HRESULT 0x801F0020 (ERROR_FLT_NO_WAITER_FOR

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a user-mode reply arrived after the kernel message waiter stopped waiting.

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ERROR_FLT_CONTEXT_ALREADY_LINKED	ERROR_FLT_REGISTRATION_BUSY

ERROR_FLT_NO_WAITER_FOR_REPLY

The reply no longer has a live kernel recipient

ERROR_FLT_NO_WAITER_FOR_REPLY occurs when user mode calls FilterReplyMessage but no kernel-mode FltSendMessage call is still waiting for that message identifier. The send can time out, the communication endpoint can close, or the minifilter can cancel its workflow before the user-mode service finishes processing.

Communication-port delivery is not buffered as a durable job queue, and a successful FilterGetMessage does not guarantee that a later reply will still be accepted. User mode should treat this result as a stale response, discard any reply-owned state, and avoid applying side effects that assume the original kernel request remains pending.

Correlating late replies

Log the Filter Manager message identifier, receive time, reply time, and configured kernel timeout.
Handle port disconnect and service shutdown as cancellation of outstanding work.
Do not retry the same reply because its identifier cannot acquire a new waiter.
If processing can exceed the timeout, redesign the protocol around acknowledgement and a separate durable operation.

References

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