local: Split out library reading and writting

- Create $XDG_DATA_DIR/mopidy/local in the local extension's validate env.
- Make sure we handle bad data causing ValueError in JSON decoding
- Initializing empty file causes more harm than good as it just leads to a
  ValueError. Switched to doing write_library(json_file, {})
- Helpers have been updated to be library oriented, not track. This paves the
  way for having {tracks: {uri: ...}, artist: {uri: ...}, ...} type
  denormalized data.

mopidy/backends/local/__init__.py

@@ -1,9 +1,13 @@
 from __future__ import unicode_literals
 
+import logging
 import os
 
 import mopidy
 from mopidy import config, ext
+from mopidy.utils import encoding, path
+
+logger = logging.getLogger('mopidy.backends.local')
 
 
 class Extension(ext.Extension):
@@ -27,7 +31,11 @@ class Extension(ext.Extension):
         return schema
 
     def validate_environment(self):
-        pass
+        try:
+            path.get_or_create_dir(b'$XDG_DATA_DIR/mopidy/local')
+        except EnvironmentError as error:
+            error = encoding.locale_decode(error)
+            logger.warning('Could not create local data dir: %s', error)
 
     def get_backend_classes(self):
         from .actor import LocalBackend

mopidy/backends/local/json/actor.py

@@ -1,13 +1,14 @@
 from __future__ import unicode_literals
 
 import logging
+import os
 
 import pykka
 
 from mopidy.backends import base
-from mopidy.utils import encoding, path
+from mopidy.utils import encoding
 
-from .library import LocalJsonLibraryProvider
+from . import library
 
 logger = logging.getLogger('mopidy.backends.local.json')
 
@@ -17,14 +18,13 @@ class LocalJsonBackend(pykka.ThreadingActor, base.Backend):
         super(LocalJsonBackend, self).__init__()
 
         self.config = config
-        self.check_dirs_and_files()
-        self.library = LocalJsonLibraryProvider(backend=self)
+        self.library = library.LocalJsonLibraryProvider(backend=self)
         self.uri_schemes = ['local']
 
-    def check_dirs_and_files(self):
-        try:
-            path.get_or_create_file(self.config['local-json']['json_file'])
-        except EnvironmentError as error:
-            logger.warning(
-                'Could not create empty json file: %s',
-                encoding.locale_decode(error))
+        if not os.path.exists(config['local-json']['json_file']):
+            try:
+                library.write_library(config['local-json']['json_file'], {})
+                logger.info('Created empty local JSON library.')
+            except EnvironmentError as error:
+                error = encoding.locale_decode(error)
+                logger.warning('Could not create local library: %s', error)

mopidy/backends/local/json/library.py

@@ -14,13 +14,31 @@ from mopidy.backends.local import search
 logger = logging.getLogger('mopidy.backends.local.json')
 
 
-def _load_tracks(json_file):
+def load_library(json_file):
     try:
         with gzip.open(json_file, 'rb') as fp:
-            result = json.load(fp, object_hook=models.model_json_decoder)
-    except IOError:
-        return []
-    return result.get('tracks', [])
+            return json.load(fp, object_hook=models.model_json_decoder)
+    except (IOError, ValueError) as e:
+        logger.warning('Loading JSON local library failed: %s', e)
+        return {}
+
+
+def write_library(json_file, data):
+    data['version'] = mopidy.__version__
+    directory, basename = os.path.split(json_file)
+
+    # TODO: cleanup directory/basename.* files.
+    tmp = tempfile.NamedTemporaryFile(
+        prefix=basename + '.', dir=directory, delete=False)
+
+    try:
+        with gzip.GzipFile(fileobj=tmp, mode='wb') as fp:
+            json.dump(data, fp, cls=models.ModelJSONEncoder,
+                      indent=2, separators=(',', ': '))
+        os.rename(tmp.name, json_file)
+    finally:
+        if os.path.exists(tmp.name):
+            os.remove(tmp.name)
 
 
 class LocalJsonLibraryProvider(base.BaseLibraryProvider):
@@ -36,7 +54,7 @@ class LocalJsonLibraryProvider(base.BaseLibraryProvider):
             'Loading local tracks from %s using %s',
             self._media_dir, self._json_file)
 
-        tracks = _load_tracks(self._json_file)
+        tracks = load_library(self._json_file).get('tracks', [])
         uris_to_remove = set(self._uri_mapping)
 
         for track in tracks:
@@ -75,7 +93,7 @@ class LocalJsonLibraryUpdateProvider(base.BaseLibraryProvider):
         self._json_file = config['local-json']['json_file']
 
     def load(self):
-        for track in _load_tracks(self._json_file):
+        for track in load_library(self._json_file).get('tracks', []):
             self._tracks[track.uri] = track
         return self._tracks.values()
 
@@ -87,19 +105,4 @@ class LocalJsonLibraryUpdateProvider(base.BaseLibraryProvider):
             del self._tracks[uri]
 
     def commit(self):
-        directory, basename = os.path.split(self._json_file)
-
-        # TODO: cleanup directory/basename.* files.
-        tmp = tempfile.NamedTemporaryFile(
-            prefix=basename + '.', dir=directory, delete=False)
-
-        try:
-            with gzip.GzipFile(fileobj=tmp, mode='wb') as fp:
-                data = {'version': mopidy.__version__,
-                        'tracks': self._tracks.values()}
-                json.dump(data, fp, cls=models.ModelJSONEncoder,
-                          indent=2, separators=(',', ': '))
-            os.rename(tmp.name, self._json_file)
-        finally:
-            if os.path.exists(tmp.name):
-                os.remove(tmp.name)
+        write_library(self._json_file, {'tracks': self._tracks.values()})