bpo-33671: efficient zero-copy for shutil.copy* functions (Linux, OSX and Win) (#7160)

* have shutil.copyfileobj use sendfile() if possible

* refactoring: use ctx manager

* add test with non-regular file obj

* emulate case where file size can't be determined

* reference _copyfileobj_sendfile directly

* add test for offset() at certain position

* add test for empty file

* add test for non regular file dst

* small refactoring

* leave copyfileobj() alone in order to not introduce any incompatibility

* minor refactoring

* remove old test

* update docstring

* update docstring; rename exception class

* detect platforms which only support file to socket zero copy

* don't run test on platforms where file-to-file zero copy is not supported

* use tempfiles

* reset verbosity

* add test for smaller chunks

* add big file size test

* add comment

* update doc

* update whatsnew doc

* update doc

* catch Exception

* remove unused import

* add test case for error on second sendfile() call

* turn docstring into comment

* add one more test

* update comment

* add Misc/NEWS entry

* get rid of COPY_BUFSIZE; it belongs to another PR

* update doc

* expose posix._fcopyfile() for OSX

* merge from linux branch

* merge from linux branch

* expose fcopyfile

* arg clinic for the win implementation

* convert path type to path_t

* expose CopyFileW

* fix windows tests

* release GIL

* minor refactoring

* update doc

* update comment

* update docstrings

* rename functions

* rename test classes

* update doc

* update doc

* update docstrings and comments

* avoid do import nt|posix modules if unnecessary

* set nt|posix modules to None if not available

* micro speedup

* update description

* add doc note

* use better wording in doc

* rename function using 'fastcopy' prefix instead of 'zerocopy'

* use :ref: in rst doc

* change wording in doc

* add test to make sure sendfile() doesn't get called aymore in case it doesn't support file to file copies

* move CopyFileW in _winapi and actually expose CopyFileExW instead

* fix line endings

* add tests for mode bits

* add docstring

* remove test file mode class; let's keep it for later when Istart addressing OSX fcopyfile() specific copies

* update doc to reflect new changes

* update doc

* adjust tests on win

* fix argument clinic error

* update doc

* OSX: expose copyfile(3) instead of fcopyfile(3); also expose flags arg to python

* osx / copyfile: use path_t instead of char

* do not set dst name in the OSError exception in order to remain consistent with platforms which cannot do that (e.g. linux)

* add same file test

* add test for same file

* have osx copyfile() pre-emptively check if src and dst are the same, otherwise it will return immedialtey and src file content gets deleted

* turn PermissionError into appropriate SameFileError

* expose ERROR_SHARING_VIOLATION in order to raise more appropriate SameFileError

* honour follow_symlinks arg when using CopyFileEx

* update Misc/NEWS

* expose CreateDirectoryEx mock

* change C type

* CreateDirectoryExW actual implementation

* provide specific makedirs() implementation for win

* fix typo

* skeleton for SetNamedSecurityInfo

* get security info for src path

* finally set security attrs

* add unit tests

* mimick os.makedirs() behavior and raise if dst dir exists

* set 2 paths for OSError object

* set 2 paths for OSError object

* expand windows test

* in case of exception on os.sendfile() set filename and filename2 exception attributes

* set 2 filenames (src, dst) for OSError in case copyfile() fails on OSX

* update doc

* do not use CreateDirectoryEx() in copytree() if source dir is a symlink (breaks test_copytree_symlink_dir); instead just create a plain dir and remain consistent with POSIX implementation

* use bytearray() and readinto()

* use memoryview() with bytearray()

* refactoring + introduce a new _fastcopy_binfileobj() fun

* remove CopyFileEx and other C wrappers

* remove code related to CopyFileEx

* Recognize binary files in copyfileobj()
...and use fastest _fastcopy_binfileobj() when possible

* set 1MB copy bufsize on win; also add a global _COPY_BUFSIZE variable

* use ctx manager for memoryview()

* update doc

* remove outdated doc

* remove last CopyFileEx remnants

* OSX - use fcopyfile(3) instead of copyfile(3)

...as an extra safety measure: in case src/dst are "exotic" files (non
regular or living on a network fs etc.) we better fail on open() instead
of copyfile(3) as we're not quite sure what's gonna happen in that
case.

* update doc

2 files changed, 437 insertions, 14 deletions

diff --git a/Lib/shutil.py b/Lib/shutil.py
index 3c02776a40..09a5727ab4 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -10,6 +10,7 @@ import stat
 import fnmatch
 import collections
 import errno
+import io
 
 try:
     import zlib
@@ -42,6 +43,16 @@ try:
 except ImportError:
     getgrnam = None
 
+posix = nt = None
+if os.name == 'posix':
+    import posix
+elif os.name == 'nt':
+    import nt
+
+COPY_BUFSIZE = 1024 * 1024 if os.name == 'nt' else 16 * 1024
+_HAS_SENDFILE = posix and hasattr(os, "sendfile")
+_HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # OSX
+
 __all__ = ["copyfileobj", "copyfile", "copymode", "copystat", "copy", "copy2",
            "copytree", "move", "rmtree", "Error", "SpecialFileError",
            "ExecError", "make_archive", "get_archive_formats",
@@ -72,14 +83,124 @@ class RegistryError(Exception):
     """Raised when a registry operation with the archiving
     and unpacking registries fails"""
 
+class _GiveupOnFastCopy(Exception):
+    """Raised as a signal to fallback on using raw read()/write()
+    file copy when fast-copy functions fail to do so.
+    """
+
+def _fastcopy_osx(fsrc, fdst, flags):
+    """Copy a regular file content or metadata by using high-performance
+    fcopyfile(3) syscall (OSX).
+    """
+    try:
+        infd = fsrc.fileno()
+        outfd = fdst.fileno()
+    except Exception as err:
+        raise _GiveupOnFastCopy(err)  # not a regular file
+
+    try:
+        posix._fcopyfile(infd, outfd, flags)
+    except OSError as err:
+        err.filename = fsrc.name
+        err.filename2 = fdst.name
+        if err.errno in {errno.EINVAL, errno.ENOTSUP}:
+            raise _GiveupOnFastCopy(err)
+        else:
+            raise err from None
+
+def _fastcopy_sendfile(fsrc, fdst):
+    """Copy data from one regular mmap-like fd to another by using
+    high-performance sendfile(2) syscall.
+    This should work on Linux >= 2.6.33 and Solaris only.
+    """
+    # Note: copyfileobj() is left alone in order to not introduce any
+    # unexpected breakage. Possible risks by using zero-copy calls
+    # in copyfileobj() are:
+    # - fdst cannot be open in "a"(ppend) mode
+    # - fsrc and fdst may be open in "t"(ext) mode
+    # - fsrc may be a BufferedReader (which hides unread data in a buffer),
+    #   GzipFile (which decompresses data), HTTPResponse (which decodes
+    #   chunks).
+    # - possibly others (e.g. encrypted fs/partition?)
+    global _HAS_SENDFILE
+    try:
+        infd = fsrc.fileno()
+        outfd = fdst.fileno()
+    except Exception as err:
+        raise _GiveupOnFastCopy(err)  # not a regular file
+
+    # Hopefully the whole file will be copied in a single call.
+    # sendfile() is called in a loop 'till EOF is reached (0 return)
+    # so a bufsize smaller or bigger than the actual file size
+    # should not make any difference, also in case the file content
+    # changes while being copied.
+    try:
+        blocksize = max(os.fstat(infd).st_size, 2 ** 23)  # min 8MB
+    except Exception:
+        blocksize = 2 ** 27  # 128MB
+
+    offset = 0
+    while True:
+        try:
+            sent = os.sendfile(outfd, infd, offset, blocksize)
+        except OSError as err:
+            # ...in oder to have a more informative exception.
+            err.filename = fsrc.name
+            err.filename2 = fdst.name
+
+            if err.errno == errno.ENOTSOCK:
+                # sendfile() on this platform (probably Linux < 2.6.33)
+                # does not support copies between regular files (only
+                # sockets).
+                _HAS_SENDFILE = False
+                raise _GiveupOnFastCopy(err)
+
+            if err.errno == errno.ENOSPC:  # filesystem is full
+                raise err from None
+
+            # Give up on first call and if no data was copied.
+            if offset == 0 and os.lseek(outfd, 0, os.SEEK_CUR) == 0:
+                raise _GiveupOnFastCopy(err)
+
+            raise err
+        else:
+            if sent == 0:
+                break  # EOF
+            offset += sent
+
+def _copybinfileobj(fsrc, fdst, length=COPY_BUFSIZE):
+    """Copy 2 regular file objects open in binary mode."""
+    # Localize variable access to minimize overhead.
+    fsrc_readinto = fsrc.readinto
+    fdst_write = fdst.write
+    with memoryview(bytearray(length)) as mv:
+        while True:
+            n = fsrc_readinto(mv)
+            if not n:
+                break
+            elif n < length:
+                fdst_write(mv[:n])
+            else:
+                fdst_write(mv)
+
+def _is_binary_files_pair(fsrc, fdst):
+    return hasattr(fsrc, 'readinto') and \
+        isinstance(fsrc, io.BytesIO) or 'b' in getattr(fsrc, 'mode', '') and \
+        isinstance(fdst, io.BytesIO) or 'b' in getattr(fdst, 'mode', '')
 
-def copyfileobj(fsrc, fdst, length=16*1024):
+def copyfileobj(fsrc, fdst, length=COPY_BUFSIZE):
     """copy data from file-like object fsrc to file-like object fdst"""
-    while 1:
-        buf = fsrc.read(length)
-        if not buf:
-            break
-        fdst.write(buf)
+    if _is_binary_files_pair(fsrc, fdst):
+        _copybinfileobj(fsrc, fdst, length=length)
+    else:
+        # Localize variable access to minimize overhead.
+        fsrc_read = fsrc.read
+        fdst_write = fdst.write
+        while 1:
+            buf = fsrc_read(length)
+            if not buf:
+                break
+            fdst_write(buf)
 
 def _samefile(src, dst):
     # Macintosh, Unix.
@@ -117,9 +238,23 @@ def copyfile(src, dst, *, follow_symlinks=True):
     if not follow_symlinks and os.path.islink(src):
         os.symlink(os.readlink(src), dst)
     else:
-        with open(src, 'rb') as fsrc:
-            with open(dst, 'wb') as fdst:
-                copyfileobj(fsrc, fdst)
+        with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
+            if _HAS_SENDFILE:
+                try:
+                    _fastcopy_sendfile(fsrc, fdst)
+                    return dst
+                except _GiveupOnFastCopy:
+                    pass
+
+            if _HAS_FCOPYFILE:
+                try:
+                    _fastcopy_osx(fsrc, fdst, posix._COPYFILE_DATA)
+                    return dst
+                except _GiveupOnFastCopy:
+                    pass
+
+            _copybinfileobj(fsrc, fdst)
+
     return dst
 
 def copymode(src, dst, *, follow_symlinks=True):
@@ -244,13 +379,12 @@ def copy(src, dst, *, follow_symlinks=True):
 
 def copy2(src, dst, *, follow_symlinks=True):
     """Copy data and all stat info ("cp -p src dst"). Return the file's
-    destination."
+    destination.
 
     The destination may be a directory.
 
     If follow_symlinks is false, symlinks won't be followed. This
     resembles GNU's "cp -P src dst".
-
     """
     if os.path.isdir(dst):
         dst = os.path.join(dst, os.path.basename(src))
@@ -1015,7 +1149,6 @@ if hasattr(os, 'statvfs'):
 
 elif os.name == 'nt':
 
-    import nt
     __all__.append('disk_usage')
     _ntuple_diskusage = collections.namedtuple('usage', 'total used free')
 
diff --git a/Lib/test/test_shutil.py b/Lib/test/test_shutil.py
index 2cb2f14643..8d51994419 100644
--- a/Lib/test/test_shutil.py
+++ b/Lib/test/test_shutil.py
@@ -12,20 +12,28 @@ import errno
 import functools
 import pathlib
 import subprocess
+import random
+import string
+import contextlib
+import io
 from shutil import (make_archive,
                     register_archive_format, unregister_archive_format,
                     get_archive_formats, Error, unpack_archive,
                     register_unpack_format, RegistryError,
                     unregister_unpack_format, get_unpack_formats,
-                    SameFileError)
+                    SameFileError, _GiveupOnFastCopy)
 import tarfile
 import zipfile
+try:
+    import posix
+except ImportError:
+    posix = None
 
 from test import support
 from test.support import TESTFN, FakePath
 
 TESTFN2 = TESTFN + "2"
-
+OSX = sys.platform.startswith("darwin")
 try:
     import grp
     import pwd
@@ -60,6 +68,24 @@ def write_file(path, content, binary=False):
     with open(path, 'wb' if binary else 'w') as fp:
         fp.write(content)
 
+def write_test_file(path, size):
+    """Create a test file with an arbitrary size and random text content."""
+    def chunks(total, step):
+        assert total >= step
+        while total > step:
+            yield step
+            total -= step
+        if total:
+            yield total
+
+    bufsize = min(size, 8192)
+    chunk = b"".join([random.choice(string.ascii_letters).encode()
+                      for i in range(bufsize)])
+    with open(path, 'wb') as f:
+        for csize in chunks(size, bufsize):
+            f.write(chunk)
+    assert os.path.getsize(path) == size
+
 def read_file(path, binary=False):
     """Return contents from a file located at *path*.
 
@@ -84,6 +110,37 @@ def rlistdir(path):
             res.append(name)
     return res
 
+def supports_file2file_sendfile():
+    # ...apparently Linux and Solaris are the only ones
+    if not hasattr(os, "sendfile"):
+        return False
+    srcname = None
+    dstname = None
+    try:
+        with tempfile.NamedTemporaryFile("wb", delete=False) as f:
+            srcname = f.name
+            f.write(b"0123456789")
+
+        with open(srcname, "rb") as src:
+            with tempfile.NamedTemporaryFile("wb", delete=False) as dst:
+                dstname = f.name
+                infd = src.fileno()
+                outfd = dst.fileno()
+                try:
+                    os.sendfile(outfd, infd, 0, 2)
+                except OSError:
+                    return False
+                else:
+                    return True
+    finally:
+        if srcname is not None:
+            support.unlink(srcname)
+        if dstname is not None:
+            support.unlink(dstname)
+
+
+SUPPORTS_SENDFILE = supports_file2file_sendfile()
+
 
 class TestShutil(unittest.TestCase):
 
@@ -1401,6 +1458,8 @@ class TestShutil(unittest.TestCase):
         self.assertRaises(SameFileError, shutil.copyfile, src_file, src_file)
         # But Error should work too, to stay backward compatible.
         self.assertRaises(Error, shutil.copyfile, src_file, src_file)
+        # Make sure file is not corrupted.
+        self.assertEqual(read_file(src_file), 'foo')
 
     def test_copytree_return_value(self):
         # copytree returns its destination path.
@@ -1749,6 +1808,7 @@ class TestCopyFile(unittest.TestCase):
 
         self.assertRaises(OSError, shutil.copyfile, 'srcfile', 'destfile')
 
+    @unittest.skipIf(OSX, "skipped on OSX")
     def test_w_dest_open_fails(self):
 
         srcfile = self.Faux()
@@ -1768,6 +1828,7 @@ class TestCopyFile(unittest.TestCase):
         self.assertEqual(srcfile._exited_with[1].args,
                          ('Cannot open "destfile"',))
 
+    @unittest.skipIf(OSX, "skipped on OSX")
     def test_w_dest_close_fails(self):
 
         srcfile = self.Faux()
@@ -1790,6 +1851,7 @@ class TestCopyFile(unittest.TestCase):
         self.assertEqual(srcfile._exited_with[1].args,
                          ('Cannot close',))
 
+    @unittest.skipIf(OSX, "skipped on OSX")
     def test_w_source_close_fails(self):
 
         srcfile = self.Faux(True)
@@ -1829,6 +1891,234 @@ class TestCopyFile(unittest.TestCase):
         finally:
             os.rmdir(dst_dir)
 
+
+class _ZeroCopyFileTest(object):
+    """Tests common to all zero-copy APIs."""
+    FILESIZE = (10 * 1024 * 1024)  # 10 MiB
+    FILEDATA = b""
+    PATCHPOINT = ""
+
+    @classmethod
+    def setUpClass(cls):
+        write_test_file(TESTFN, cls.FILESIZE)
+        with open(TESTFN, 'rb') as f:
+            cls.FILEDATA = f.read()
+            assert len(cls.FILEDATA) == cls.FILESIZE
+
+    @classmethod
+    def tearDownClass(cls):
+        support.unlink(TESTFN)
+
+    def tearDown(self):
+        support.unlink(TESTFN2)
+
+    @contextlib.contextmanager
+    def get_files(self):
+        with open(TESTFN, "rb") as src:
+            with open(TESTFN2, "wb") as dst:
+                yield (src, dst)
+
+    def zerocopy_fun(self, *args, **kwargs):
+        raise NotImplementedError("must be implemented in subclass")
+
+    def reset(self):
+        self.tearDown()
+        self.tearDownClass()
+        self.setUpClass()
+        self.setUp()
+
+    # ---
+
+    def test_regular_copy(self):
+        with self.get_files() as (src, dst):
+            self.zerocopy_fun(src, dst)
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+        # Make sure the fallback function is not called.
+        with self.get_files() as (src, dst):
+            with unittest.mock.patch('shutil.copyfileobj') as m:
+                shutil.copyfile(TESTFN, TESTFN2)
+            assert not m.called
+
+    def test_same_file(self):
+        self.addCleanup(self.reset)
+        with self.get_files() as (src, dst):
+            with self.assertRaises(Exception):
+                self.zerocopy_fun(src, src)
+        # Make sure src file is not corrupted.
+        self.assertEqual(read_file(TESTFN, binary=True), self.FILEDATA)
+
+    def test_non_existent_src(self):
+        name = tempfile.mktemp()
+        with self.assertRaises(FileNotFoundError) as cm:
+            shutil.copyfile(name, "new")
+        self.assertEqual(cm.exception.filename, name)
+
+    def test_empty_file(self):
+        srcname = TESTFN + 'src'
+        dstname = TESTFN + 'dst'
+        self.addCleanup(lambda: support.unlink(srcname))
+        self.addCleanup(lambda: support.unlink(dstname))
+        with open(srcname, "wb"):
+            pass
+
+        with open(srcname, "rb") as src:
+            with open(dstname, "wb") as dst:
+                self.zerocopy_fun(src, dst)
+
+        self.assertEqual(read_file(dstname, binary=True), b"")
+
+    def test_unhandled_exception(self):
+        with unittest.mock.patch(self.PATCHPOINT,
+                                 side_effect=ZeroDivisionError):
+            self.assertRaises(ZeroDivisionError,
+                              shutil.copyfile, TESTFN, TESTFN2)
+
+    def test_exception_on_first_call(self):
+        # Emulate a case where the first call to the zero-copy
+        # function raises an exception in which case the function is
+        # supposed to give up immediately.
+        with unittest.mock.patch(self.PATCHPOINT,
+                                 side_effect=OSError(errno.EINVAL, "yo")):
+            with self.get_files() as (src, dst):
+                with self.assertRaises(_GiveupOnFastCopy):
+                    self.zerocopy_fun(src, dst)
+
+    def test_filesystem_full(self):
+        # Emulate a case where filesystem is full and sendfile() fails
+        # on first call.
+        with unittest.mock.patch(self.PATCHPOINT,
+                                 side_effect=OSError(errno.ENOSPC, "yo")):
+            with self.get_files() as (src, dst):
+                self.assertRaises(OSError, self.zerocopy_fun, src, dst)
+
+
+@unittest.skipIf(not SUPPORTS_SENDFILE, 'os.sendfile() not supported')
+class TestZeroCopySendfile(_ZeroCopyFileTest, unittest.TestCase):
+    PATCHPOINT = "os.sendfile"
+
+    def zerocopy_fun(self, fsrc, fdst):
+        return shutil._fastcopy_sendfile(fsrc, fdst)
+
+    def test_non_regular_file_src(self):
+        with io.BytesIO(self.FILEDATA) as src:
+            with open(TESTFN2, "wb") as dst:
+                with self.assertRaises(_GiveupOnFastCopy):
+                    self.zerocopy_fun(src, dst)
+                shutil.copyfileobj(src, dst)
+
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+
+    def test_non_regular_file_dst(self):
+        with open(TESTFN, "rb") as src:
+            with io.BytesIO() as dst:
+                with self.assertRaises(_GiveupOnFastCopy):
+                    self.zerocopy_fun(src, dst)
+                shutil.copyfileobj(src, dst)
+                dst.seek(0)
+                self.assertEqual(dst.read(), self.FILEDATA)
+
+    def test_exception_on_second_call(self):
+        def sendfile(*args, **kwargs):
+            if not flag:
+                flag.append(None)
+                return orig_sendfile(*args, **kwargs)
+            else:
+                raise OSError(errno.EBADF, "yo")
+
+        flag = []
+        orig_sendfile = os.sendfile
+        with unittest.mock.patch('os.sendfile', create=True,
+                                 side_effect=sendfile):
+            with self.get_files() as (src, dst):
+                with self.assertRaises(OSError) as cm:
+                    shutil._fastcopy_sendfile(src, dst)
+        assert flag
+        self.assertEqual(cm.exception.errno, errno.EBADF)
+
+    def test_cant_get_size(self):
+        # Emulate a case where src file size cannot be determined.
+        # Internally bufsize will be set to a small value and
+        # sendfile() will be called repeatedly.
+        with unittest.mock.patch('os.fstat', side_effect=OSError) as m:
+            with self.get_files() as (src, dst):
+                shutil._fastcopy_sendfile(src, dst)
+                assert m.called
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+
+    def test_small_chunks(self):
+        # Force internal file size detection to be smaller than the
+        # actual file size. We want to force sendfile() to be called
+        # multiple times, also in order to emulate a src fd which gets
+        # bigger while it is being copied.
+        mock = unittest.mock.Mock()
+        mock.st_size = 65536 + 1
+        with unittest.mock.patch('os.fstat', return_value=mock) as m:
+            with self.get_files() as (src, dst):
+                shutil._fastcopy_sendfile(src, dst)
+                assert m.called
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+
+    def test_big_chunk(self):
+        # Force internal file size detection to be +100MB bigger than
+        # the actual file size. Make sure sendfile() does not rely on
+        # file size value except for (maybe) a better throughput /
+        # performance.
+        mock = unittest.mock.Mock()
+        mock.st_size = self.FILESIZE + (100 * 1024 * 1024)
+        with unittest.mock.patch('os.fstat', return_value=mock) as m:
+            with self.get_files() as (src, dst):
+                shutil._fastcopy_sendfile(src, dst)
+                assert m.called
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+
+    def test_blocksize_arg(self):
+        with unittest.mock.patch('os.sendfile',
+                                 side_effect=ZeroDivisionError) as m:
+            self.assertRaises(ZeroDivisionError,
+                              shutil.copyfile, TESTFN, TESTFN2)
+            blocksize = m.call_args[0][3]
+            # Make sure file size and the block size arg passed to
+            # sendfile() are the same.
+            self.assertEqual(blocksize, os.path.getsize(TESTFN))
+            # ...unless we're dealing with a small file.
+            support.unlink(TESTFN2)
+            write_file(TESTFN2, b"hello", binary=True)
+            self.addCleanup(support.unlink, TESTFN2 + '3')
+            self.assertRaises(ZeroDivisionError,
+                              shutil.copyfile, TESTFN2, TESTFN2 + '3')
+            blocksize = m.call_args[0][3]
+            self.assertEqual(blocksize, 2 ** 23)
+
+    def test_file2file_not_supported(self):
+        # Emulate a case where sendfile() only support file->socket
+        # fds. In such a case copyfile() is supposed to skip the
+        # fast-copy attempt from then on.
+        assert shutil._HAS_SENDFILE
+        try:
+            with unittest.mock.patch(
+                    self.PATCHPOINT,
+                    side_effect=OSError(errno.ENOTSOCK, "yo")) as m:
+                with self.get_files() as (src, dst):
+                    with self.assertRaises(_GiveupOnFastCopy):
+                        shutil._fastcopy_sendfile(src, dst)
+                assert m.called
+            assert not shutil._HAS_SENDFILE
+
+            with unittest.mock.patch(self.PATCHPOINT) as m:
+                shutil.copyfile(TESTFN, TESTFN2)
+                assert not m.called
+        finally:
+            shutil._HAS_SENDFILE = True
+
+
+@unittest.skipIf(not OSX, 'OSX only')
+class TestZeroCopyOSX(_ZeroCopyFileTest, unittest.TestCase):
+    PATCHPOINT = "posix._fcopyfile"
+
+    def zerocopy_fun(self, src, dst):
+        return shutil._fastcopy_osx(src, dst, posix._COPYFILE_DATA)
+
+
 class TermsizeTests(unittest.TestCase):
     def test_does_not_crash(self):
         """Check if get_terminal_size() returns a meaningful value.