Add copy-on-write support to shutil

Author: illia-v

Doc/library/shutil.rst

@@ -48,7 +48,7 @@ Directory and files operations
    be copied.
 
 
-.. function:: copyfile(src, dst, *, follow_symlinks=True)
+.. function:: copyfile(src, dst, *, follow_symlinks=True, allow_reflink=True)
 
    Copy the contents (no metadata) of the file named *src* to a file named
    *dst* and return *dst* in the most efficient way possible.
@@ -67,6 +67,10 @@ Directory and files operations
    a new symbolic link will be created instead of copying the
    file *src* points to.
 
+   *allow_reflink* enables copy-on-write on supported Linux filesystems.
+   The :c:func:`copy_file_range` system call is used internally when
+   *allow_reflink* is true.
+
    .. audit-event:: shutil.copyfile src,dst shutil.copyfile
 
    .. versionchanged:: 3.3
@@ -83,6 +87,10 @@ Directory and files operations
       copy the file more efficiently. See
       :ref:`shutil-platform-dependent-efficient-copy-operations` section.
 
+   .. versionchanged:: 3.12
+      Added *allow_reflink* argument. Copy-on-write is enabled by default on
+      supported Linux filesystems.
+
 .. exception:: SameFileError
 
    This exception is raised if source and destination in :func:`copyfile`
@@ -155,7 +163,7 @@ Directory and files operations
    .. versionchanged:: 3.3
       Added *follow_symlinks* argument and support for Linux extended attributes.
 
-.. function:: copy(src, dst, *, follow_symlinks=True)
+.. function:: copy(src, dst, *, follow_symlinks=True, allow_reflink=True)
 
    Copies the file *src* to the file or directory *dst*.  *src* and *dst*
    should be :term:`path-like objects <path-like object>` or strings.  If
@@ -168,6 +176,10 @@ Directory and files operations
    is true and *src* is a symbolic link, *dst* will be a copy of
    the file *src* refers to.
 
+   *allow_reflink* enables copy-on-write on supported Linux filesystems.
+   The :c:func:`copy_file_range` system call is used internally when
+   *allow_reflink* is true.
+
    :func:`~shutil.copy` copies the file data and the file's permission
    mode (see :func:`os.chmod`).  Other metadata, like the
    file's creation and modification times, is not preserved.
@@ -187,7 +199,11 @@ Directory and files operations
       copy the file more efficiently. See
       :ref:`shutil-platform-dependent-efficient-copy-operations` section.
 
-.. function:: copy2(src, dst, *, follow_symlinks=True)
+   .. versionchanged:: 3.12
+      Added *allow_reflink* argument. Copy-on-write is enabled by default on
+      supported Linux filesystems.
+
+.. function:: copy2(src, dst, *, follow_symlinks=True, allow_reflink=True)
 
    Identical to :func:`~shutil.copy` except that :func:`copy2`
    also attempts to preserve file metadata.
@@ -201,6 +217,10 @@ Directory and files operations
    it can; :func:`copy2` never raises an exception because it
    cannot preserve file metadata.
 
+   *allow_reflink* enables copy-on-write on supported Linux filesystems.
+   The :c:func:`copy_file_range` system call is used internally when
+   *allow_reflink* is true.
+
    :func:`copy2` uses :func:`copystat` to copy the file metadata.
    Please see :func:`copystat` for more information
    about platform support for modifying symbolic link metadata.
@@ -219,6 +239,10 @@ Directory and files operations
       copy the file more efficiently. See
       :ref:`shutil-platform-dependent-efficient-copy-operations` section.
 
+   .. versionchanged:: 3.12
+      Added *allow_reflink* argument. Copy-on-write is enabled by default on
+      supported Linux filesystems.
+
 .. function:: ignore_patterns(*patterns)
 
    This factory function creates a function that can be used as a callable for

Lib/shutil.py

@@ -43,6 +43,7 @@
 # This should never be removed, see rationale in:
 # https://bugs.python.org/issue43743#msg393429
 _USE_CP_SENDFILE = hasattr(os, "sendfile") and sys.platform.startswith("linux")
+_USE_CP_COPY_FILE_RANGE = hasattr(os, "copy_file_range")
 _HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS
 
 # CMD defaults in Windows 10
@@ -122,6 +123,47 @@ def _determine_linux_fastcopy_blocksize(infd):
         blocksize = min(blocksize, 2 ** 30)
     return blocksize
 
+def _fastcopy_copy_file_range(fsrc, fdst):
+    """Copy data from one regular mmap-like fd to another by using
+    a high-performance copy_file_range(2) syscall that gives filesystems
+    an opportunity to implement the use of reflinks or server-side copy.
+
+    This should work on Linux >= 4.5 only.
+    """
+    try:
+        infd = fsrc.fileno()
+        outfd = fdst.fileno()
+    except Exception as err:
+        raise _GiveupOnFastCopy(err)  # not a regular file
+
+    blocksize = _determine_linux_fastcopy_blocksize(infd)
+    offset = 0
+    while True:
+        try:
+            n_copied = os.copy_file_range(infd, outfd, blocksize, offset_dst=offset)
+        except OSError as err:
+            # ...in oder to have a more informative exception.
+            err.filename = fsrc.name
+            err.filename2 = fdst.name
+
+            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 n_copied == 0:
+                # If no bytes have been copied yet, copy_file_range
+                # might silently fail.
+                # https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
+                if offset == 0:
+                    raise _GiveupOnFastCopy()
+                break
+            offset += n_copied
+
 def _fastcopy_sendfile(fsrc, fdst):
     """Copy data from one regular mmap-like fd to another by using
     high-performance sendfile(2) syscall.
@@ -230,7 +272,7 @@ def _stat(fn):
 def _islink(fn):
     return fn.is_symlink() if isinstance(fn, os.DirEntry) else os.path.islink(fn)
 
-def copyfile(src, dst, *, follow_symlinks=True):
+def copyfile(src, dst, *, follow_symlinks=True, allow_reflink=True):
     """Copy data from src to dst in the most efficient way possible.
 
     If follow_symlinks is not set and src is a symbolic link, a new
@@ -271,12 +313,20 @@ def copyfile(src, dst, *, follow_symlinks=True):
                         except _GiveupOnFastCopy:
                             pass
                     # Linux
-                    elif _USE_CP_SENDFILE:
-                        try:
-                            _fastcopy_sendfile(fsrc, fdst)
-                            return dst
-                        except _GiveupOnFastCopy:
-                            pass
+                    elif _USE_CP_SENDFILE or _USE_CP_COPY_FILE_RANGE:
+                        # reflink may be implicit in copy_file_range.
+                        if _USE_CP_COPY_FILE_RANGE and allow_reflink:
+                            try:
+                                _fastcopy_copy_file_range(fsrc, fdst)
+                                return dst
+                            except _GiveupOnFastCopy:
+                                pass
+                        if _USE_CP_SENDFILE:
+                            try:
+                                _fastcopy_sendfile(fsrc, fdst)
+                                return dst
+                            except _GiveupOnFastCopy:
+                                pass
                     # Windows, see:
                     # https://github.com/python/cpython/pull/7160#discussion_r195405230
                     elif _WINDOWS and file_size > 0:
@@ -409,7 +459,7 @@ def lookup(name):
             else:
                 raise
 
-def copy(src, dst, *, follow_symlinks=True):
+def copy(src, dst, *, follow_symlinks=True, allow_reflink=True):
     """Copy data and mode bits ("cp src dst"). Return the file's destination.
 
     The destination may be a directory.
@@ -423,11 +473,11 @@ def copy(src, dst, *, follow_symlinks=True):
     """
     if os.path.isdir(dst):
         dst = os.path.join(dst, os.path.basename(src))
-    copyfile(src, dst, follow_symlinks=follow_symlinks)
+    copyfile(src, dst, follow_symlinks=follow_symlinks, allow_reflink=allow_reflink)
     copymode(src, dst, follow_symlinks=follow_symlinks)
     return dst
 
-def copy2(src, dst, *, follow_symlinks=True):
+def copy2(src, dst, *, follow_symlinks=True, allow_reflink=True):
     """Copy data and metadata. Return the file's destination.
 
     Metadata is copied with copystat(). Please see the copystat function
@@ -440,7 +490,7 @@ def copy2(src, dst, *, follow_symlinks=True):
     """
     if os.path.isdir(dst):
         dst = os.path.join(dst, os.path.basename(src))
-    copyfile(src, dst, follow_symlinks=follow_symlinks)
+    copyfile(src, dst, follow_symlinks=follow_symlinks, allow_reflink=allow_reflink)
     copystat(src, dst, follow_symlinks=follow_symlinks)
     return dst
 

Lib/test/test_shutil.py

@@ -2467,12 +2467,8 @@ def test_filesystem_full(self):
                 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)
+class _ZeroCopyFileLinuxTest(_ZeroCopyFileTest):
+    BLOCKSIZE_INDEX = None
 
     def test_non_regular_file_src(self):
         with io.BytesIO(self.FILEDATA) as src:
@@ -2493,65 +2489,65 @@ def test_non_regular_file_dst(self):
                 self.assertEqual(dst.read(), self.FILEDATA)
 
     def test_exception_on_second_call(self):
-        def sendfile(*args, **kwargs):
+        def syscall(*args, **kwargs):
             if not flag:
                 flag.append(None)
-                return orig_sendfile(*args, **kwargs)
+                return orig_syscall(*args, **kwargs)
             else:
                 raise OSError(errno.EBADF, "yo")
 
         flag = []
-        orig_sendfile = os.sendfile
-        with unittest.mock.patch('os.sendfile', create=True,
-                                 side_effect=sendfile):
+        orig_syscall = eval(self.PATCHPOINT)
+        with unittest.mock.patch(self.PATCHPOINT, create=True,
+                                 side_effect=syscall):
             with self.get_files() as (src, dst):
                 with self.assertRaises(OSError) as cm:
-                    shutil._fastcopy_sendfile(src, dst)
+                    self.zerocopy_fun(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.
+        # a system call 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)
+                self.zerocopy_fun(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
+        # actual file size. We want to force a system call 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)
+                self.zerocopy_fun(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
+        # the actual file size. Make sure a system call 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)
+                self.zerocopy_fun(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',
+        with unittest.mock.patch(self.PATCHPOINT,
                                  side_effect=ZeroDivisionError) as m:
             self.assertRaises(ZeroDivisionError,
                               shutil.copyfile, TESTFN, TESTFN2)
-            blocksize = m.call_args[0][3]
+            blocksize = m.call_args[0][self.BLOCKSIZE_INDEX]
             # Make sure file size and the block size arg passed to
             # sendfile() are the same.
             self.assertEqual(blocksize, os.path.getsize(TESTFN))
@@ -2561,9 +2557,19 @@ def test_blocksize_arg(self):
             self.addCleanup(os_helper.unlink, TESTFN2 + '3')
             self.assertRaises(ZeroDivisionError,
                               shutil.copyfile, TESTFN2, TESTFN2 + '3')
-            blocksize = m.call_args[0][3]
+            blocksize = m.call_args[0][self.BLOCKSIZE_INDEX]
             self.assertEqual(blocksize, 2 ** 23)
 
+
+@unittest.skipIf(not SUPPORTS_SENDFILE, 'os.sendfile() not supported')
+@unittest.mock.patch.object(shutil, "_USE_CP_COPY_FILE_RANGE", False)
+class TestZeroCopySendfile(_ZeroCopyFileLinuxTest, unittest.TestCase):
+    PATCHPOINT = "os.sendfile"
+    BLOCKSIZE_INDEX = 3
+
+    def zerocopy_fun(self, fsrc, fdst):
+        return shutil._fastcopy_sendfile(fsrc, fdst)
+
     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
@@ -2586,6 +2592,29 @@ def test_file2file_not_supported(self):
             shutil._USE_CP_SENDFILE = True
 
 
+@unittest.skipUnless(shutil._USE_CP_COPY_FILE_RANGE, "os.copy_file_range() not supported")
+class TestZeroCopyCopyFileRange(_ZeroCopyFileLinuxTest, unittest.TestCase):
+    PATCHPOINT = "os.copy_file_range"
+    BLOCKSIZE_INDEX = 2
+
+    def zerocopy_fun(self, fsrc, fdst):
+        return shutil._fastcopy_copy_file_range(fsrc, fdst)
+
+    def test_empty_file(self):
+        srcname = f"{TESTFN}src"
+        dstname = f"{TESTFN}dst"
+        self.addCleanup(lambda: os_helper.unlink(srcname))
+        self.addCleanup(lambda: os_helper.unlink(dstname))
+        with open(srcname, "wb"):
+            pass
+
+        with open(srcname, "rb") as src, open(dstname, "wb") as dst:
+            # _fastcopy_copy_file_range gives up copying empty files due
+            # to a bug in older Linux.
+            with self.assertRaises(shutil._GiveupOnFastCopy):
+                self.zerocopy_fun(src, dst)
+
+
 @unittest.skipIf(not MACOS, 'macOS only')
 class TestZeroCopyMACOS(_ZeroCopyFileTest, unittest.TestCase):
     PATCHPOINT = "posix._fcopyfile"

Misc/ACKS

@@ -1878,6 +1878,7 @@ Johannes Vogel
 Michael Vogt
 Radu Voicilas
 Alex Volkov
+Illia Volochii
 Ruben Vorderman
 Guido Vranken
 Martijn Vries

Misc/NEWS.d/next/Library/2022-05-23-21-23-29.gh-issue-81340.D11RkZ.rst

@@ -0,0 +1,5 @@
+Use the :c:func:`copy_file_range` Linux system call in :func:`shutil.copy`,
+:func:`shutil.copy2`, and :func:`shutil.copyfile` functions by default. The
+system call gives filesystems an opportunity to implement the use of
+copy-on-write or server-side copy. The functions have a new *allow_reflink*
+argument to control the functionality. Patch by Illia Volochii.