[PATCH 5 of 8] ui: do not try readline support if fin/fout aren't standard streams

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1520325146 21600
#      Tue Mar 06 02:32:26 2018 -0600
# Node ID 63a13b91e1ab4d9fa0a713935be58794b9cadab5
# Parent  10e3ea3d8b1dc671f6302ebed4489773c7d79458
ui: do not try readline support if fin/fout aren't standard streams

It's unlikely for a non-stdio stream to be a tty. Minimizing readline support
makes it much simpler to work around the unicode input() function of Python 3.

This also works on chg which duplicates client's tty to stdio fds.

diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -1264,7 +1264,9 @@ class ui(object):
         return i
 
     def _readline(self):
-        if self._isatty(self.fin):
+        usereadline = (self._isatty(self.fin) and self._isatty(self.fout)
+                       and util.isstdin(self.fin) and util.isstdout(self.fout))
+        if usereadline:
             try:
                 # magically add command line editing support, where
                 # available
@@ -1273,7 +1275,7 @@ class ui(object):
                 readline.read_history_file
                 # windows sometimes raises something other than ImportError
             except Exception:
-                pass
+                usereadline = False
 
         # prompt ' ' must exist; otherwise readline may delete entire line
         # - http://bugs.python.org/issue12833