Allow a file to be moved cross-device.

With `os.rename` you get a `OSError: [Errno 18] Invalid cross-device link`
if you attempt to move a file across filesystems.

`shutil.move` covers the basic case:

> If the destination is on the current filesystem, then os.rename() is used.

and additionally handles the cross-device case.

If the destination is on a different filesystem,
we approximate atomicity by first copying the source
to a temporary file on the target filesystem, followed
by a rename. Finally the source file is removed.

luigi/file.py

@@ -25,6 +25,7 @@
 import tempfile
 import io
 import warnings
+import errno
 
 from luigi.format import FileWrapper, get_default_format
 from luigi.target import FileAlreadyExists, MissingParentDirectory, NotADirectory, FileSystem, FileSystemTarget, AtomicLocalFile
@@ -92,12 +93,26 @@ def remove(self, path, recursive=True):
             os.remove(path)
 
     def move(self, old_path, new_path, raise_if_exists=False):
+        """
+        Move file atomically. If source and destination are located
+        on different filesystems, atomicity is approximated
+        but cannot be guaranteed.
+        """
         if raise_if_exists and os.path.exists(new_path):
             raise RuntimeError('Destination exists: %s' % new_path)
         d = os.path.dirname(new_path)
         if d and not os.path.exists(d):
             self.mkdir(d)
-        os.rename(old_path, new_path)
+        try:
+            os.rename(old_path, new_path)
+        except OSError as err:
+            if err.errno == errno.EXDEV:
+                new_path_tmp = '%s-%09d' % (new_path, random.randint(0, 999999999))
+                shutil.copy(old_path, new_path_tmp)
+                os.rename(new_path_tmp, new_path)
+                os.remove(old_path)
+            else:
+                raise err
 
 
 class LocalTarget(FileSystemTarget):

test/file_test.py

@@ -32,7 +32,7 @@
 
 import itertools
 import io
-from errno import EEXIST
+from errno import EEXIST, EXDEV
 
 
 class LocalTargetTest(unittest.TestCase, FileSystemTargetTestMixin):
@@ -130,6 +130,32 @@ def test_move(self):
         self.assertFalse(os.path.exists(self.path))
         self.assertTrue(os.path.exists(self.copy))
 
+    def test_move_across_filesystems(self):
+        t = LocalTarget(self.path)
+        with t.open('w') as f:
+            f.write('test_data')
+
+        def rename_across_filesystems(src, dst):
+            err = OSError()
+            err.errno = EXDEV
+            raise err
+
+        real_rename = os.rename
+
+        def mockrename(src, dst):
+            if '-across-fs' in src:
+                real_rename(src, dst)
+            else:
+                rename_across_filesystems(src, dst)
+
+        copy = '%s-across-fs' % self.copy
+        with mock.patch('os.rename', mockrename):
+            t.move(copy)
+
+        self.assertFalse(os.path.exists(self.path))
+        self.assertTrue(os.path.exists(copy))
+        self.assertEqual('test_data', LocalTarget(copy).open('r').read())
+
     def test_format_chain(self):
         UTF8WIN = luigi.format.TextFormat(encoding='utf8', newline='\r\n')
         t = LocalTarget(self.path, UTF8WIN >> luigi.format.Gzip)