Use a context manager for vls_cache_lock

src/calibre/db/cache.py

@@ -29,7 +29,9 @@ from calibre.db.categories import get_categories
 from calibre.db.errors import NoSuchBook, NoSuchFormat
 from calibre.db.fields import IDENTITY, InvalidLinkTable, create_field
 from calibre.db.lazy import FormatMetadata, FormatsList, ProxyMetadata
-from calibre.db.locking import LockingError, DowngradeLockError, SafeReadLock, create_locks
+from calibre.db.locking import (
+    DowngradeLockError, LockingError, SafeReadLock, create_locks, try_lock
+)
 from calibre.db.search import Search
 from calibre.db.tables import VirtualTable
 from calibre.db.utils import type_safe_sort_key_function
@@ -2232,12 +2234,10 @@ class Cache(object):
         if self.vls_for_books_cache is None:
             # Using a list is slightly faster than a set.
             c = defaultdict(list)
-            got_lock = False
+            # use a primitive lock to ensure that only one thread is updating
-            try:
+            # the cache and that recursive calls don't do the update. This
-                # use a primitive lock to ensure that only one thread is updating
+            # method can recurse via self._search()
-                # the cache and that recursive calls don't do the update. This
+            with try_lock(self.vls_cache_lock) as got_lock:
-                # method can recurse via self._search()
-                got_lock = self.vls_cache_lock.acquire(blocking=False)
                 if not got_lock:
                     # We get here if resolving the books in a VL triggers another VL
                     # calculation. This can be 'real' recursion, in which case the
@@ -2258,9 +2258,6 @@ class Cache(object):
                         if book:
                             c[book].append(_('[Error in Virtual library {0}: {1}]').format(lib, str(e)))
                 self.vls_for_books_cache = {b:tuple(sorted(libs, key=sort_key)) for b, libs in c.items()}
-            finally:
-                if got_lock:
-                    self.vls_cache_lock.release()
         if not book_ids:
             book_ids = self._all_book_ids()
         # book_ids is usually 1 long. The loop will be faster than a comprehension

src/calibre/db/locking.py

@@ -8,9 +8,20 @@ __docformat__ = 'restructuredtext en'
 
 import traceback, sys
 from threading import Lock, Condition, current_thread
+from contextlib import contextmanager
 from calibre.utils.config_base import tweaks
 
 
+@contextmanager
+def try_lock(lock):
+    got_lock = lock.acquire(blocking=False)
+    try:
+        yield got_lock
+    finally:
+        if got_lock:
+            lock.release()
+
+
 class LockingError(RuntimeError):
 
     is_locking_error = True