Fix division by zero in CubicCongestionControlGetNetworkStatistics

[Copilot]

SmoothedRtt is zero-initialized and only set after the first RTT sample, so calling GetNetworkStatistics early in connection lifetime (e.g., QUIC_PARAM_CONN_STATISTICS_V2) triggers STATUS_INTEGER_DIVIDE_BY_ZERO on Windows.

Description

• src/core/cubic.c: Guard the bandwidth calculation against zero SmoothedRtt:

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

Testing

New unit test CubicTest.GetNetworkStatistics_ZeroRtt in src/core/unittest/CubicTest.cpp calls GetNetworkStatistics before any RTT sample is received (SmoothedRtt == 0) and asserts Bandwidth == 0.

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 CubicCongestionControlGetNetworkStatistics</issue_title>
<issue_description># Location: src/core/cubic.c, line 433
NetworkStatistics->Bandwidth = Cubic->CongestionWindow / Path->SmoothedRtt;

Root cause: GetNetworkStatistics divides CongestionWindow by SmoothedRtt without checking whether SmoothedRtt is zero. SmoothedRtt is initialized to 0 (via CxPlatZeroMemory in connection init) and only set to a non-zero value after the first RTT sample is received.

Impact: Any call to GetNetworkStatistics before the first RTT sample (e.g., via MsQuicGetParam with QUIC_PARAM_CONN_STATISTICS_V2 early in connection lifetime) causes a STATUS_INTEGER_DIVIDE_BY_ZERO structured exception, crashing the process.

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 case to src/core/unittest/CubicTest.cpp

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

    InitializeMockConnection(Connection, 1280);
    // SmoothedRtt is 0 by default (no RTT sample received yet)
    // GotFirstRttSample is FALSE by default

    CubicCongestionControlInitialize(&Connection.CongestionControl, &Settings);

    QUIC_NETWORK_STATISTICS NetworkStats;
    CxPlatZeroMemory(&NetworkStats, sizeof(NetworkStats));

    // This will crash with STATUS_INTEGER_DIVIDE_BY_ZERO
    // because SmoothedRtt=0 and line 433 does: CongestionWindow / SmoothedRtt
    Connection.CongestionControl.QuicCongestionControlGetNetworkStatistics(
        &Connection,
        &Connection.CongestionControl,
        &NetworkStats);

    // If the bug were fixed, we'd expect:
    ASSERT_EQ(NetworkStats.Bandwidth, 0u); // Graceful when no RTT sample
}

2. Build and run the test case with filter CubicTest.DeepTest_Bug_GetNetworkStatsDivByZero

Expected behavior

Should return Bandwidth=0 when SmoothedRtt=0 (graceful degradation)

Actual outcome

STATUS_INTEGER_DIVIDE_BY_ZERO crash

Additional details

No response</issue_description>

<agent_instructions>Fix and add test</agent_instructions>

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

• Fixes [DeepTest] Division by zero in CubicCongestionControlGetNetworkStatistics #5833

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