)
	at TestEmbeddedMultiThreading.access$100(TestEmbeddedMultiThreading.java:25)
	at TestEmbeddedMultiThreading$ViewCreatorDropper.run(TestEmbeddedMultiThreading.java:188)
	at java.lang.Thread.run(Thread.java:619)

These lines in CreatViewNode#bindViewDefinition account for both thread stacks:

(2) ProviderInfo[] providerInfos = dm.getPersistentProviderInfos(apl);
      // need to clear the column info in case the same table descriptor
      // is reused, eg., in multiple target only view definition
(1) dm.clearColumnInfoInProviders(apl);

Thread 1 is excuting line marked (1), thread 2 is executing line marked (2).


> 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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