natsdotnet/tests/NATS.Server.Gateways.Tests/Gateways/GatewayReconnectionTests.cs

using Microsoft.Extensions.Logging.Abstractions;
using NATS.Server.Configuration;
using NATS.Server.Gateways;
using Shouldly;

namespace NATS.Server.Gateways.Tests.Gateways;

/// <summary>
/// Tests for GatewayReconnectPolicy and GatewayManager reconnection tracking.
/// Go reference: server/gateway.go reconnectGateway / solicitGateway (gateway.go:1700+).
/// </summary>
public class GatewayReconnectionTests
{
    // ── GatewayReconnectPolicy delay calculation ──────────────────────────

    // Go: server/gateway.go solicitGateway delay=0 on first attempt
    [Fact]
    public void CalculateDelay_first_attempt_is_initial_delay()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromSeconds(1),
            MaxDelay = TimeSpan.FromSeconds(30),
        };

        var delay = policy.CalculateDelay(0);

        delay.ShouldBe(TimeSpan.FromSeconds(1));
    }

    // Go: server/gateway.go reconnectGateway exponential back-off doubling
    [Fact]
    public void CalculateDelay_doubles_each_attempt()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromSeconds(1),
            MaxDelay = TimeSpan.FromSeconds(1000),
        };

        var delay0 = policy.CalculateDelay(0);
        var delay1 = policy.CalculateDelay(1);
        var delay2 = policy.CalculateDelay(2);
        var delay3 = policy.CalculateDelay(3);

        delay1.ShouldBe(delay0 * 2);
        delay2.ShouldBe(delay0 * 4);
        delay3.ShouldBe(delay0 * 8);
    }

    // Go: server/gateway.go reconnectGateway maxDelay cap
    [Fact]
    public void CalculateDelay_caps_at_max_delay()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromSeconds(1),
            MaxDelay = TimeSpan.FromSeconds(5),
        };

        // At attempt=10 (2^10 = 1024 seconds * 1s initial), should be capped
        var delay = policy.CalculateDelay(10);

        delay.ShouldBe(TimeSpan.FromSeconds(5));
    }

    // Go: server/gateway.go reconnectGateway jitter added to avoid thundering herd
    [Fact]
    public void CalculateDelayWithJitter_adds_jitter()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromSeconds(1),
            MaxDelay = TimeSpan.FromSeconds(30),
            JitterFactor = 0.2,
        };

        var baseDelay = policy.CalculateDelay(3);

        // Run several times to increase the chance of observing jitter
        var observed = false;
        for (var i = 0; i < 20; i++)
        {
            var jittered = policy.CalculateDelayWithJitter(3);
            jittered.ShouldBeGreaterThanOrEqualTo(baseDelay);
            jittered.ShouldBeLessThanOrEqualTo(TimeSpan.FromMilliseconds(
                baseDelay.TotalMilliseconds * (1 + policy.JitterFactor) + 1));

            if (jittered > baseDelay)
                observed = true;
        }

        observed.ShouldBeTrue("at least one jittered delay should exceed base delay");
    }

    // ── GatewayManager reconnect attempt tracking ──────────────────────────

    // Go: server/gateway.go initial state has no reconnect history
    [Fact]
    public void GetReconnectAttempts_starts_at_zero()
    {
        var manager = BuildManager();

        manager.GetReconnectAttempts("gw-east").ShouldBe(0);
        manager.GetReconnectAttempts("gw-west").ShouldBe(0);
    }

    // Go: server/gateway.go reconnectGateway increments attempt counter each cycle
    [Fact]
    public async Task ReconnectAttempts_incremented_on_reconnect()
    {
        var manager = BuildManager();
        using var cts = new CancellationTokenSource();
        cts.Cancel(); // Cancel immediately so Task.Delay throws before any real wait

        // Counter is incremented before the delay, so it reaches 1 even when cancelled.
        await Should.ThrowAsync<OperationCanceledException>(
            () => manager.ReconnectGatewayAsync("gw-east", cts.Token));

        manager.GetReconnectAttempts("gw-east").ShouldBe(1);
    }

    // Go: server/gateway.go solicitGateway resets counter after successful connect
    [Fact]
    public async Task ResetReconnectAttempts_clears_count()
    {
        var manager = BuildManager();

        // Seed the counter with one cancelled attempt
        await ReconnectAsync(manager, "gw-east");

        manager.GetReconnectAttempts("gw-east").ShouldBe(1);

        manager.ResetReconnectAttempts("gw-east");

        manager.GetReconnectAttempts("gw-east").ShouldBe(0);
    }

    // Go: server/gateway.go configurable initial delay via options
    [Fact]
    public void Custom_initial_delay_respected()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromMilliseconds(500),
            MaxDelay = TimeSpan.FromSeconds(30),
        };

        policy.CalculateDelay(0).ShouldBe(TimeSpan.FromMilliseconds(500));
        policy.CalculateDelay(1).ShouldBe(TimeSpan.FromMilliseconds(1000));
        policy.CalculateDelay(2).ShouldBe(TimeSpan.FromMilliseconds(2000));
    }

    // Go: server/gateway.go configurable max delay cap
    [Fact]
    public void Custom_max_delay_caps_correctly()
    {
        var policy = new GatewayReconnectPolicy
        {
            InitialDelay = TimeSpan.FromSeconds(2),
            MaxDelay = TimeSpan.FromSeconds(10),
        };

        // 2s * 2^10 = 2048s >> 10s cap
        policy.CalculateDelay(10).ShouldBe(TimeSpan.FromSeconds(10));
        // 2s * 2^2 = 8s < 10s cap
        policy.CalculateDelay(2).ShouldBe(TimeSpan.FromSeconds(8));
    }

    // Go: server/gateway.go independent reconnect state per remote gateway
    [Fact]
    public async Task Multiple_gateways_tracked_independently()
    {
        var manager = BuildManager();

        // Increment east twice, west once
        await ReconnectAsync(manager, "gw-east");
        await ReconnectAsync(manager, "gw-east");
        await ReconnectAsync(manager, "gw-west");

        manager.GetReconnectAttempts("gw-east").ShouldBe(2);
        manager.GetReconnectAttempts("gw-west").ShouldBe(1);

        // Reset east should not affect west
        manager.ResetReconnectAttempts("gw-east");
        manager.GetReconnectAttempts("gw-east").ShouldBe(0);
        manager.GetReconnectAttempts("gw-west").ShouldBe(1);
    }

    // ── Helpers ─────────────────────────────────────────────────────────────

    private static GatewayManager BuildManager() =>
        new GatewayManager(
            new GatewayOptions { Name = "TEST", Host = "127.0.0.1", Port = 0 },
            new ServerStats(),
            "S1",
            _ => { },
            _ => { },
            NullLogger<GatewayManager>.Instance);

    /// <summary>
    /// Triggers a single ReconnectGatewayAsync cycle with an immediately-cancelled token so
    /// the attempt counter is incremented without waiting for any real delay.
    /// The expected OperationCanceledException is asserted via Shouldly.
    /// </summary>
    private static async Task ReconnectAsync(GatewayManager manager, string gatewayName)
    {
        using var cts = new CancellationTokenSource();
        cts.Cancel();
        await Should.ThrowAsync<OperationCanceledException>(
            () => manager.ReconnectGatewayAsync(gatewayName, cts.Token));
    }
}