Work around a RocksDB shutdown bug

zebra-state/src/service/finalized_state/disk_db.rs

@@ -684,6 +684,12 @@ impl DiskDb {
         self.db.flush().expect("flush is successful");
 
         // But we should call `cancel_all_background_work` before Zebra exits.
+        //
+        // In some tests, we need to drop() the state service before the test function returns,
+        // and sleep() until all the other threads exit.
+        // (This seems to be a bug in RocksDB: cancel_all_background_work() should wait until
+        // all the threads have cleaned up.)
+        //
         // If we don't, we see these kinds of errors:
         // ```
         // pthread lock: Invalid argument

zebra-state/src/service/tests.rs

@@ -465,10 +465,14 @@ proptest! {
                 expected_transparent_pool
             );
         }
+
+        // Work around a RocksDB shutdown bug, see DiskDb::shutdown() for details.
+        std::mem::drop(state_service);
+        std::thread::sleep(Duration::from_secs(1));
     }
 }
 
-// This test sleeps, so we only ever want to run it once
+// This test sleeps for every block, so we only ever want to run it once
 proptest! {
     #![proptest_config(
         proptest::test_runner::Config::with_cases(1)
@@ -539,6 +543,10 @@ proptest! {
             prop_assert_eq!(latest_chain_tip.best_tip_height(), Some(expected_block.height));
             prop_assert_eq!(chain_tip_change.last_tip_change(), Some(expected_action));
         }
+
+        // Work around a RocksDB shutdown bug, see DiskDb::shutdown() for details.
+        std::mem::drop(state_service);
+        std::thread::sleep(Duration::from_secs(1));
     }
 }