Fix division by zero in CUBIC NetStats Bandwidth computation when SmoothedRtt is 0

[Copilot]

Description

CongestionWindow / SmoothedRtt in two CUBIC paths crashes with STATUS_INTEGER_DIVIDE_BY_ZERO when an ACK or stats query arrives before the first RTT sample is recorded (SmoothedRtt == 0).

• cubic.c — CubicCongestionControlOnDataAcknowledged: Guard the Bandwidth assignment in the NetStatsEventEnabled path against zero SmoothedRtt
• cubic.c — CubicCongestionControlGetNetworkStatistics: Same guard applied to the identical pattern

// Before (both sites)
NetworkStatistics->Bandwidth = Cubic->CongestionWindow / Path->SmoothedRtt;

// After
NetworkStatistics->Bandwidth = Path->SmoothedRtt == 0 ? 0 : Cubic->CongestionWindow / Path->SmoothedRtt;

Testing

New test CubicTest.OnDataAcknowledged_NetStatsEventDivByZero exercises the crash path: initializes a connection with NetStatsEventEnabled = TRUE and SmoothedRtt = 0, then fires an ACK event — previously causing a divide-by-zero, now returning cleanly.

Documentation

No documentation impact.

Original prompt

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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 and add test case</agent_instructions>

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

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

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