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WININET_E_REQUEST_PENDING
WININET_E_REQUEST_PENDING means that the requested operation cannot start because one or more operations are already pending for the relevant WinINet handle. It is a concurrency and sequencing signal, not a server-side HTTP error.
Correct asynchronous pattern
When asynchronous mode is enabled, a function can return ERROR_IO_PENDING; completion is then reported through INTERNET_STATUS_REQUEST_COMPLETE. Do not issue a competing operation on the same handle until the prior operation has reached its completion callback and the result has been inspected.
Design options
- Serialize operations per handle through a small state machine or queue.
- Use separate request handles when the protocol and connection model allow it.
- Keep callbacks short and do not call into a reentrant operation path without a clear state transition.
Microsoft: setting up asynchronous WinINet operations · Microsoft: WinINet error messages
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