Fix CancelOnLossSend test race by arming loss helper before send

[guhetier]

Description

The Misc/WithCancelOnLossArgs.CancelOnLossSend/1 test (the DropPackets: true variant) was racy. It armed the loss helper after calling MsQuicStream::Send(QUIC_SEND_FLAG_CANCEL_ON_LOSS):

TEST_QUIC_SUCCEEDED(ClientContext.Stream->Send(&MessageBuffer, 1, QUIC_SEND_FLAG_CANCEL_ON_LOSS));
if (DropPackets) {
    LossHelper.DropPackets(1);   // armed too late
}

Send is asynchronous: it queues the data to an MsQuic worker thread. On a fast loopback datapath (this manifested in the UseXdp + UseQtip BVT configuration), the worker can transmit the stream data and the server can dispatch QUIC_STREAM_EVENT_RECEIVE before the test thread executes DropPackets(1). The server callback then sets ExitCode = SuccessExitCode (42) and signals SendPhaseEndedEvent; the main test thread wakes and asserts 42 != ErrorExitCode (24).

Trace from the failing run (quic.log lines, same TID 2784.2064):

Time	Event
22:28:27.381556	QUIC_STREAM_EVENT_RECEIVE [23 bytes] (server)
22:28:27.407382	[test][hook] Selective packet drop (26 ms too late)
22:28:27.468536	QUIC_STREAM_EVENT_PEER_SEND_ABORTED (0x18) (server)

The fix moves the DropPackets(1) call to before Send, so the very next packet hitting the server receive path (the stream data) is the one dropped. This causes the client to detect loss and fire QUIC_STREAM_EVENT_CANCEL_ON_LOSS as intended, which in turn delivers PEER_SEND_ABORTED to the server with ErrorExitCode.

Fixes #6065

Testing

Existing test Misc/WithCancelOnLossArgs.CancelOnLossSend/* covers this change. Ran the parameterized test 20 times locally with the Debug + schannel build — 20/20 passed (previously intermittent in CI).

Documentation

No documentation impact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.62%. Comparing base (a40688d) to head (f51ca8d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6079      +/-   ##
==========================================
+ Coverage   85.32%   85.62%   +0.29%     
==========================================
  Files          60       60              
  Lines       18798    18841      +43     
==========================================
+ Hits        16040    16132      +92     
+ Misses       2758     2709      -49     

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Pull request overview

This PR fixes a race in the Misc/WithCancelOnLossArgs.CancelOnLossSend/1 test by ensuring the selective packet-drop hook is armed before calling MsQuicStream::Send(..., QUIC_SEND_FLAG_CANCEL_ON_LOSS), so the stream data packet is reliably the one dropped on fast loopback datapaths.

Changes:

• Move LossHelper.DropPackets(1) to occur before Send(...) in the DropPackets: true variant to eliminate a timing window.
• Expand the in-test comment explaining why arming after Send is racy.