[foreign-jextract] jextract-generated MethodHandle is null

[Filip Krakowski]

Hi,

I had some time to code again and found the following bug in jextract. 
Since jextract's source mode is not working at the moment 
(https://bugs.openjdk.java.net/browse/JDK-8252799) I switched to class 
file mode. The following code (only the important parts) was generated 
using source mode for better explanation.

    static final FunctionDescriptor ibv_post_send$FUNC_ =FunctionDescriptor.of(C_INT,
         C_POINTER,
         C_POINTER,
         C_POINTER );

    static final MethodHandle ibv_post_send$MH_ =RuntimeHelper.downcallHandle(
         LIBRARIES,"ibv_post_send",
         "(Ljdk/incubator/foreign/MemoryAddress;Ljdk/incubator/foreign/MemoryAddress;Ljdk/incubator/foreign/MemoryAddress;)I",
         ibv_post_send$FUNC_,false );

    public static java.lang.invoke.MethodHandle ibv_post_send$MH() {return ibv_post_send$MH_; }

    public static MethodHandle ibv_post_send$MH() {
         return header_h$constants$0.ibv_post_send$MH();
    }

    public static @C("int")int ibv_post_send (@C("struct ibv_qp*")Addressable qp, at C("struct ibv_send_wr*")Addressable wr, at C("struct ibv_send_wr**")Addressable bad_wr) {
         try {
             return (int)header_h$constants$0.ibv_post_send$MH().invokeExact(qp.address(), wr.address(), bad_wr.address());
         }catch (Throwable ex) {
             throw new AssertionError(ex);
         }
    }

Calling the generated "ibv_post_send" 
(https://linux.die.net/man/3/ibv_post_send) method results in a 
NullPointerException since RuntimeHelper#downcallHandle returns null 
instead the correct MethodHandle.

    /java.lang.AssertionError: java.lang.NullPointerException: Cannot
    invoke
    "java.lang.invoke.MethodHandle.invokeExact(jdk.incubator.foreign.MemoryAddress,
    jdk.incubator.foreign.MemoryAddress,
    jdk.incubator.foreign.MemoryAddress)" because the return value of
    "org.linux.rdma.header_h$constants$0.ibv_post_send$MH()" is null/

I also tried creating my own MethodHandle using 
RuntimeHelper#downcallHandle which also returned null. Interestingly, 
all previous calls to methods from the same shared library work without 
problems.

Best regards,
Filip