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From "Dyre Tjeldvoll (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3198) Using setQueryTimeout will leak sections
Date Thu, 22 Nov 2007 19:40:43 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3198?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12544883
] 

Dyre Tjeldvoll commented on DERBY-3198:
---------------------------------------

Based on what I see methods similar to writeSetSpecialRegister do, I think a safe and reasonable
way of getting a Section is as follows:

1) If the surrounding code has set the newSection variable, use that. This will usually be
the case when writeSetSpecialRegister is used to request session data, as it happens after
this variable has been assigned.

2) If Statement.section_ is not null, it is probably safe to use it.

3) if Statement.section_ is null, request a new one from the SectionManager and assign that
to section_. Assigning it to section_ will ensure that it is freed later.

Doing so does, however, require a change to the signature of the writeSetSpecialRegister()
methods in the Net-layer, so that the Section to use can be passed in as a parameter (this
seems to be how it is done in other write methods). The change is necessary because the decision
about which Section to use need to happen in the am-layer. It cannot be done in  NetStatementRequest
as it currently is, because there we don't have access to the Section variables mentioned
earlier. 

I'm wondering if the ability to access the SectionManager from the Net-layer is an unintended
loophole. Seems like NetStatementReply.writeSetSpecialRegister() is the only place in the
net package where it is used. The SectionManager is accessed through a public member in am.Agent
which has the following comment:
     public SectionManager sectionManager_ = null; // temporarily public, make friendly at
least !!

The Net-layer gets at this member because NetAgent extends am.Agent.

> Using setQueryTimeout will leak sections 
> -----------------------------------------
>
>                 Key: DERBY-3198
>                 URL: https://issues.apache.org/jira/browse/DERBY-3198
>             Project: Derby
>          Issue Type: Bug
>          Components: JDBC, Network Client
>    Affects Versions: 10.3.1.4
>            Reporter: Dyre Tjeldvoll
>         Attachments: repro.diff
>
>
> The implementation of setQueryTimeout relies on NetStatementReply.writeSetSpecialRegister()
which will allocate a dynamic section when called. No reference to this Section object is
kept, and so Section.free() never gets called on it. Executing the same statment repeatedly
with a query timeout set results in the client driver throwing an exception because the number
of Sections exceeding 32000.

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