Minor cleanup of InnoDB shutdown

os_thread_active(): Remove.

srv_shutdown_all_bg_threads(): Assert that high-level threads
have already exited. Do not sleep if os_thread_count=0.

storage/innobase/include/os0thread.h

@@ -151,10 +151,4 @@ os_thread_sleep(
 /*============*/
 	ulint	tm);	/*!< in: time in microseconds */
 
-/*****************************************************************//**
-Check if there are threads active.
-@return true if the thread count > 0. */
-bool
-os_thread_active();
-
 #endif

storage/innobase/os/os0thread.cc

@@ -237,21 +237,3 @@ os_thread_sleep(
 	select(0, NULL, NULL, NULL, &t);
 #endif /* _WIN32 */
 }
-
-/*****************************************************************//**
-Check if there are threads active.
-@return true if the thread count > 0. */
-bool
-os_thread_active()
-/*==============*/
-{
-	/* All the threads have exited or are just exiting;
-	NOTE that the threads may not have completed their
-	exit yet. Should we use pthread_join() to make sure
-	they have exited? If we did, we would have to
-	remove the pthread_detach() from
-	os_thread_exit().  Now we just sleep 0.1
-	seconds and hope that is enough! */
-
-	return(my_atomic_loadlint(&os_thread_count) > 0);
-}

storage/innobase/srv/srv0start.cc

@@ -1234,6 +1234,7 @@ static
 void
 srv_shutdown_all_bg_threads()
 {
+	ut_ad(!srv_undo_sources);
 	srv_shutdown_state = SRV_SHUTDOWN_EXIT_THREADS;
 
 	/* All threads end up waiting for certain events. Put those events
@@ -1285,13 +1286,7 @@ srv_shutdown_all_bg_threads()
 
 			os_event_set(buf_flush_event);
 
-			/* f. dict_stats_thread is signaled from
-			logs_empty_and_mark_files_at_shutdown() and
-			should have already quit or is quitting right
-			now. */
-
 			if (srv_use_mtflush) {
-				/* g. Exit the multi threaded flush threads */
 				buf_mtflu_io_thread_exit();
 			}
 		}
@@ -1300,13 +1295,11 @@ srv_shutdown_all_bg_threads()
 			os_aio_wake_all_threads_at_shutdown();
 		}
 
-		const bool active = os_thread_active();
-
-		os_thread_sleep(100000);
-
-		if (!active) {
+		if (!os_thread_count) {
 			return;
 		}
+
+		os_thread_sleep(100000);
 	}
 
 	ib::warn() << os_thread_count << " threads created by InnoDB"