BUG: function pointers inside a struct do not generate SegmentAllocators when needed

[Jorn Vernee]

Hello Marko,

Thanks for the report! I can reproduce it here as well. I think the 
problem is the lambda in the `ofAddress` factory that is generated in 
the interface for the callback type. Note that the getter will just end 
up calling that factory. (you can see that also in your stack trace: `at 
Foo$bar.lambda$ofAddress$0(Foo.java:32)`).

I've filed: https://bugs.openjdk.org/browse/CODETOOLS-7903239

Cheers,
Jorn

On 28/07/2022 02:00, Marko wrote:
> Hello again!
> When trying to call a function pointer inside a struct which returns a
> MemorySegment, I receive the following error:
> ```
> Exception in thread "main" java.lang.AssertionError: should not reach here
> at Foo$bar.lambda$ofAddress$0(Foo.java:34)
> at Main.main(Main.java:8)
> Caused by: java.lang.invoke.WrongMethodTypeException: expected
> (NativeSymbol,SegmentAllocator)MemorySegment but found
> (NativeSymbol)MemorySegment
> at java.base/java.lang.invoke.Invokers.newWrongMethodTypeException(Invokers.java:523)
> at java.base/java.lang.invoke.Invokers.checkExactType(Invokers.java:532)
> at Foo$bar.lambda$ofAddress$0(Foo.java:32)
> ... 1 more
> ```
> I believe this is due to
> `FunctionalInterfaceBuilder#emitFunctionalFactoryForPointer` not
> checking to see if the returnType is a MemorySegment, as it does in
> `HeaderFileBuilder` for regular functions, see
> https://github.com/openjdk/jextract/blob/0582eaf1b4cdba95f0ee8c2480767433bb647d0d/src/main/java/org/openjdk/jextract/impl/HeaderFileBuilder.java#L122-L127.
>
> I began implementing a fix by copying this functionality over, however
> there is a bit of an issue I came across. When jextract emits the
> getter for the functional interface in a struct (StructBuilder), only
> the name of the interface is known. AFAIK, it does not know whether it
> needs a SegmentAllocator and cannot currently generate the correct
> code.
>
> I thought about adding another field to the method
> `emitFunctionalFactoryForPointer`, similar to fiName, like a boolean
> for whether a segment allocator is needed or not, however, I can't help but
> feel it is a bit hacky. I wonder what your opinions are on this.
>
> A repro can be found here:
> https://github.com/lost-illusi0n/jextract-func-ptr-seg-alloc-repro
> Just run the run script if you are on Linux. You will have to change the
> library output in the script if you are on another os.