Explicitly destroy c++ IOBuf from cython IOBuf

Summary: This seems to not happen reliably and causes rss to grow. The `__dealloc__` cannot fix it completely, although I verified it is called. So calling it explicitly seems to guarantee it to happen.

Reviewed By: yfeldblum

Differential Revision: D20206887

fbshipit-source-id: 6285a8e8943a53d20714e54efd3220a545010102

folly/python/iobuf.pxd

@@ -64,6 +64,7 @@ cdef class IOBuf:
     cdef Py_ssize_t strides[1]
     @staticmethod
     cdef IOBuf create(cIOBuf* this, object parent)
+    cdef void cleanup(self)
     cdef unique_ptr[cIOBuf] c_clone(self)
 
 cdef unique_ptr[cIOBuf] from_python_buffer(memoryview view)

folly/python/iobuf.pyx

@@ -54,6 +54,9 @@ cdef class IOBuf:
         self._hash = None
         __cache[(<unsigned long>self._this, id(self))] = self
 
+    def __dealloc__(self):
+        self._ours.reset()
+
     @staticmethod
     cdef IOBuf create(cIOBuf* this, object parent):
         key = (<unsigned long>this, id(parent))
@@ -65,6 +68,9 @@ cdef class IOBuf:
             __cache[key] = inst
         return inst
 
+    cdef void cleanup(self):
+        self._ours.reset()
+
     cdef unique_ptr[cIOBuf] c_clone(self):
         return move(self._this.clone())