[DB-SIG] conn.close() idempotence

[M.-A. Lemburg]

Federico Di Gregorio wrote:
> On 19/10/11 09:59, M.-A. Lemburg wrote:
> [snip]
>>> risks to become a problem, with the close() that may raise an error
>>>> because the connection has been implicitly closed by a communication
>>>> error. Note that close() in itself often is a quite safe operation,
>>>> not involving database communication, so it is not expected to fail
>>>> and may not be well guarded.
>> That last sentence is not quite correct: .close() issues an implicit
>> .rollback() and usually also tells the database backend to free up
>> resources maintained for the connection, so it does require communication
>> with the backend.
> 
> Here is the problem. Does .close() always issue an implicit .rollback()?

Yes.

> The DBAPI says yes but, as noted previously, the driver can choose to
> NOT send a rollback.

Not if it complies to the DB-API.

> In fact on the second .close() the driver SHOULD
> NOT send a rollback because there is no transaction in progress.

The second .close() would in that case raise an exception
as per the definition in the database, since it has become
unusable :-)

BTW: Regardless of whether the driver explicitly issues a rollback
or not, the database backend will roll back the connection if the
connection closes and there's a pending transaction.

> I'd vote for idempotent .close() interpretation too.

Please read my full reply: the implicit rollback is not
the only operation that can fail. Silencing errors is not
a good idea.

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Marc-Andre Lemburg
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