Fix division by zero in cubic.c NetStats bandwidth calculation when SmoothedRtt is 0

[Copilot]

Description

Bandwidth = CongestionWindow / SmoothedRtt in cubic.c crashes with STATUS_INTEGER_DIVIDE_BY_ZERO when an ACK is processed before the first RTT sample is available (SmoothedRtt == 0). This affects two code paths:

• CubicCongestionControlGetNetworkStatistics — queried bandwidth when RTT not yet sampled
• CubicCongestionControlOnDataAcknowledged — NetStatsEventEnabled path on early ACK

Both sites now return Bandwidth = 0 when SmoothedRtt == 0:

// Before
NetworkStatistics->Bandwidth = Cubic->CongestionWindow / Path->SmoothedRtt;

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

Testing

Two new unit tests were added to src/core/unittest/CubicTest.cpp to cover both fixed paths:

• CubicTest.GetNetworkStatistics_ZeroSmoothedRtt_BandwidthIsZero — calls GetNetworkStatistics with SmoothedRtt == 0 and asserts Bandwidth == 0 without crashing.
• CubicTest.OnDataAcknowledged_NetStats_ZeroSmoothedRtt_BandwidthIsZero — fires an ACK with NetStatsEventEnabled = TRUE and SmoothedRtt == 0 and asserts no divide-by-zero crash occurs.

All 52 CubicTest cases pass.

Documentation

No documentation impact.

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 issue. </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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[saikat107]

@copilot add test case(s) in the src/core/unittest/CubicTest.cpp to illustrate the change

[Copilot]

@copilot add test case(s) in the src/core/unittest/CubicTest.cpp to illustrate the change

Added two test cases in commit 2bb5c96:

• CubicTest.GetNetworkStatistics_ZeroSmoothedRtt_BandwidthIsZero — calls GetNetworkStatistics with SmoothedRtt == 0 and asserts Bandwidth == 0 (no crash).
• CubicTest.OnDataAcknowledged_NetStats_ZeroSmoothedRtt_BandwidthIsZero — fires an ACK with NetStatsEventEnabled = TRUE and SmoothedRtt == 0 and asserts no divide-by-zero crash occurs.

Both tests are placed alongside the existing GetNetworkStatistics_RetrieveStats test and all 52 CubicTest cases pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.78%. Comparing base (0071278) to head (4fe4893).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5888      +/-   ##
==========================================
- Coverage   86.23%   84.78%   -1.45%     
==========================================
  Files          60       60              
  Lines       18732    18732              
==========================================
- Hits        16153    15882     -271     
- Misses       2579     2850     +271     

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[guhetier]

I am not sure we can ever reach that point in the code without having at least one RTT sample (since we got an Ack, we likely updated the RTT right before), but since the path is not trivial, the validation won't hurt and maybe avoid an issue in a corner case.

Thanks for the PR