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Don't cancel the command after QueryUnbufferedAsync completes normally - #2245
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QueryUnbufferedAsync's finally block calls DbCommand.Cancel() whenever the reader is still open, and on the success path it always is. Npgsql sends that cancel over a new connection, so every completed query pays for an extra connection to cancel a command that has already finished. Close the reader and clear it after reading to the end, as the sync Query and QueryRow paths already do. Do the same on the early exit for statements with no columns; sync QueryImpl still cancels there. Cancel() now only runs when the caller stops early or an error is thrown. Fixes DapperLib#2188
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October 20, 2026 16:32
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Fixes #2188.
Problem
QueryUnbufferedAsynccallsDbCommand.Cancel()in itsfinallywhenever!reader.IsClosed. On the success path the reader is still open at that point, so every query that is read to the end gets cancelled. Npgsql sends that cancel over a new connection, so each completed query costs an extra connection, plus server work to cancel a command that has already finished. #2188 reports similar behaviour with MySqlConnector.Fix
Close the reader and set it to
nullbefore thefinallyruns. The read-to-end exit now mirrors the syncQueryImplandQueryRowImpl, comment included. The early exit for statements that return no columns gets the same treatment. SyncQueryImplstill cancels on that exit and could get the same fix in a follow-up.Cancel()still runs when the caller stops enumerating early or an exception is thrown, which is the case #2121 added it for.command.OnCompleted()now runs after the reader is closed, as it does on the sync path. Providers fill in output parameters once the reader closes, so they're now populated when that callback runs.#2207 tried a drained flag and was closed by its author without review. This PR mirrors the sync code instead and adds tests.
Tests
The tests use SQLite: it needs no server and still reproduces the bug. They count
Cancel()calls through a pass-throughDbConnection/DbCommandwrapper:QueryUnbufferedAsync_ReadToEnd_DoesNotCancelQueryUnbufferedAsync_NoColumns_DoesNotCancelQueryUnbufferedAsync_StopEarly_CancelsRelated: #2220 removes
Cancel()from the sync single-row cleanup.I agree to the terms in NonCLA.md.