bpo-33671 / shutil.copyfile: use memoryview() with dynamic size on Windows (#7681)

bpo-33671
* use memoryview() with size == file size on Windows, see https://github.com/python/cpython/pull/7160#discussion_r195405230
* release intermediate (sliced) memoryview immediately
* replace "OSX" occurrences with "macOS"
* add some unittests for copyfileobj()

2 files changed, 22 insertions, 12 deletions

diff --git a/Doc/library/shutil.rst b/Doc/library/shutil.rst
index a3b87ee..c692cf4 100644
--- a/Doc/library/shutil.rst
+++ b/Doc/library/shutil.rst
@@ -407,11 +407,15 @@ efficiently (see :issue:`33671`).
 "fast-copy" means that the copying operation occurs within the kernel, avoiding
 the use of userspace buffers in Python as in "``outfd.write(infd.read())``".
 
-On OSX `fcopyfile`_ is used to copy the file content (not metadata).
+On macOS `fcopyfile`_ is used to copy the file content (not metadata).
 
 On Linux, Solaris and other POSIX platforms where :func:`os.sendfile` supports
 copies between 2 regular file descriptors :func:`os.sendfile` is used.
 
+On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+instead of 16 KiB) and a :func:`memoryview`-based variant of
+:func:`shutil.copyfileobj` is used.
+
 If the fast-copy operation fails and no data was written in the destination
 file then shutil will silently fallback on using less efficient
 :func:`copyfileobj` function internally.
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 58be90f..32c45ec 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -95,20 +95,18 @@ Optimizations
 
 * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
   :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
-  "fast-copy" syscalls on Linux, OSX and Solaris in order to copy the file more
-  efficiently.
+  "fast-copy" syscalls on Linux, macOS and Solaris in order to copy the file
+  more efficiently.
   "fast-copy" means that the copying operation occurs within the kernel,
   avoiding the use of userspace buffers in Python as in
   "``outfd.write(infd.read())``".
-  All other platforms not using such technique will rely on a faster
-  :func:`shutil.copyfile` implementation using :func:`memoryview`,
-  :class:`bytearray` and
-  :meth:`BufferedIOBase.readinto() <io.BufferedIOBase.readinto>`.
-  Finally, :func:`shutil.copyfile` default buffer size on Windows was increased
-  from 16KB to 1MB.
-  The speedup for copying a 512MB file within the same partition is about +26%
-  on Linux, +50% on OSX and +38% on Windows. Also, much less CPU cycles are
-  consumed.
+  On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+  instead of 16 KiB) and a :func:`memoryview`-based variant of
+  :func:`shutil.copyfileobj` is used.
+  The speedup for copying a 512 MiB file within the same partition is about
+  +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
+  are consumed.
+  See :ref:`shutil-platform-dependent-efficient-copy-operations` section.
   (Contributed by Giampaolo Rodola' in :issue:`25427`.)
 
 * The default protocol in the :mod:`pickle` module is now Protocol 4,
@@ -179,6 +177,14 @@ Changes in the Python API
 * The :class:`cProfile.Profile` class can now be used as a context
   manager. (Contributed by Scott Sanderson in :issue:`29235`.)
 
+* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
+  :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
+  "fast-copy" syscalls (see
+  :ref:`shutil-platform-dependent-efficient-copy-operations` section).
+
+* :func:`shutil.copyfile` default buffer size on Windows was changed from
+  16 KiB to 1 MiB.
+
 CPython bytecode changes
 ------------------------