Introduce atomic move and write of file

The idea of this changes is simple: let move file to some temporary name
inside distance folder, and after the file is already copy it renames to
expected name.

When someone tries to save anything it also moves file to trigger OS
level notification for change FS.

This commit also enforce that `beets.util.move` shouldn't be used to
move directories as it described in comment.

Thus, this is fixed beetbox#3849

beets/util/__init__.py

@@ -19,6 +19,7 @@
 import errno
 import locale
 import re
+import tempfile
 import shutil
 import fnmatch
 import functools
@@ -478,6 +479,10 @@ def move(path, dest, replace=False):
     instead, in which case metadata will *not* be preserved. Paths are
     translated to system paths.
     """
+    if os.path.isdir(path):
+        raise FilesystemError(u'source is directory', 'move', (path, dest))
+    if os.path.isdir(dest):
+        raise FilesystemError(u'destination is directory', 'move', (path, dest))
     if samefile(path, dest):
         return
     path = syspath(path)
@@ -487,15 +492,23 @@ def move(path, dest, replace=False):
 
     # First, try renaming the file.
     try:
-        os.rename(path, dest)
+        os.replace(path, dest)
     except OSError:
-        # Otherwise, copy and delete the original.
+        tmp = tempfile.mktemp(suffix='.beets',
+                              prefix=py3_path(b'.' + os.path.basename(dest)),
+                              dir=py3_path(os.path.dirname(dest)))
+        tmp = syspath(tmp)
         try:
-            shutil.copyfile(path, dest)
+            shutil.copyfile(path, tmp)
+            os.replace(tmp, dest)
+            tmp = None
             os.remove(path)
         except OSError as exc:
             raise FilesystemError(exc, 'move', (path, dest),
                                   traceback.format_exc())
+        finally:
+            if tmp is not None:
+                os.remove(tmp)
 
 
 def link(path, dest, replace=False):