RFR: 8304265: Implementation of Foreign Function and Memory API (Third Preview) [v8]

[Maurizio Cimadamore]

On Thu, 23 Mar 2023 10:28:23 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Yes - although there is a distinction between passed by value and by ref. I wouldn't be surprised if we just threw NPE for by-value.
>
> Just did a test with jshell:
> 
> 
> jshell> Linker linker = Linker.nativeLinker();
> 
> jshell> MethodHandle handleValue = linker.downcallHandle(FunctionDescriptor.ofVoid(MemoryLayout.structLayout(ValueLayout.JAVA_INT)));
> handleValue ==> MethodHandle(MemorySegment,MemorySegment)void
> 
> jshell> handleValue.invoke(MemorySegment.ofAddress(1), null);
> |  Exception java.lang.NullPointerException
> |        at (#5:1)
> 
> 
> So, yes, passing `null` for a "by-value" struct issues NPEs. I also get same exception when calling a method handle which passes a struct by reference:
> 
> 
> jshell> MethodHandle handleRef = linker.downcallHandle(FunctionDescriptor.ofVoid(ValueLayout.ADDRESS));
> handleRef ==> MethodHandle(MemorySegment,MemorySegment)void
> 
> jshell> handleRef.invoke(MemorySegment.ofAddress(1), null);
> |  Exception java.lang.NullPointerException
> |        at (#7:1)
> 
> 
> Given what the impl does, to be fair I don't mind the NPE (it seems a good exception to throw if we're passing a raw `null`). But we should document it at such.
> 
> It also seems like the first parameter of the downcall has somewhat better checks (but still NPEs):
> 
> 
> jshell> handleRef.invoke(null, MemorySegment.ofAddress(1));
> |  Exception java.lang.NullPointerException
> |        at Objects.requireNonNull (Objects.java:233)
> |        at SharedUtils.checkSymbol (SharedUtils.java:351)
> |        at (#8:1)
> 
> 
> And:
> 
> 
> jshell> handleRef.invoke(MemorySegment.NULL, MemorySegment.ofAddress(1));
> |  Exception java.lang.IllegalArgumentException: Symbol is NULL: MemorySegment{ heapBase: Optional.empty address:0 limit: 0 }
> |        at SharedUtils.checkSymbol (SharedUtils.java:353)
> |        at (#9:1)
> 
> 
> I think this last case is the _only_ one where IAE is warranted - as the role of the first parameter is special, and we can't tolerate a `MemorySegment::NULL` there. For all other reference parameters, the downcall handle should just check that they are non-null, and throw if so.

The returned method handle will throw an {@link IllegalArgumentException} if the {@link MemorySegment}
representing the target address of the foreign function is the {@link MemorySegment#NULL} address.
The returned method handle will additionally throw {@link NullPointerException} if any parameter of type {@link MemorySegment} is {@code null}.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13079#discussion_r1145982159