Silently abort libspotify calls after global lock is freed

Fixes #202

docs/changelog.rst

@@ -2,10 +2,20 @@
 Changelog
 *********
 
+v2.1.2 (UNRELEASED)
+===================
+
+Maintenance release.
+
+- Silently abort libspotify `sp_*_release()` function calls that happen during
+  process shutdown, after pyspotify's global lock is freed. (Fixes:
+  :issue:`202`)
+
+
 v2.1.1 (2019-11-17)
 ===================
 
-Maintanance release.
+Maintenance release.
 
 - Add support for Python 3.8. No changes was required, but the test suite now
   runs on this version too.

spotify/__init__.py

@@ -56,6 +56,13 @@ def serialized(f):
 
     @functools.wraps(f)
     def wrapper(*args, **kwargs):
+        if _lock is None:
+            # During process teardown, objects wrapped with `ffi.gc()` might be
+            # freed and their libspotify release functions called after the lock
+            # has been freed. When this happens, `_lock` will be `None`.
+            # Since we're already shutting down the process, we just abort the
+            # call when the lock is gone.
+            return
         with _lock:
             return f(*args, **kwargs)