mgr/DaemonState: Minimise time we hold the DaemonStateIndex lock

Calling back into python functions whilst holding the lock can result in
this thread being queued for the GIL and resulting in extended delays
for threads waiting to acquire the lock.

Fixes: https://tracker.ceph.com/issues/72337
Signed-off-by: Brad Hubbard <[email protected]>

Author: badone

src/mgr/DaemonState.h

@@ -261,9 +261,12 @@ class DaemonStateIndex
   template<typename Callback, typename...Args>
   auto with_daemons_by_server(Callback&& cb, Args&&... args) const ->
     decltype(cb(by_server, std::forward<Args>(args)...)) {
-    std::shared_lock l{lock};
-    
-    return std::forward<Callback>(cb)(by_server, std::forward<Args>(args)...);
+    const decltype(by_server) by_server_copy = [&] {
+      // Don't hold the lock any longer than necessary
+      std::shared_lock l{lock};
+      return by_server;
+    }();
+    return std::forward<Callback>(cb)(by_server_copy, std::forward<Args>(args)...);
   }
 
   template<typename Callback, typename...Args>