Re: [PATCH v4 2/3] module: add Error arguments to module_load_one and module_load_qom_one

[Claudio Fontana]

On 9/22/22 15:20, Markus Armbruster wrote:
> Claudio Fontana <cfontana@suse.de> writes:
> 
> [...]
> 
>> I think it would be better to completely make the return value separate from 
>> the Error,
>> and really treat Error as an exception and not mix it up with the regular 
>> execution,
>>
>> but if it is the general consensus that I am the only one seeing this 
>> conflation problem we can model it this way too.
> 
> It's a matter of language pragmatics.  In Java, you throw an exception
> on error.  In C, you return an error value.
> 
> Trying to emulate exceptions in C might be even more unadvisable than
> trying to avoid them in Java.  Best to work with the language, not
> against it.
> 
> Trouble is the error values we can conveniently return in C can't convey
> enough information.  So we use Error for that.  Just like GLib uses

Right, we use Error for that and that's fine, but we should use it _only Error_ 
for that.

Ie map the Exceptions directly to Error.

So:

try {

  rv = function_call(...);

  use_return_value(rv);

} catch (Exception e) {

  /* handle exceptional case */

}

becomes

rv = function_call(..., Error **errp);

if (errp) {
  /* handle exceptional case */
}

use_return_value(rv);


Instead we mix up the Exception code path and the regular code path, by having 
rv carry a mix of the Exception and regular return value,
and this creates problems and confusion.

If we put a hard line between the two, I think more clarity emerges.


> GError.
> 
> More modern languages do "return error value" much better than C can.  C
> is what it is.
> 
> We could certainly argue how to do better than we do now in QEMU's C
> code.  However, the Error API is used all over the place, which makes
> changing it expensive.  "Rethinking the whole Error API" (your words)
> would have to generate benefits worth this expense.  Which seems
> unlikely.
> 
> [...]
> 

This is all fine, the problem is how we remodel this in C.

This is how I see the next steps to clarify my position:

short term:

- keep the existing API with the existing assumptions, use a separate argument 
to carry the pointer to the actual return value (where the function return 
value as provided by the language is used to return if an exception was 
generated or not, as we assume today).

- long term (maybe): fix the existing API by detaching completely the return 
value from the exception.


Wdyt?

C