Mongo's existing RPC system (i.e., NetworkInterface) may use ASIO timers to enforce timeouts. The behavior, however, is racy as threads may set and cancel a timer without synchronization. Consider the following:

• We schedule a remote command through startCommand, but since there are no connections available, the invocation returns before starting any remote command.
• We decide to cancel the scheduled remote command through cancelCommand, which attempts to cancel the timer from here.
• At the same time, the connection pool provides a connection, so NetworkInterface starts sending the remote command and setting the timer as a prerequisite from here.
• Now, we have two threads that are concurrency reading and writing an ASIO-internal value without synchronization:
  • The thread canceling the remote command is reading this value.
  • The thread that is trying to send the remote command is writing this value.

A possible solution to address this issue is to synchronize setting/canceling timers at NetworkInterface layer.