jextract generates downcallHandles where I need upcallStubs

[Maurizio Cimadamore]

On 26/12/2020 09:23, Sebastian Stenzel wrote:
>> On 15. Dec 2020, at 12:31, Maurizio Cimadamore 
>> <maurizio.cimadamore at oracle.com 
>> <mailto:maurizio.cimadamore at oracle.com>> wrote:
>>
>> Now, it's true that the function pointer type behind "readdir" takes 
>> its own function pointer (filler), but that shouldn't be a concern 
>> when creating the upcall for (1) - a function pointer is just some 
>> MemoryAddress parameter that the upcall receives (and that the 
>> runtime of the libfuse library will provide). And, it is true that, 
>> in this case jextract will also generate functional interfaces for 
>> e.g. the filler upcall, but you don't have to use them in this case 
>> (since the filler function pointer will never be generated by you - 
>> but by the library) - of course jextract sees a function pointer and 
>> generates the corresponding functional interface, it has no way to 
>> know which party will allocate the function pointer.
>
> After experimenting with the latest snapshot version of Panama, I was 
> now able to implement aforementioned readdir() method. However I still 
> don't manage to use the filler function.
>
> jextract will generate a Java method interface from readdir [1] that 
> looks like this:
>
> ```
> int readdir(MemoryAddress path, MemoryAddress buf, MemoryAddress 
> filler, long offset, MemoryAddress fi)
> ```
>
> Note that the filler is merely a MemoryAddress. From within my Java 
> implementation I'd need to call filler, however in order to create a 
> downcallHandle from this address, I'd need to know 
> the FunctionDescriptor, which is kept package-private within the code 
> generated by jextract.
>
> It would be even easier, if there was some way to retrieve a 
> MethodHandle from a given MemoryAddress and a known FunctionDescriptor.
>
> Or am I missing something, again?

So, if I understand correctly, what you need here is to pass a function 
pointer to the routine that does the "filling" - which is probably a 
Java upcall.

Since `fuse_fill_dir_t` is defined as a function type, there should be a 
functional interface generated for that parameter (probably called 
something like readdir$filler). If you don't see the functional 
interface, it is possible you might be missing the fix for:

https://github.com/openjdk/panama-foreign/pull/424

Which, judging from the date in which it has been integrated, it is not 
part of the EA.

Are you in a position to do a build of the Panama repo? If so, you could 
try if the functional interface is generated in that case.

We plan to spin up another EA as soon as IntelliJ is updated to support 
Java 16 in full.

Cheers
Maurizio

>
>
> [1]: 
> https://github.com/libfuse/libfuse/blob/fuse_2_9_5/include/fuse.h#L304 
> <https://urldefense.com/v3/__https://github.com/libfuse/libfuse/blob/fuse_2_9_5/include/fuse.h*L304__;Iw!!GqivPVa7Brio!KkNaiLpgV2IUiPSEnWuo04SmoGAhT7N5sgbRsGpPJ6OgM7cyleyhPer4wfes780CgDr32fY$>