SRE-18741: improve shutdown handling

Don't release the lock before cancelling pending locks. There might or
might not have been a race condition there, but either way, it's best to
have signal handlers be minimal. We can release any existing locks
later.

We do still need to cancel the lock in the handler, in order to raise an
exception to break out of synchronous code.

Also, don't try self.zoo.close if self.zoo is None.

tds/scripts/daemon.py

@@ -211,7 +211,13 @@ def main(self):
         if self.end_callback:
             self.end_callback()
 
-        self.zoo.close()
+        if self.zoo is not None:
+            if self.lock is not None:
+                self.release_lock()
+
+            self.zoo.stop()
+            self.zoo.close()
+
         log.info("Stopped.")
 
     def release_lock(self):
@@ -255,8 +261,6 @@ def shutdown_handler(self, signum, frame):
 
         self._should_stop = True
         if self.lock is not None:
-            self.release_lock()
+            # Cancel any pending lock acquisition. This will cause kazoo to
+            # raise a CancelledError, which we catch.
             self.cancel_lock()
-
-        if self.zoo is not None:
-            self.zoo.stop()