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/library.py

@@ -760,6 +760,8 @@ def write(self, path=None, tags=None, id3v23=None):
         mediafile.update(item_tags)
         try:
             mediafile.save()
+            # Move file to trigger OS event if some processes subscribed to it
+            util.move(path, path, force=True)
         except UnreadableFileError as exc:
             raise WriteError(self.path, exc)
 

beets/util/__init__.py

@@ -21,6 +21,7 @@
 import errno
 import locale
 import re
+import tempfile
 import shutil
 import fnmatch
 import functools
@@ -474,32 +475,45 @@ def copy(path, dest, replace=False):
                               traceback.format_exc())
 
 
-def move(path, dest, replace=False):
+def move(path, dest, replace=False, force=False):
     """Rename a file. `dest` may not be a directory. If `dest` already
     exists, raises an OSError unless `replace` is True. Has no effect if
-    `path` is the same as `dest`. If the paths are on different
-    filesystems (or the rename otherwise fails), a copy is attempted
-    instead, in which case metadata will *not* be preserved. Paths are
-    translated to system paths.
+    `path` is the same as `dest` unless `force` is True. If the paths are
+    on different filesystems (or the rename otherwise fails), a copy
+    is attempted instead, in which case metadata will *not* be preserved.
+    Paths are translated to system paths.
     """
-    if samefile(path, dest):
+    if os.path.isdir(path):
+        raise FilesystemError(u'source is directory', 'move', (path, dest))
+    if samefile(path, dest) and not force:
         return
     path = syspath(path)
     dest = syspath(dest)
-    if os.path.exists(dest) and not replace:
+    if os.path.exists(dest) and not replace and not force:
         raise FilesystemError(u'file exists', 'rename', (path, dest))
 
     # First, try renaming the file.
     try:
-        os.rename(path, dest)
+        os.replace(path, dest)
     except OSError:
         # Otherwise, copy and delete the original.
+        if os.path.isdir(dest):
+            dest = os.path.join(dest, os.path.basename(path))
+        tmp = tempfile.mktemp(suffix='.beets',
+                              prefix=as_string(b'.' + os.path.basename(dest)),
+                              dir=as_string(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, IOError) 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):