core: Update BaseLibraryProvider to not require refresh, search or find_exact.

These methods may now return None, and the core code has been updated to filter
out missing SearchResults.

mopidy/backends/base.py

@@ -57,9 +57,9 @@ class BaseLibraryProvider(object):
         """
         See :meth:`mopidy.core.LibraryController.find_exact`.
 
-        *MUST be implemented by subclass.*
+        *MAY be implemented by subclass.*
         """
-        raise NotImplementedError
+        pass
 
     def lookup(self, uri):
         """
@@ -73,17 +73,17 @@ class BaseLibraryProvider(object):
         """
         See :meth:`mopidy.core.LibraryController.refresh`.
 
-        *MUST be implemented by subclass.*
+        *MAY be implemented by subclass.*
         """
-        raise NotImplementedError
+        pass
 
     def search(self, **query):
         """
         See :meth:`mopidy.core.LibraryController.search`.
 
-        *MUST be implemented by subclass.*
+        *MAY be implemented by subclass.*
         """
-        raise NotImplementedError
+        pass
 
 
 class BasePlaybackProvider(object):

mopidy/backends/stream/actor.py

@@ -46,12 +46,3 @@ class StreamLibraryProvider(base.BaseLibraryProvider):
         # Note that we would only want the stream metadata at this stage,
         # not the currently playing track's.
         return [Track(uri=uri, name=uri)]
-
-    def find_exact(self, **query):
-        return SearchResult()
-
-    def search(self, **query):
-        return SearchResult()
-
-    def refresh(self, uri=None):
-        pass

mopidy/core/library.py

@@ -41,7 +41,7 @@ class LibraryController(object):
         query = query or kwargs
         futures = [
             b.library.find_exact(**query) for b in self.backends.with_library]
-        return pykka.get_all(futures)
+        return [result for result in pykka.get_all(futures) if result]
 
     def lookup(self, uri):
         """
@@ -101,4 +101,4 @@ class LibraryController(object):
         query = query or kwargs
         futures = [
             b.library.search(**query) for b in self.backends.with_library]
-        return pykka.get_all(futures)
+        return [result for result in pykka.get_all(futures) if result]