[WIP] [DeepTest] Division by zero in OnDataAcknowledged NetStats event path

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[DeepTest] Division by zero in OnDataAcknowledged NetStats event path</issue_title>
<issue_description>### Describe the bug
Location: src/core/cubic.c, line 701
Event.NETWORK_STATISTICS.Bandwidth = Cubic->CongestionWindow / Path->SmoothedRtt;

Root cause: Same division-by-zero pattern as #5833, but in the OnDataAcknowledged function's NetStatsEventEnabled path. If NetStatsEventEnabled is TRUE and an ACK is processed before SmoothedRtt is set, the same crash occurs.

Affected OS

• Windows
• Linux
• macOS
• Other (specify below)

Additional OS information

No response

MsQuic version

main

Steps taken to reproduce bug

1. Add the following test in src/core/unittest/CubicTest.cpp

TEST(CubicTest, DeepTest_Bug_NetStatsEventDivByZero)
{
    QUIC_CONNECTION Connection;
    QUIC_SETTINGS_INTERNAL Settings{};
    Settings.InitialWindowPackets = 10;
    Settings.SendIdleTimeoutMs = 1000;

    InitializeMockConnection(Connection, 1280);
    Connection.Settings.NetStatsEventEnabled = TRUE;
    Connection.ClientCallbackHandler = DummyConnectionCallback;
    // SmoothedRtt is 0 by default

    CubicCongestionControlInitialize(&Connection.CongestionControl, &Settings);

    QUIC_CONGESTION_CONTROL_CUBIC* Cubic = &Connection.CongestionControl.Cubic;
    Cubic->BytesInFlight = 5000;

    QUIC_ACK_EVENT AckEvent;
    CxPlatZeroMemory(&AckEvent, sizeof(AckEvent));
    AckEvent.TimeNow = 1000000;
    AckEvent.LargestAck = 5;
    AckEvent.LargestSentPacketNumber = 10;
    AckEvent.NumRetransmittableBytes = 1000;
    AckEvent.NumTotalAckedRetransmittableBytes = 1000;
    AckEvent.SmoothedRtt = 0; // No RTT sample
    AckEvent.MinRtt = 0;
    AckEvent.MinRttValid = FALSE;
    AckEvent.AckedPackets = NULL;

    // This will crash with STATUS_INTEGER_DIVIDE_BY_ZERO
    // because Path->SmoothedRtt=0 and line 701 does: CongestionWindow / SmoothedRtt
    Connection.CongestionControl.QuicCongestionControlOnDataAcknowledged(
        &Connection.CongestionControl, &AckEvent);
}

2. Build and run the test with filter CubicTest.DeepTest_Bug_NetStatsEventDivByZero

Expected behavior

Should not crash.

Actual outcome

STATUS_INTEGER_DIVIDE_BY_ZERO crash

Additional details

No response</issue_description>

<agent_instructions>Fix this bug and add the test case in appropriate file</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [DeepTest] Division by zero in OnDataAcknowledged NetStats event path #5836

---

📱 Kick off Copilot coding agent tasks wherever you are with GitHub Mobile, available on iOS and Android.