Adding a comment to explain reason for mutex locking and map copying.

server/context.go

@@ -140,6 +140,9 @@ func (s *SessionManager) SetDB(conn *mysql.Conn, db string) error {
 
 // Iter iterates over the active sessions and executes the specified callback function on each one.
 func (s *SessionManager) Iter(f func(session sql.Session) (stop bool, err error)) error {
+	// Lock the mutex guarding the sessions map while we make a copy of it to prevent errors from
+	// mutating a map while iterating over it. Making a copy of the map also allows us to guard
+	// against long running callback functions being passed in that could cause long mutex blocking.
 	s.mu.Lock()
 	sessionsCopy := make(map[uint32]*managedSession)
 	for key, value := range s.sessions {