Parallelize db shutdown a bit more

src/calibre/db/backend.py

@@ -980,6 +980,10 @@ class DB:
     def shutdown_fts(self):
         if self.fts_enabled:
             self.fts.shutdown()
+
+    def join_fts(self):
+        if self.fts:
+            self.fts.pool.join()
             self.fts = None
 
     def get_connection(self):

src/calibre/db/cache.py

@@ -2387,6 +2387,7 @@ class Cache:
             self.close_called = True
             self.shutting_down = True
             self.event_dispatcher.close()
+            self.backend.shutdown_fts()
             if self.fts_queue_thread is not None:
                 self.fts_job_queue.put(None)
             from calibre.customize.ui import available_library_closed_plugins
@@ -2398,10 +2399,10 @@ class Cache:
                     traceback.print_exc()
         # the fts supervisor thread could be in the middle of committing a
         # result to the db, so holding a lock here will cause a deadlock
-        self.backend.shutdown_fts()
         if self.fts_queue_thread is not None:
             self.fts_queue_thread.join()
             self.fts_queue_thread = None
+        self.backend.join_fts()
         with self.write_lock:
             self.backend.close()
 

src/calibre/db/fts/pool.py

@@ -210,11 +210,13 @@ class Pool:
                 w.keep_going = False
                 for i in range(2*len(self.workers)):
                     self.jobs_queue.put(quit)
-            self.supervisor_thread.join()
-            for w in self.workers:
-                w.join()
-            self.workers = []
             self.initialized.clear()
+
+    def join(self):
+        self.supervisor_thread.join()
+        for w in self.workers:
+            w.join()
+        self.workers = []
     # }}}
 
     def do_check_for_work(self):