Re: postgres_fdw - cached connection leaks if the associated user mapping/foreign server is dropped - Mailing list pgsql-hackers

On 2020/12/24 15:42, Bharath Rupireddy wrote:
> On Thu, Dec 24, 2020 at 7:21 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>> On 2020/12/23 23:40, Bharath Rupireddy wrote:
>>> On Wed, Dec 23, 2020 at 7:31 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>>>> I agree to make pgfdw_xact_callback() close the connection when
>>>> entry->invalidated == true. But I think that it's better to get rid of
>>>> have_invalid_connections flag and make pgfdw_inval_callback() close
>>>> the connection immediately if entry->xact_depth == 0, to avoid unnecessary
>>>> scan of the hashtable during COMMIT of transaction not accessing to
>>>> foreign servers. Attached is the POC patch that I'm thinking. Thought?
>>>
>>> We could do that way as well. It seems okay to me. Now the disconnect
>>> code is spread in pgfdw_inval_callback() and pgfdw_xact_callback().
>>> There's also less burden on pgfdw_xact_callback() to close a lot of
>>> connections on a single commit. The behaviour is like this - If
>>> entry->xact_depth == 0, then the entries wouldn't have got any
>>> connection in the current txn so they can be immediately closed in
>>> pgfdw_inval_callback() and pgfdw_xact_callback() can exit immediately
>>> as xact_got_connection is false. If entry->xact_depth > 0 which means
>>> that probably pgfdw_inval_callback() came from a sub txn, we would
>>> have got a connection in the txn i.e. xact_got_connection becomes true
>>> due to which it can get invalidated in pgfdw_inval_callback() and get
>>> closed in pgfdw_xact_callback() at the end of the txn.
>>>
>>> And there's no chance of entry->xact_depth > 0 and xact_got_connection false.
>>>
>>> I think we need to change the comment before pgfdw_inval_callback() a bit:
>>>
>>>    * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
>>>    * mark connections depending on that entry as needing to be remade.
>>>    * We can't immediately destroy them, since they might be in the midst of
>>>    * a transaction, but we'll remake them at the next opportunity.
>>>
>>> to
>>>
>>>    * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
>>> *  try to close the cached connections associated with them if they
>>> are not in the
>>> *  midst of a transaction otherwise mark them as invalidated. We will
>>> destroy the
>>>    * invalidated connections in pgfdw_xact_callback() at the end of the main xact.
>>>    * Closed connections will be remade at the next opportunity.
>>
>> Yes, I agree that we need to update that comment.
>>
>>>
>>> Any thoughts on the earlier point in [1] related to a test case(which
>>> becomes unnecessary with this patch) coverage?
>>>
>>
>> ISTM that we can leave that test as it is because it's still useful to test
>> the case where the cached connection is discarded in pgfdw_inval_callback().
>> Thought?
> 
> Yes, that test case covers the code this patch adds i.e. closing the
> connection in pgfdw_inval_callback() while committing alter foreign
> server stmt.
> 
>> By applying the patch, probably we can get rid of the code to discard
>> the invalidated cached connection in GetConnection(). But at least for
>> the back branches, I'd like to leave the code as it is so that we can make
>> sure that the invalidated cached connection doesn't exist when getting
>> new connection. Maybe we can improve that in the master, but I'm not
>> sure if it's really worth doing that against the gain. Thought?
> 
> +1 to keep that code as is even after this patch is applied(at least
> it works as an assertion that we don't have any leftover invalid
> connections). I'm not quite sure, we may need that in some cases, say
> if we don't hit disconnect_pg_server() code in pgfdw_xact_callback()
> and pgfdw_inval_callback() due to some errors in between. I can not
> think of an exact use case though.
> 
> Attaching v2 patch that adds the comments and the other test case that
> covers disconnecting code in pgfdw_xact_callback. Please review it.

Thanks for updating the patch! It basically looks good to me except
the following minor things.

+ * After a change to a pg_foreign_server or pg_user_mapping catalog entry, try
+ * to close the cached connections associated with them if they are not in the
+ * midst of a transaction otherwise mark them as invalid. We will destroy the
+ * invalidated connections in pgfdw_xact_callback() at the end of the main
+ * xact. Closed connections will be remade at the next opportunity.

Even when we're in the midst of transaction, if that transaction has not used
the cached connections yet, we close them immediately. So, to make the
comment more precise, what about updating the comment as follows?

---------------------
     After a change to a pg_foreign_server or pg_user_mapping catalog entry,
     close connections depending on that entry immediately if current
     transaction has not used those connections yet. Otherwise, mark those
     connections as invalid and then make pgfdw_xact_callback() close them
     at the end of current transaction, since they cannot be closed in the midst
     of a transaction using them. Closed connections will be remade at the next
     opportunity if necessary.
---------------------

+            /*
+             * Close the connection if it's not in midst of a xact. Otherwise
+             * mark it as invalid so that it can be disconnected at the end of
+             * main xact in pgfdw_xact_callback().
+             */

Because of the same reason as the above, what about updating this comment
as follows?

---------------------
     Close the connection immediately if it's not used yet in this transaction.
     Otherwise mark it as invalid so that pgfdw_xact_callback() can close it
     at the end of this transaction.
---------------------

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION