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From Phil Steitz <>
Subject Re: [DBCP] Connection just obtained from datasource is invalid
Date Tue, 09 Jan 2018 18:50:59 GMT
On 1/8/18 4:23 PM, Shawn Heisey wrote:
> On 11/22/2017 5:00 PM, Phil Steitz wrote:
>> If the problem is the evictor closing a connection and having that
>> connection delivered to a client, the problem is almost certainly in
>> pool.  The thread-safety of the pool in this regard is engineered in
>> DefaultPooledObject, which is the wrapper that pool manages and
>> delivers to DBCP.  When the evictor visits a PooledObject (in
>> GenericObjectPool#evict) it tries to start the eviction test on the
>> object by calling its startEvictionTest method.  This method is
>> synchronized on the DefaultPooledObject.  Look at the code in that
>> method.  It checks to make sure that the object is in fact idle in
>> the pool.  The other half of the protection here is in
>> GenericObjectPool#borrowObject, which is what PoolingDataSource
>> calls to get a connection.  That method tries to get a PooledObject
>> from the pool and before handing it out (or validating it), it calls
>> the PooledObject's allocate method.  Look at the code for that in
>> DefaultPooledObject.  That method (also synchronized on the
>> PooledObject) checks that the object is not under eviction and sets
>> its state to allocated.  That is the core sync protection that
>> *should* make it impossible for the evictor to do anything to an
>> object that has been handed out to a client.
> I see the synchronization you're talking about here.  It appears that
> all of the critical methods in DefaultPooledObject are synchronized (on
> the object).
> If you're absolutely certain that DefaultPooledObject is involved with
> all of the implementation my code is using, it all looks pretty complete
> to me. 

Yes, the code you posted at the top of the thread uses a
PoolableConnectionFactory as the object factory for the pool.  You
can see that PCF's makeObject returns a DefaultPooledObject, so that
much is certain.
>  So I'm really curious as to why the connection is getting
> closed.  I have seen the problem only minutes after restarting my
> program, so it seems unlikely that the server side is closing the
> connection, since the timeout for that is 8 hours.

I looked back at the initial stack trace and I noticed something
that I had not noticed before.

This line


means that checkOpen() succeeded.  That, combined with your
statement above that isClosed() returns true on a failed connection
means that there might be concurrent access to the
DelegatingConnections happening.  It looks like the sequence might
have been:

thread 1: checkOpen - sees true
thread 2: close the DelegatingConnection  (there is no sync to
prevent this)
thread1 : createStatement - bang!
thread1 : isClosed() returns true

DBCP is not really safe to use that way - i.e., really the intended
setup is that individual connection handles are not concurrently
accessed by multiple threads.  Is it possible something like this is
going on?  Note that what I am talking about here is two different
threads holding references to the same connection handle - i.e., no
trips back through the pool.

> I did add the code a while back to test on create, borrow, return, and
> while idle, but it turns out that I hadn't actually pulled it down to
> the test server and recompiled.  That is now done, so we'll see if that
> makes any difference.
> If testing the connection on pool actions does make a difference, then
> what is your speculation about what was happening when I ran into the
> closed connection only minutes after restart, and would it be worthy of
> an issue in Jira?  The only theory I had was a race condition between
> eviction and borrowing, but unless there's something amiss in how all
> the object inheritance works out, it looks like that's probably not it. 
> Some kind of issue with the TCP stack in Linux (either on the machines
> running my code or the MySQL server) is the only other idea I can think
> of.  Or maybe a hardware/firmware issue, since it's likely that at least
> one of the NICs involved is doing TCP offload.  I think that virtually
> every NIC in our infrastructure has that feature and that Linux enables it.

> Thanks,
> Shawn
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