[Libguestfs] libnbd: When are callbacks freed

[Tage Johansson]

On 7/13/2023 3:26 PM, Richard W.M. Jones wrote:
> On Thu, Jul 13, 2023 at 08:01:09AM -0500, Eric Blake wrote:
>> On Thu, Jul 13, 2023 at 07:13:37AM -0500, Eric Blake wrote:
>>>>> 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.
>> Eww, the bug is bigger than just nbd_aio_opt* not always calling
>> completion.callback exactly once.  With just this diff (to be folded
>> into the larger patch I'm working on), I'm getting an assertion
>> failure that we fail to call completion.callback for
>> nbd_aio_pread_structured when calling nbd_close() prior to the command
>> running to completion, so I'll have to fix that too.
> Just to be clear about this, are we really sure the completion
> callback should always be called once?  I'm not clear why that should
> be the case.  (The .free callback however should be.)
>
> For example, if I call a function with bogus invalid parameters so
> that it fails very early on (or when the handle is in an incorrect
> state), should I expect the completion callback to be called?  I would
> expect not.
>
> Rich.


The user needs a way to know if an error occurred. So the completion 
callback must be called if the asynchronous function did not fail 
(returned 0). If the completion callback should be called, with the 
error parameter set, even when the asynchronous function immediately 
failed with a non-zero return value is another question. I see two 
possibilities: Either the completion callback should always be called. 
Or it should be called iff the asynchronous function returned 0 (did not 
fail).


By the way, if the error parameter is set in the completion callback, 
how can the user retrieve the error text? Is it possible to call 
`get_nbd_error(3)` from inside the completion callback?


Best regards,

Tage


>> diff --git i/tests/closure-lifetimes.c w/tests/closure-lifetimes.c
>> index 9bb4e120..c5ad4c10 100644
>> --- i/tests/closure-lifetimes.c
>> +++ w/tests/closure-lifetimes.c
>> @@ -66,6 +66,7 @@ read_cb (void *opaque,
>>            uint64_t offset, unsigned status, int *error)
>>   {
>>     assert (!read_cb_freed);
>> +  assert (!completion_cb_called);
>>     read_cb_called++;
>>     return 0;
>>   }
>> @@ -73,6 +74,7 @@ read_cb (void *opaque,
>>   static void
>>   read_cb_free (void *opaque)
>>   {
>> +  assert (completion_cb_called);
>>     read_cb_freed++;
>>   }
>>
>> @@ -81,6 +83,7 @@ block_status_cb (void *opaque, const char *meta, uint64_t offset,
>>                    uint32_t *entries, size_t nr_entries, int *error)
>>   {
>>     assert (!block_status_cb_freed);
>> +  assert (!completion_cb_called);
>>     block_status_cb_called++;
>>     return 0;
>>   }
>> @@ -88,6 +91,7 @@ block_status_cb (void *opaque, const char *meta, uint64_t offset,
>>   static void
>>   block_status_cb_free (void *opaque)
>>   {
>> +  assert (completion_cb_called);
>>     block_status_cb_freed++;
>>   }
>>
>> @@ -102,6 +106,7 @@ completion_cb (void *opaque, int *error)
>>   static void
>>   completion_cb_free (void *opaque)
>>   {
>> +  assert (completion_cb_called);
>>     completion_cb_freed++;
>>   }
>>
>>
>> -- 
>> Eric Blake, Principal Software Engineer
>> Red Hat, Inc.           +1-919-301-3266
>> Virtualization:  qemu.org | libvirt.org