Add weakref support for pyjlwrap types

I have added [weak reference support][1] for `pyjlwrap` types, following the
documentation for Python 3+. Adding weak reference support for Python 2.x seems
much the same as in 3.x, however 3.x does not use Py_TPFLAGS_HAVE_WEAKREFS,
which also does not seem necessary for 2.x. Simple testing suggests that this is
working as expected.

Closes JuliaPy#21, JuliaPy#158

[1]: https://docs.python.org/3/extending/newtypes.html#weak-reference-support

src/pyinit.jl

@@ -27,7 +27,7 @@ const pyxrange = Ref{PyPtr}(0)
 function pyjlwrap_init()
     # PyMemberDef stores explicit pointers, hence must be initialized at runtime
     push!(pyjlwrap_members, PyMemberDef(pyjlwrap_membername,
-                                        T_PYSSIZET, sizeof_PyObject_HEAD, READONLY,
+                                        T_PYSSIZET, sizeof_pyjlwrap_head, READONLY,
                                         pyjlwrap_doc),
                             PyMemberDef(C_NULL,0,0,0,C_NULL))
 
@@ -57,6 +57,7 @@ function pyjlwrap_init()
         t.tp_iter = pyjlwrap_getiter_ptr
         t.tp_hash = sizeof(Py_hash_t) < sizeof(Int) ?
                     pyjlwrap_hash32_ptr : pyjlwrap_hash_ptr
+        t.tp_weaklistoffset = fieldoffset(Py_jlWrap, 3)
     end
 end
 

src/pytype.jl

@@ -168,6 +168,7 @@ const Py_TPFLAGS_HAVE_STACKLESS_EXTENSION_ = (0x00000003<<15)
 #  -- most fields can default to 0 except where noted
 
 const sizeof_PyObject_HEAD = sizeof(Int) + sizeof(PyPtr)
+const sizeof_pyjlwrap_head = sizeof_PyObject_HEAD + sizeof(PyPtr)
 
 mutable struct PyTypeObject
     # PyObject_HEAD (for non-Py_TRACE_REFS build):
@@ -325,17 +326,22 @@ struct Py_jlWrap
     ob_refcnt::Int
     ob_type::PyPtr
 
+    ob_weakrefs::PyPtr
     jl_value::Any
 end
 
 # destructor for jlwrap instance, assuming it was created with pyjlwrap_new
 function pyjlwrap_dealloc(o::PyPtr)
+    p = convert(Ptr{PyPtr}, o)
+    if unsafe_load(p, 3) != PyPtr_NULL
+        ccall((@pysym :PyObject_ClearWeakRefs), Cvoid, (PyPtr,), o)
+    end
     delete!(pycall_gc, o)
     return nothing
 end
 
 unsafe_pyjlwrap_to_objref(o::PyPtr) =
-  unsafe_pointer_to_objref(unsafe_load(convert(Ptr{Ptr{Cvoid}}, o), 3))
+  unsafe_pointer_to_objref(unsafe_load(convert(Ptr{Ptr{Cvoid}}, o), 4))
 
 function pyjlwrap_repr(o::PyPtr)
     try
@@ -440,7 +446,8 @@ function pyjlwrap_new(pyT::PyTypeObject, value::Any)
         pycall_gc[o.o] = value
         ptr = pointer_from_objref(value)
     end
-    unsafe_store!(p, ptr, 3)
+    unsafe_store!(p, C_NULL, 3)
+    unsafe_store!(p, ptr, 4)
     return o
 end
 

test/runtests.jl

@@ -393,6 +393,15 @@ const PyInt = pyversion < v"3" ? Int : Clonglong
         @test get(weakdict(Dict(3=>weakdict)),3) == weakdict
     end
 
+    # Weak ref support for pyjlwrap types
+    let weakref = pyimport("weakref")
+        bar = TestConstruct(1)
+        o = PyObject(bar)
+        @test PyCall.is_pyjlwrap(o)
+        r = weakref[:ref](o)
+        @test weakref[:getweakrefcount](o) == 1
+    end
+
     # Expose python docs to Julia doc system
     py"""
     def foo():