From patchwork Tue Jun  2 18:07:33 2020
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Subject: [stable] py3: always flush ui streams on Python 3
From: Manuel Jacob <me@manueljacob.de>
X-Patchwork-Id: 46458
Message-Id: <ebbc45544673c33ea3be.1591121253@tmp>
To: mercurial-devel@mercurial-scm.org
Date: Tue, 02 Jun 2020 20:07:33 +0200

# HG changeset patch
# User Manuel Jacob <me@manueljacob.de>
# Date 1591120869 -7200
#      Tue Jun 02 20:01:09 2020 +0200
# Branch stable
# Node ID ebbc45544673c33ea3beb887ed4d5230b015102a
# Parent  91e509a12dbc4cd6cf2dcb9dae3ed383932132ac
py3: always flush ui streams on Python 3

On Python 2, we rely on that stdout and stderr are line buffered. Python 3’s
io module doesn’t offer line buffered binary streams.

diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -1198,9 +1198,14 @@ class ui(object):
                     label = opts.get('label', b'')
                     msg = self.label(msg, label)
                 dest.write(msg)
+            # On Python 2, stdout and stderr are usually line buffered, but
             # stderr may be buffered under win32 when redirected to files,
             # including stdout.
-            if dest is self._ferr and not getattr(self._ferr, 'closed', False):
+            # On Python 3, we use the underlying binary buffer, which does not
+            # support line buffering.
+            if (pycompat.ispy3 or dest is self._ferr) and not getattr(
+                self._ferr, 'closed', False
+            ):
                 dest.flush()
         except IOError as err:
             if dest is self._ferr and err.errno in (