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From Phil Steitz <phil.ste...@gmail.com>
Subject Re: [DBCP] Connection just obtained from datasource is invalid
Date Tue, 09 Jan 2018 20:56:28 GMT
On 1/9/18 11:50 AM, Phil Steitz wrote:
> 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
>
> org.apache.commons.dbcp2.DelegatingConnection.createStatement(DelegatingConnection.java:262)
>
> 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 just noticed another thing in [pool] that might have something to
do with this.  It's probably best to investigate what I have in mind
on the dev list.  I will post a summary / ticket reference here if
it turns out I this is a bug.

Phil
>
> Phil
>> 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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