On Thu, Jul 13, 2023 at 12:03:24PM +0100, Richard W.M. Jones wrote:

On Thu, Jul 13, 2023 at 09:53:58AM +0000, Tage Johansson wrote:

Apologize if resending, but I'm not sure my previous email was
actually delivered.


On 7/12/2023 10:33 PM, Eric Blake wrote:

On Wed, Jul 12, 2023 at 03:19:59PM +0000, Tage Johansson wrote:

Hello,


While writing some tests for the Rust bindings, I discovered a
memory leak
with Valgrind due to a completion callback not being freed. More
specificly,
the completion callback of `aio_opt_info()` doesn't seem to be if
`aio_opt_info()` failes. In this case, `aio_opt_info()` was called in the
connected state, so it should indeed fail, but it should perhaps
also call
the free function associated with the callback.

Can you paste the valgrind output showing the leak?

The Valgrind output is very Rust specific and only shows the Rust
allocations which goes into the completion callback and are lost.


But if you apply the following patch (which modifies
tests/opt-info.c) you shall see that the completion callback is not
called.

Ah. I see.  We are testing the synchronous command, but not the aio
counterpart command.  That is indeed a hole in the testsuite (even
after my patch yesterday).

I have replaced a call to `nbd_opt_info()` with a call to
`nbd_aio_opt_info()` and passed in a completion callback which just
calls `exit(EXIT_FAILURE)`. So if the completion callback is called
the test should fail, which it doesn't, at least not on my machine.

Isn't that OK?  Only .free is required to be called.

For the context callback (for opt_set_meta), .callback may be called
zero, one, or multiple times, but .free should be called exactly once.
But for the completion callback, I agree that the docs state that both
.callback and .free should each be called exactly once ("On the other
hand, if a completion callback is supplied (only possible with
asynchronous commands), it will always be reached exactly once, and
the completion callback must not ignore the value pointed to by
C<error>."); we are indeed missing the call to .callback.  I'll work
on a patch.

Ok, so if I understand correctly the completion callback should actually be called exactly once, right?

Then I will not have to adjust my code. I am storing the completion callback as an `FnOnce`, so it should free itself when it is called. If it will be called zero or one time I could store it in an `Option` and take it when it is called. The free callback would then free that Option.

But if the completion callback will be called exactly once that'll be unnecessary.

Best regards,

Tage