diff -r 71dcd4a4fa2f -r db823e38a61c mercurial/ui.py
--- a/mercurial/ui.py	Tue Apr 11 14:54:12 2017 -0700
+++ b/mercurial/ui.py	Tue Apr 11 14:54:12 2017 -0700
@@ -768,6 +768,8 @@
         try:
             for a in msgs:
                 self.fout.write(a)
+        except IOError as err:
+            raise error.StdioError(err)
         finally:
             self._blockedtimes['stdio_blocked'] += \
                 (util.timer() - starttime) * 1000