GH-139416: Fix portability of sendfile(2) support detection for Lustre filesystems

Lib/shutil.py

@@ -206,11 +206,24 @@ def _fastcopy_sendfile(fsrc, fdst):
             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).
+            if err.errno in (errno.ENOTSOCK, errno.ENODATA):
+                # ENOTSOCK: sendfile() on this platform (probably
+                # Linux < 2.6.33) does not support copies between
+                # regular files (only sockets).
+                #
+                # ENODATA: In rare cases, sendfile() on Linux Lustre
+                # returns ENODATA.
                 _USE_CP_SENDFILE = False
+
+                dstpos = os.lseek(outfd, 0, os.SEEK_CUR)
+                if dstpos > 0:
+                    # Some data has already been written but we use
+                    # sendfile in a mode that does not update the
+                    # input fd position when reading. Hence seek the
+                    # input fd to the correct position before falling
+                    # back on POSIX read/write method
+                    os.lseek(infd, dstpos, os.SEEK_SET)
+
                 raise _GiveupOnFastCopy(err)
 
             if err.errno == errno.ENOSPC:  # filesystem is full

Lib/test/test_shutil.py

@@ -3365,6 +3365,42 @@ def test_file2file_not_supported(self):
         finally:
             shutil._USE_CP_SENDFILE = True
 
+    def test_exception_on_enodata_call(self):
+        # Test logic when sendfile(2) call returns ENODATA error on
+        # the not-first call on the file and we need to fall back to
+        # traditional POSIX while preserving the position of where we
+        # got to in writing
+        def syscall(*args, **kwargs):
+            if not flag:
+                flag.append(None)
+                return orig_syscall(*args, **kwargs)
+            else:
+                raise OSError(errno.ENODATA, "yo")
+
+        flag = []
+        orig_syscall = eval(self.PATCHPOINT)
+        # Reduce block size so that multiple syscalls are needed
+        mock = unittest.mock.Mock()
+        mock.st_size = 65536 + 1
+        with unittest.mock.patch('os.fstat', return_value=mock) as m:
+            with unittest.mock.patch(self.PATCHPOINT, create=True,
+                                     side_effect=syscall) as m2:
+                with self.get_files() as (src, dst):
+                    with self.assertRaises(_GiveupOnFastCopy) as cm:
+                        self.zerocopy_fun(src, dst)
+
+            # Reset flag so that second syscall fails again
+            flag = []
+            with unittest.mock.patch(self.PATCHPOINT, create=True,
+                                     side_effect=syscall) as m2:                        
+                shutil._USE_CP_SENDFILE = True
+                shutil.copyfile(TESTFN, TESTFN2)
+                assert m2.called
+        shutil._USE_CP_SENDFILE = True
+        assert flag
+        self.assertEqual(read_file(TESTFN2, binary=True), self.FILEDATA)
+
+
 
 @unittest.skipUnless(shutil._USE_CP_COPY_FILE_RANGE, "os.copy_file_range() not supported")
 class TestZeroCopyCopyFileRange(_ZeroCopyFileLinuxTest, unittest.TestCase):

Misc/NEWS.d/next/Library/2025-09-29-14-30-00.gh-issue-139416.bnzz33.rst

@@ -0,0 +1,3 @@
+:func:`shutil.copyfile`: Detect problem seen on Lustre filesystems
+giving "Errno 61" due to the sendfile(2) optimised implementation and
+fall back to the Posix standard read/write implementation.