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https://issues.apache.org/jira/browse/DERBY-3260?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12555295#action_12555295
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Knut Anders Hatlen commented on DERBY-3260:
-------------------------------------------
The fix looks correct to me. We could probably make the synchronized block even
smaller, since the last part of it only modifies the state of the activation,
which is not shared between statements.
It would be interesting, though, if someone could shed some light on why the
synchronization was commented out in the first place. This piece of code hasn't
been changed since the code was donated to Apache, so the commit log doesn't
tell us anything about it.
The synchronization statement that is commented out, enclosed the entire if
statement, whereas the suggested fix only synchronizes the body of the if
statement. I'm not sure, but I think this is fine, since the only risk is that
we miss that someone else has recompiled/invalidated the plan, for which there
are plenty of tests when we start executing the plan.
> NullPointerException caused by race condition in GenericActivationHolder
> ------------------------------------------------------------------------
>
> Key: DERBY-3260
> URL: https://issues.apache.org/jira/browse/DERBY-3260
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.0.2.1, 10.1.1.0, 10.1.2.1, 10.1.3.1, 10.2.1.6,
> 10.2.2.0, 10.3.1.4
> Reporter: Robert Yokota
> Priority: Blocker
> Attachments: GenericActivationHolder.java.diff
>
>
> I have a stress test using Derby 10.3.1.4 which is executing the same
> PreparedStatement using multiple threads concurrently and I consistently get
> the following NPE after several hours of running:
> 2007-12-07 00:48:10.914 GMT Thread[pool-5-thread-25,5,main] (XID = 1219661),
> (SESSIONID = 377), (DATABASE = /usr/ironhide/var/db/opera/derby), (DRDAID =
> null), Failed Statement is: select rdbmsvaria0_.GUID_AND_INDEX as GUID1_3_0_,
> rdbmsvaria0_.VALUE2 as VALUE2_3_0_, rdbmsvaria0_.HOLDER_GUID as HOLDER3_3_0_,
> rdbmsvaria0_.VALUE_TYPE as VALUE4_3_0_, rdbmsvaria0_.VALUE_GUID as
> VALUE5_3_0_, rdbmsvaria0_.DELETED as DELETED3_0_ from VARIABLE rdbmsvaria0_
> where rdbmsvaria0_.GUID_AND_INDEX=? with 1 parameters begin parameter #1:
> 9C202AB9E8356288A9320C9C383A4D2F-11 :end parameter
> java.lang.NullPointerException
> at
> org.apache.derby.impl.sql.GenericActivationHolder.execute(GenericActivationHolder.java:271)
> at
> org.apache.derby.impl.sql.GenericPreparedStatement.execute(GenericPreparedStatement.java:368)
> at
> org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(EmbedStatement.java:1203)
> at
> org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeStatement(EmbedPreparedStatement.java:1652)
> at
> org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeQuery(EmbedPreparedStatement.java:275)
> at
> org.hibernate.jdbc.AbstractBatcher.getResultSet(AbstractBatcher.java:186)
> at org.hibernate.loader.Loader.getResultSet(Loader.java:1787)
> at org.hibernate.loader.Loader.doQuery(Loader.java:674)
> at
> org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:236)
> at org.hibernate.loader.Loader.loadEntity(Loader.java:1860)
> at
> org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:48)
> at
> org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:42)
> at
> org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3044)
> at
> org.hibernate.event.def.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:395)
> at
> org.hibernate.event.def.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:375)
> at
> org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:139)
> at
> org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:179)
> at
> org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:103)
> at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:878)
> at org.hibernate.impl.SessionImpl.get(SessionImpl.java:815)
> at org.hibernate.impl.SessionImpl.get(SessionImpl.java:808)
> at sun.reflect.GeneratedMethodAccessor69.invoke(Unknown Source)
> at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> at java.lang.reflect.Method.invoke(Method.java:597)
> at
> org.hibernate.context.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:301)
> at $Proxy41.get(Unknown Source)
> at
> com.approuter.maestro.opera.rdbms.RdbmsContextHolder.getRdbmsVariable(RdbmsContextHolder.java:108)
> at
> com.approuter.maestro.opera.rdbms.RdbmsContextHolder.getVariable(RdbmsContextHolder.java:94)
> at com.approuter.maestro.vm.Frame.getParameter(Frame.java:218)
> at com.approuter.maestro.vm.Task.getParameter(Task.java:1267)
> at
> com.approuter.maestro.vm.CallContextImpl.setOutputParameter(CallContextImpl.java:195)
> at
> com.approuter.maestro.vm.CallContextImpl.getOutputParameterWriter(CallContextImpl.java:264)
> at
> com.approuter.maestro.sdk.mpi.DynamicExecutableActivity$3$1.getWriter(DynamicExecutableActivity.java:249)
> at
> com.approuter.module.xml.XmlSerializeTextActivity$XmlSerializeTextActivityInstance.serialize(XmlSerializeTextActivity.java:43)
> at sun.reflect.GeneratedMethodAccessor161.invoke(Unknown Source)
> at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> at java.lang.reflect.Method.invoke(Method.java:597)
> at
> com.approuter.maestro.sdk.mpi.DynamicExecutableActivity.execute(DynamicExecutableActivity.java:332)
> at com.approuter.maestro.activities.Invoke.call(Invoke.java:199)
> at sun.reflect.GeneratedMethodAccessor157.invoke(Unknown Source)
> at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> at java.lang.reflect.Method.invoke(Method.java:597)
> at
> com.approuter.maestro.activities.Instruction.call(Instruction.java:22)
> at com.approuter.maestro.vm.Program.call(Program.java:456)
> at com.approuter.maestro.vm.Task.run(Task.java:580)
> at com.approuter.maestro.vm.Task.run(Task.java:531)
> at
> java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
> at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
> at java.util.concurrent.FutureTask.run(FutureTask.java:138)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:98)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:207)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
> at java.lang.Thread.run(Thread.java:619)
> Cleanup action completed
> Here is the relevant code from GenericActivationHolder.java
> // synchronized (ps)
> {
> /* Has the activation class changed? */
> if (gc != ps.getActivationClass())
> {
> // ensure the statement is valid by rePreparing it.
> ps.rePrepare(getLanguageConnectionContext()); // <-- line 260
> /*
> ** If we get here, it means the PreparedStatement has been
> ** recompiled. Get a new Activation and check whether the
> ** parameters are compatible. If so, transfer the parameters
> ** from the old Activation to the new one, and make that the
> ** current Activation. If not, throw an exception.
> */
> GeneratedClass newGC = ps.getActivationClass();
> BaseActivation newAC = (BaseActivation)
> newGC.newInstance(lcc); // <-- line 271
> DataTypeDescriptor[] newParamTypes =
> ps.getParameterTypes();
> As you can see, the activation class returned by the ExecPreparedStatement is
> null as of line 271, even after having called a rePrepare(...) on the
> prepared statement (line 260). This is most likely due to a race condition.
> Also note that the containing block was formerly synchronized, but has since
> been commented out. From browsing the source in previous versions, the
> synchronized statement appears to have been commented out since before
> 10.0.2.1. It appears the fix is to reintroduce the synchronized block, or at
> least make the block inside the if statement synchronized.
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