[Config] Prevent symlinked config files being overwritten

If a user keeps the deluge config file under source control and symlinks
to the config files in deluge config dir then when deluge saves config
files it will replace the symlink with actual file.

Using realpath will resolve these symlinks and file will be updated in
the correct location.

Use a temporary file for new config new before moving it to the
resolved location.

Co-authored-by: Calum Lind <calumlind+deluge@gmail.com>

deluge/config.py

@@ -47,6 +47,7 @@ import os
 import shutil
 from codecs import getwriter
 from io import open
+from tempfile import NamedTemporaryFile
 
 import six.moves.cPickle as pickle
 
@@ -475,8 +476,11 @@ class Config(object):
 
         # Save the new config and make sure it's written to disk
         try:
-            log.debug('Saving new config file %s', filename + '.new')
-            with open(filename + '.new', 'wb') as _file:
+            with NamedTemporaryFile(
+                prefix=os.path.basename(filename) + '.', delete=False
+            ) as _file:
+                filename_tmp = _file.name
+                log.debug('Saving new config file %s', filename_tmp)
                 json.dump(self.__version, getwriter('utf8')(_file), **JSON_FORMAT)
                 json.dump(self.__config, getwriter('utf8')(_file), **JSON_FORMAT)
                 _file.flush()
@@ -485,6 +489,9 @@ class Config(object):
             log.error('Error writing new config file: %s', ex)
             return False
 
+        # Resolve symlinked config files before backing up and saving.
+        filename = os.path.realpath(filename)
+
         # Make a backup of the old config
         try:
             log.debug('Backing up old config file to %s.bak', filename)
@@ -495,8 +502,8 @@ class Config(object):
         # The new config file has been written successfully, so let's move it over
         # the existing one.
         try:
-            log.debug('Moving new config file %s to %s..', filename + '.new', filename)
-            shutil.move(filename + '.new', filename)
+            log.debug('Moving new config file %s to %s', filename_tmp, filename)
+            shutil.move(filename_tmp, filename)
         except IOError as ex:
             log.error('Error moving new config file: %s', ex)
             return False