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Fix CancelOnLossSend test race by arming loss helper before send #6079

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Fix CancelOnLossSend test race by arming loss helper before send #6079

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Fix CancelOnLossSend test race by arming loss helper before send
guhetier Jun 9, 2026
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guhetier Jun 9, 2026
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guhetier Jun 11, 2026
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14 changes: 11 additions & 3 deletions src/test/lib/DataTest.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1724,12 +1724,20 @@ QuicCancelOnLossSend(
&ClientContext);
TEST_TRUE(ClientContext.Stream->IsValid());
TEST_QUIC_SUCCEEDED(ClientContext.Stream->Start());
TEST_QUIC_SUCCEEDED(ClientContext.Stream->Send(&MessageBuffer, 1, QUIC_SEND_FLAG_CANCEL_ON_LOSS));

// If requested, drop packets.
// Arm the loss helper to drop one packet from the send operation.
// There is a small chance the wrong packet is dropped if a timer triggers a send flush just at the wrong time.
//
// Arm the loss helper BEFORE sending so the stream data packet itself is
// the one that gets dropped. Arming after Send is racy: on a fast loopback
// datapath, the worker thread can transmit the data and the server can
// dispatch QUIC_STREAM_EVENT_RECEIVE before the test thread arms the drop,
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// causing the test to see SuccessExitCode instead of the expected
// ErrorExitCode triggered by QUIC_STREAM_EVENT_CANCEL_ON_LOSS.
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//
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if (DropPackets) {
LossHelper.DropPackets(1);
}
TEST_QUIC_SUCCEEDED(ClientContext.Stream->Send(&MessageBuffer, 1, QUIC_SEND_FLAG_CANCEL_ON_LOSS));

// Wait for the send phase to conclude.
if (!ClientContext.SendPhaseEndedEvent.WaitTimeout(EventWaitTimeoutMs)) {
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