[foreign] RFR: add support for callbacks in struct fields/globals

[Sundararajan Athijegannathan]

The patch builds fine on Mac. Tests are passing as well. But I tried a 
simple global function pointer example. Found two issues:

* jextract change needed to implement java.nicl.types.Callback for 
generated functional interfaces. Without this, binder threw exception 
(as expected)

diff -r 46db2ebe439d 
src/jdk.jextract/share/classes/com/sun/tools/jextract/AsmCodeFactory.java
--- 
a/src/jdk.jextract/share/classes/com/sun/tools/jextract/AsmCodeFactory.java    
Tue Jul 03 19:42:25 2018 +0530
+++ 
b/src/jdk.jextract/share/classes/com/sun/tools/jextract/AsmCodeFactory.java    
Wed Jul 04 13:56:15 2018 +0530
@@ -319,7 +319,7 @@
          global_cw.visitInnerClass(name, internal_name, intf, 
ACC_PUBLIC | ACC_STATIC | ACC_ABSTRACT | ACC_INTERFACE);
          ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
          cw.visit(V1_8, ACC_PUBLIC | ACC_ABSTRACT | ACC_INTERFACE,
-                name, null, "java/lang/Object", null);
+                name, null, "java/lang/Object", new String[] { 
"java/nicl/types/Callback" });
          if (dcl != null) {
              annotateNativeLocation(cw, dcl);
          }

* After making that change, binder went fine - but I still got NPE at 
calling the functional interface method.

     static long checkPointer(Pointer<?> ptr) throws Throwable {
         Scope s = ((BoundedPointer<?>)ptr).scope();
         if (s != null) s.checkAlive();  // <-- scope is null for global 
function pointer
         return ptr.addr();
     }


With those changes, I got global function pointer type variable working.

PS. Attached the simple header/C code + Java main that I used to test.

-Sundar

On 03/07/18, 7:28 PM, Maurizio Cimadamore wrote:
> Hi,
> this patch adds binder support for function pointers in struct 
> fields/global variables.
>
> This adds a new kind of reference abstraction which handles 
> marshalling/unmarshalling of function pointers.
>
> When a function pointer is retrieved from a  struct/global var, we 
> need to create a synthetic implementation for a target functional 
> interface whose method implementation will do a native call to some 
> native function. Note: in some cases, the native function being called 
> could be a binder-generated native stub - we could detect this and 
> save the object creation and the indirect allocation, but I have not 
> done this as part of this patch.
>
> When a function pointer is set, we need to obtain an address from a 
> functional interface instance; now, if such instance is 
> binder-generated, we have a way to retrieve such address (by calling 
> Callback::resource). Otherwise, the instance truly comes from Java 
> code, so we need to allocate a stub and set its address onto the 
> struct field.
>
> I had to do some reshuffling of the code generation logic - there's 
> now a new code generator (namely CallbackImplGenerator) which uses 
> some of the features also used by HeaderImplGenerator, so I moved 
> these shared stuff in the common superclass BinderImplGenerator.
>
> Also, I've tweaked NativeInvoker NOT to eagerly bind the underlying 
> handle to a function address. That's because when we work with header 
> symbols, we always know the address statically, and we can bind to it; 
> but when we work with callbacks, the address is stored in the callback 
> implementation, so it's a dynamic property. As a result, it's better 
> not to bind the address parameter of the method handle, and have the 
> caller pass it explicitly - which works well in both cases (and saves 
> one bound MH creation).
>
> I've also added some methods to BoundedPointer in order to create an 
> unbounded pointer with given scope: we want the pointer set on the 
> callback implementation to have the same scope/lifecycle as the struct 
> field it corresponds to - this allows us to throw an exception when we 
> attempt to call a callback implementation coming from a struct field 
> whose scope has expired.
>
> The added test checks some of the assertions mentioned above.
>
> Webrev:
> http://cr.openjdk.java.net/~mcimadamore/panama/callback_in_structs/
>
>
> Maurizio
>
>
>
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