[PATCH 1 of 3 V2] windows: seek to the end of posixfile when opening in append mode

[Matt Harbison]

# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1422725984 18000
#      Sat Jan 31 12:39:44 2015 -0500
# Node ID 05678c22e04da407ab610765248636a125e1348a
# Parent  a9b61dbdb827165a9fd9d44dd42b892dbd9fa07c
windows: seek to the end of posixfile when opening in append mode

The position is implementation defined when opening in append mode, and it seems
like Linux sets it to EOF while Windows keeps it at zero.  This has caused
problems in the past when a file is opened and tell() is immediately called,
such as 48c232873a54 and 6bf93440a717.

Since the only caller of osutil.posixfile is this windows module, this seems
like a better place to fix the issue than in osutil.c and pure.osutil.

diff --git a/mercurial/windows.py b/mercurial/windows.py
--- a/mercurial/windows.py
+++ b/mercurial/windows.py
@@ -26,11 +26,19 @@
 unlink = win32.unlink
 
 umask = 0022
+_SEEK_END = 2 # os.SEEK_END was introduced in Python 2.5
 
 # wrap osutil.posixfile to provide friendlier exceptions
 def posixfile(name, mode='r', buffering=-1):
     try:
-        return osutil.posixfile(name, mode, buffering)
+        fp = osutil.posixfile(name, mode, buffering)
+
+        # The position when opening in append mode is implementation defined, so
+        # make it consistent with other platforms, which position at EOF.
+        if 'a' in mode:
+            fp.seek(0, _SEEK_END)
+
+        return fp
     except WindowsError, err:
         raise IOError(err.errno, '%s: %s' % (name, err.strerror))
 posixfile.__doc__ = osutil.posixfile.__doc__