[PATCH] util: increase the copy buffer size in shutil's copyfile

[Tony Tung]

# HG changeset patch
# User Tony Tung <ttung at fb.com>
# Date 1462566734 25200
#      Fri May 06 13:32:14 2016 -0700
# Branch stable
# Node ID edee9b1c3a2849fcb11900959ad785b202608c4f
# Parent  97811ff7964710d32cae951df1da8019b46151a2
util: increase the copy buffer size in shutil's copyfile

The default copy buffer size in shutil is 16KB.  This is far too
conservative for modern systems.  Use a 4MB buffer instead to copy files
more efficiently.

This manifests prominently with journaling large dirstates, which can be
multi-second operations.

diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -20,6 +20,7 @@
 import collections
 import datetime
 import errno
+import functools
 import gc
 import hashlib
 import imp
@@ -1010,6 +1011,23 @@
 
     return check
 
+# The default copy buffer in python's shutil is 16KB.  This is far too
+# conservative in modern systems.  Copy more at a time so we don't incur so much
+# system call overhead.
+def wrap(container, funcname, newfunc):
+    origfunc = getattr(container, funcname)
+    wrap = functools.partial(newfunc, origfunc)
+    functools.update_wrapper(wrap, origfunc)
+    setattr(container, funcname, wrap)
+
+def copyfileobj_lgbuffer(orig, *args, **kwargs):
+    if len(args) == 2 and 'length' not in kwargs:
+        args = (args[0], args[1], (4 * 1024 * 1024))
+
+    orig(*args, **kwargs)
+
+wrap(shutil, 'copyfileobj', copyfileobj_lgbuffer)
+
 def copyfile(src, dest, hardlink=False, copystat=False):
     '''copy a file, preserving mode and optionally other stat info like
     atime/mtime'''