libgolang: Start providing interfaces

- interface is analog of interface{} in Go.
- error is analog of error in Go.

For now the interfaces are implemented via classic C++ scheme via
inheritance and additional vtable field in pointed object.

In the future we might want to consider reworking that to Go-style
interfaces without predeclaring which interfaces a class implements.

Top-level documentation is TODO.

Interfaces will be soon needed to describe Context interface in C
context package.

golang/_golang.pxd

@@ -46,6 +46,7 @@ In addition to Cython/nogil API, golang.pyx provides runtime for golang.py:
 
 from libcpp cimport nullptr_t, nullptr as nil
 from libcpp.utility cimport pair
+from libcpp.string  cimport string
 from libc.stdint cimport uint64_t
 cdef extern from *:
     ctypedef bint cbool "bool"
@@ -151,6 +152,27 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
         void  incref()
         int   refcnt() const
 
+
+    # empty interface ~ interface{}
+    cppclass _interface:
+        void incref()
+        void decref()
+
+    cppclass interface (refptr[_interface]):
+        # interface.X = interface->X in C++
+        void incref "_ptr()->incref" ()
+        void decref "_ptr()->decref" ()
+
+
+    # error interface
+    cppclass _error (_interface):
+        string Error()
+
+    cppclass error (refptr[_error]):
+        # error.X = error->X in C++
+        string Error    "_ptr()->Error" ()
+
+
 # ---- python bits ----
 
 cdef void topyexc() except *

golang/libgolang.h

@@ -475,6 +475,8 @@ template<typename T> refptr<T> newref  (T *_obj);
 //
 // T must provide incref/decref methods for example via inheriting from object.
 // incref/decref must be safe to use from multiple threads simultaneously.
+//
+// See also interface.
 template<typename T>
 class refptr {
     T *_obj;
@@ -606,6 +608,28 @@ public:
     LIBGOLANG_API int  refcnt() const;
 };
 
+// interface is empty interface a-la interface{} in Go.
+//
+// It is the base class of all interfaces.
+//
+// Even if the interface is empty, it requires memory-management methods to be
+// present. See refptr for details.
+struct _interface {
+    virtual void incref() = 0;
+    virtual void decref() = 0;
+
+protected:
+    // don't use destructor -> use decref
+    ~_interface();
+};
+typedef refptr<_interface> interface;
+
+
+// error is the interface describing errors.
+struct _error : _interface {
+    virtual std::string Error() = 0;
+};
+typedef refptr<_error> error;
 
 }   // golang::
 

golang/runtime/libgolang.cpp

@@ -1199,6 +1199,8 @@ void _blockforever() {
     bug("_blockforever: woken up");
 }
 
+_interface::~_interface() {}
+
 // ---- for tests ----
 
 // _tchanlenrecvqlen returns len(_ch._recvq)