Make cursor visitor abide to the cgo pointer passing rules

Solution uses a callback registry map which stores the go function
values. GoClangCursorVisitor now accepts the id of the function
it must call in its arguments, instead of the function pointer
to the go function
Fixes #17

cursor.go

@@ -4,6 +4,7 @@ package clang
 // #include "go-clang.h"
 import "C"
 import (
+	"sync"
 	"unsafe"
 )
 
@@ -795,24 +796,60 @@ type CursorVisitor func(cursor, parent Cursor) (status ChildVisitResult)
  */
 func (c Cursor) Visit(visitor CursorVisitor) bool {
 	forceEscapeVisitor = &visitor
-	o := C._go_clang_visit_children(c.c, unsafe.Pointer(&visitor))
+	id := visitorCallbacks.add(visitor)
+	defer visitorCallbacks.remove(id)
+
+	o := C._go_clang_visit_children(c.c, C.uintptr_t(id))
 	if o != C.uint(0) {
 		return false
 	}
 	return true
 }
 
+type visitorCallbackRegistry struct {
+	lock       sync.Mutex
+	callbacks  map[uintptr]CursorVisitor
+	generation uintptr
+}
+
+var visitorCallbacks = visitorCallbackRegistry{
+	callbacks: map[uintptr]CursorVisitor{},
+}
+
+func (r *visitorCallbackRegistry) add(cb CursorVisitor) uintptr {
+	r.lock.Lock()
+	defer r.lock.Unlock()
+
+	r.generation++
+	r.callbacks[r.generation] = cb
+	return r.generation
+}
+
+func (r *visitorCallbackRegistry) remove(id uintptr) {
+	r.lock.Lock()
+	defer r.lock.Unlock()
+
+	delete(r.callbacks, id)
+}
+
+func (r *visitorCallbackRegistry) get(id uintptr) CursorVisitor {
+	r.lock.Lock()
+	defer r.lock.Unlock()
+
+	return r.callbacks[id]
+}
+
 // forceEscapeVisitor is write-only: to force compiler to escape the address
 // (else the address can become stale if the goroutine stack needs to grow
 // and is forced to move)
 // Explained by rsc in https://golang.org/issue/9125
 var forceEscapeVisitor *CursorVisitor
 
 //export GoClangCursorVisitor
-func GoClangCursorVisitor(cursor, parent C.CXCursor, cfct unsafe.Pointer) (status ChildVisitResult) {
-	fct := *(*CursorVisitor)(cfct)
-	o := fct(Cursor{cursor}, Cursor{parent})
-	return o
+func GoClangCursorVisitor(cursor, parent C.CXCursor, callback_id unsafe.Pointer) (status ChildVisitResult) {
+	id := uintptr(callback_id)
+	cb := visitorCallbacks.get(id)
+	return cb(Cursor{cursor}, Cursor{parent})
 }
 
 /**

go-clang.h

@@ -30,7 +30,7 @@ _goclang_get_platform_availability_at(CXPlatformAvailability* array, int idx) {
   return array[idx];
 }
 
-unsigned _go_clang_visit_children(CXCursor c, void *fct);
+unsigned _go_clang_visit_children(CXCursor c, uintptr_t callback_id);
 
 CXPlatformAvailability
 _goclang_get_platform_availability_at(CXPlatformAvailability* array, int idx);

visitorwrap.c

@@ -5,9 +5,9 @@
 #include "go-clang.h"
 
 unsigned
-_go_clang_visit_children(CXCursor c, void *fct)
+_go_clang_visit_children(CXCursor c, uintptr_t callback_id)
 {
-  return clang_visitChildren(c, (CXCursorVisitor)&GoClangCursorVisitor, fct);
+  return clang_visitChildren(c, (CXCursorVisitor)&GoClangCursorVisitor, (CXClientData)callback_id);
 }
 
 /* EOF */