config: Add support for 'all' loglevel

Equal to logging.NOTSET or 0 in the logging module.

docs/changelog.rst

@@ -26,6 +26,12 @@ v0.20.0 (UNRELEASED)
 - Make the ``mopidy`` command print a friendly error message if the
   :mod:`gobject` Python module cannot be imported. (Fixes: :issue:`836`)
 
+**Configuration**
+
+- Add support for the log level value ``all`` to the loglevels configurations.
+  This can be used to show absolutely all log records, including those at
+  custom levels below ``DEBUG``.
+
 **Local backend**
 
 - Add cover URL to all scanned files with MusicBrainz album IDs. (Fixes:
@@ -50,7 +56,7 @@ v0.20.0 (UNRELEASED)
 
 - Enable browsing of artist references, in addition to albums and playlists.
   (PR: :issue:`884`)
-  
+
 - Share a single mapping between names and URIs across all MPD sessions. (Fixes:
   :issue:`934`, PR: :issue:`968`)
 

mopidy/config/types.py

@@ -200,8 +200,8 @@ class List(ConfigValue):
 class LogLevel(ConfigValue):
     """Log level value.
 
-    Expects one of ``critical``, ``error``, ``warning``, ``info``, ``debug``
+    Expects one of ``critical``, ``error``, ``warning``, ``info``, ``debug``,
-    with any casing.
+    or ``all``, with any casing.
     """
     levels = {
         b'critical': logging.CRITICAL,
@@ -209,6 +209,7 @@ class LogLevel(ConfigValue):
         b'warning': logging.WARNING,
         b'info': logging.INFO,
         b'debug': logging.DEBUG,
+        b'all': logging.NOTSET,
     }
 
     def deserialize(self, value):

tests/config/test_types.py

@@ -281,11 +281,14 @@ class ListTest(unittest.TestCase):
 
 
 class LogLevelTest(unittest.TestCase):
-    levels = {'critical': logging.CRITICAL,
+    levels = {
-              'error': logging.ERROR,
+        'critical': logging.CRITICAL,
-              'warning': logging.WARNING,
+        'error': logging.ERROR,
-              'info': logging.INFO,
+        'warning': logging.WARNING,
-              'debug': logging.DEBUG}
+        'info': logging.INFO,
+        'debug': logging.DEBUG,
+        'all': logging.NOTSET,
+    }
 
     def test_deserialize_conversion_success(self):
         value = types.LogLevel()