Update xargs.partition with platform information

Change how xargs.partition computes the command length (including
arguments) depending on the plataform.
More specifically, 'win32' uses the amount of characters while posix
system uses the byte count.

pre_commit/xargs.py

@@ -1,41 +1,54 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+import sys
+
 from pre_commit import parse_shebang
 from pre_commit.util import cmd_output
 
 
-# Limit used previously to avoid "xargs ... Bad file number" on windows
-# This is slightly less than the posix mandated minimum
-MAX_LENGTH = 4000
+# TODO: properly compute max_length value
+def _get_platform_max_length():
+    # posix minimum
+    return 4 * 1024
+
+
+def _get_command_length(command, arg):
+    parts = command + (arg,)
+    full_cmd = ' '.join(parts)
+
+    # win32 uses the amount of characters, more details at:
+    # https://blogs.msdn.microsoft.com/oldnewthing/20031210-00/?p=41553/
+    if sys.platform == 'win32':
+        return len(full_cmd)
+
+    return len(full_cmd.encode(sys.getdefaultencoding()))
 
 
 class ArgumentTooLongError(RuntimeError):
     pass
 
 
-def partition(cmd, varargs, _max_length=MAX_LENGTH):
+def partition(cmd, varargs, _max_length=None):
+    _max_length = _max_length or _get_platform_max_length()
     cmd = tuple(cmd)
     ret = []
 
     ret_cmd = []
-    total_len = len(' '.join(cmd))
     # Reversed so arguments are in order
     varargs = list(reversed(varargs))
 
     while varargs:
         arg = varargs.pop()
 
-        if total_len + 1 + len(arg) <= _max_length:
+        if _get_command_length(cmd + tuple(ret_cmd), arg) <= _max_length:
             ret_cmd.append(arg)
-            total_len += len(arg)
         elif not ret_cmd:
             raise ArgumentTooLongError(arg)
         else:
             # We've exceeded the length, yield a command
             ret.append(cmd + tuple(ret_cmd))
             ret_cmd = []
-            total_len = len(' '.join(cmd))
             varargs.append(arg)
 
     ret.append(cmd + tuple(ret_cmd))