[Vm-dev] FFI Plugin | Proposal: Add #doesNotCoerce:for: (like #doesNotUnderstand:)

[Fabio Niephaus]

Hi Marcel:

On Fri, 19 Jun 2020 at 1:15 pm, Marcel Taeumel <marcel.taeumel at hpi.de>
wrote:

>
> Hi all!
>
> We have #doesNotUnderstand:, which is sent to the receiver when a message
> cannot be understood. Default reaction in Object is to raise
> MessageNotUnderstood exception.
>
> What if we could have a similar mechanism for FFI calls:
> #doesNotCoerce:for:. The receiver would be the one that made that call,
> should be an instance of ExternalLibrary, but could be any object. The
> arguments would be a pair of (argObject, argType) and the message (like the
> argument for doesNotUnderstand:) that represents the FFI call.
>

I don't like the idea of adding a plugin-specific mechanism like this to
the VM. I know it's not as convenient, but maybe something similar can be
achieved with error codes in fallback code? There already is BAD_ARGUMENT,
maybe there's a way to encode which argument is bad in the error code?

Fabio


> The pair of (argObject, argType) could be expressed as FFICallArgument:
>
> FFICallArgument
>    argObject  ... <Object>
>    argType ... <ExternalType>
>
> I suppose that such a new class would have to be reserved in the special
> objects array. An alternative would be to just use Array. Keep it simple.
> ;-)
>
> Default reaction could be to implement #doesNotCoerce:for: in Object (as
> *FFI-Kernel extension) to raise FFICallArgumentNotCoerced exception (to be
> implemented). Also, we could implement manual coercion in ExternaLibrary
> to, for example, wrap atomics into their aliasing structures.
>
> ExternalLibrary >> #doesNotCoerce: callArg for: message
>    | argObject argType argIndex wrappedArg |
>    argObject := callArg first. "or argObject accessor"
>    argType := callArg second. "or argType accessor"
>    argIndex := message arguments indexOf: argObject.
>    wrappedArg := argType referentClass fromHandle: argObject.
>    message arguments beWritableObject. "Just in case ;-)"
>    message at: argIndex put: wrappedArg.
>    ^ message sendTo: self
>
> Please share your thoughts. :-)
>
> Best,
> Marcel
>
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