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

[Maurizio Cimadamore]

On Wed, 22 Mar 2023 20:07:01 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Ah. True. This is true about all the MS parameters as well.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13079#discussion_r1145978702