[PATCH] ui: flush stderr after printing a non-chained exception for Windows

[Matt Harbison]

# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1434161381 14400
#      Fri Jun 12 22:09:41 2015 -0400
# Node ID a9a90e8afc80f372d62f7f6eacc163bc8f48cce9
# Parent  e8075329c5fbb651508777c15cb6bc9ae33b5dff
ui: flush stderr after printing a non-chained exception for Windows

There were consistent test failures in test-bad-extension.t, because Windows
buffers stderr when redirected to a file (per the comment in ui.write_err()).
That resulted in failures like this:

  --- c:/Users/Matt/Projects/hg/tests/test-bad-extension.t
  +++ c:/Users/Matt/Projects/hg/tests/test-bad-extension.t.err
  @@ -23,11 +23,11 @@
     Traceback (most recent call last):
     Exception: bit bucket overflow
     *** failed to import extension badext2: No module named badext2
  -  Traceback (most recent call last):
  -  ImportError: No module named badext2
     hg help [-ec] [TOPIC]

     show help for a given topic or a help overview
  +  Traceback (most recent call last):
  +  ImportError: No module named badext2

   show traceback for ImportError of hgext.name if debug is set
   (note that --debug option isn't applied yet when loading extensions)

Instead of inserting another flush immediately after the print, to go along with
the one recently added prior to the print (see 3ff4b07412ad), funnel the output
through ui.write_err().  The flush prior to printing the traceback only mentions
that stdout needs to be flushed, and only stderr needs to be flushed after
printing the traceback.  ui.write_err() does both for us without needing to
redocument the quirky Windows behavior.  It will also clear any progress bar.

diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -6,7 +6,7 @@
 # GNU General Public License version 2 or any later version.
 
 from i18n import _
-import errno, getpass, os, socket, sys, tempfile, traceback
+import cStringIO, errno, getpass, os, socket, sys, tempfile, traceback
 import config, scmutil, util, error, formatter, progress
 from node import hex
 
@@ -869,9 +869,10 @@
                                ''.join(causetb),
                                ''.join(exconly))
             else:
-                self.flush()  # flush debug or status message
+                output = cStringIO.StringIO()
                 traceback.print_exception(exc[0], exc[1], exc[2],
-                                          file=self.ferr)
+                                          file=output)
+                self.write_err(output.getvalue())
         return self.tracebackflag or force
 
     def geteditor(self):