diff --git a/mercurial/worker.py b/mercurial/worker.py
--- a/mercurial/worker.py
+++ b/mercurial/worker.py
@@ -109,8 +109,10 @@
 
             while pos < size:
                 ret = self._wrapped.readinto(view[pos:])
-                if not ret:
-                    break
+                if ret is None:
+                    continue  # nonblocking, and no data
+                elif ret == 0:
+                    break  # 0 -> EOF
                 pos += ret
 
             del view