Use recursive locks for the cover caches

Performance hit of a few nanoseconds per lock operation, however
eliminates one source of deadlocks.

src/calibre/db/utils.py

@@ -13,7 +13,7 @@ import tempfile
 from collections import OrderedDict, namedtuple
 from contextlib import suppress
 from locale import localeconv
-from threading import Lock
+from threading import RLock
 
 from calibre import as_unicode, prints
 from calibre.constants import cache_dir, get_windows_number_formats, iswindows, preferred_encoding
@@ -136,7 +136,7 @@ class ThumbnailCache:
         self.group_id = 'group'
         self.thumbnail_size = thumbnail_size
         self.size_changed = False
-        self.lock = Lock()
+        self.lock = RLock()
         self.min_disk_cache = min_disk_cache
         if test_mode:
             self.log = self.fail_on_error

src/calibre/gui2/library/caches.py

@@ -12,7 +12,7 @@ from collections import OrderedDict
 from collections.abc import Iterable, Iterator, MutableMapping
 from functools import partial
 from queue import Empty, LifoQueue, Queue, ShutDown
-from threading import Event, Lock, Thread, current_thread
+from threading import Event, RLock, Thread, current_thread
 from time import monotonic
 from typing import TypeVar
 
@@ -46,7 +46,7 @@ class RAMCache(MutableMapping[int, T]):
 
     def __init__(self, limit=100):
         self.items = OrderedDict[int, T]()
-        self.lock = Lock()
+        self.lock = RLock()
         self.limit = limit
         self.pixmap_staging: list[T] = []
         self.gui_thread = current_thread()