Ensure deserialized numpy arrays are immutable (#7181)

* ensure numpy arrays are immutable when deserialized from the memory buffer

python/ray/includes/serialization.pxi

@@ -98,6 +98,10 @@ cdef class SubBuffer:
         """
         return self.len
 
+    @property
+    def readonly(self):
+        return self.readonly
+
     def tobytes(self):
         """
         Return this buffer as a Python bytes object. Memory is copied.
@@ -212,7 +216,9 @@ cdef class Pickle5Writer:
                                    cpython.PyBUF_FULL_RO)
         buffer.set_length(view.len)
         buffer.set_ndim(view.ndim)
-        buffer.set_readonly(view.readonly)
+        # It should be 'view.readonly'. But for the sake of shared memory,
+        # we have to make it immutable.
+        buffer.set_readonly(1)
         buffer.set_itemsize(view.itemsize)
         if view.format:
             buffer.set_format(view.format)