natsdotnet/tests/NATS.E2E.Cluster.Tests/GatewayFailoverTests.cs

using NATS.Client.Core;
using NATS.E2E.Cluster.Tests.Infrastructure;

namespace NATS.E2E.Cluster.Tests;

public class GatewayFailoverTests(GatewayPairFixture fixture) : IClassFixture<GatewayPairFixture>
{
    /// <summary>
    /// Kills gateway B, restarts it, waits for the gateway connection to re-establish,
    /// then verifies a message published on A is delivered to a subscriber on B.
    /// Go ref: TestGatewayReconnectAfterKill (server/gateway_test.go)
    /// </summary>
    [Fact]
    public async Task Gateway_Disconnect_Reconnects()
    {
        await fixture.KillNode(1);
        await fixture.RestartNode(1);
        await fixture.WaitForGatewayConnectionAsync();

        await using var clientA = fixture.CreateClientA();
        await using var clientB = fixture.CreateClientB();

        await clientA.ConnectAsync();
        await clientB.ConnectAsync();

        const string subject = "e2e.gw.reconnect";

        await using var sub = await clientB.SubscribeCoreAsync<string>(subject);

        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));

        // Probe-publish until B receives it, confirming interest has propagated across the gateway
        await WaitForPropagationAsync(clientA, sub, subject, "after-reconnect", cts.Token);
    }

    /// <summary>
    /// Verifies that after killing and restarting gateway B, a fresh subscription on B
    /// receives updated interest from A, and a published message is correctly delivered.
    /// Go ref: TestGatewayInterestAfterReconnect (server/gateway_test.go)
    /// </summary>
    [Fact]
    public async Task Gateway_InterestUpdated_AfterReconnect()
    {
        const string subject = "e2e.gw.interest";

        // --- Phase 1: baseline delivery before kill ---
        await using var clientA1 = fixture.CreateClientA();
        await using var clientB1 = fixture.CreateClientB();

        await clientA1.ConnectAsync();
        await clientB1.ConnectAsync();

        await using var sub1 = await clientB1.SubscribeCoreAsync<string>(subject);

        using var cts1 = new CancellationTokenSource(TimeSpan.FromSeconds(30));
        await WaitForPropagationAsync(clientA1, sub1, subject, "before-kill", cts1.Token);

        // --- Phase 2: kill B, restart, re-subscribe, verify delivery ---
        await fixture.KillNode(1);
        await fixture.RestartNode(1);
        await fixture.WaitForGatewayConnectionAsync();

        await using var clientA2 = fixture.CreateClientA();
        await using var clientB2 = fixture.CreateClientB();

        await clientA2.ConnectAsync();
        await clientB2.ConnectAsync();

        await using var sub2 = await clientB2.SubscribeCoreAsync<string>(subject);

        using var cts2 = new CancellationTokenSource(TimeSpan.FromSeconds(30));
        await WaitForPropagationAsync(clientA2, sub2, subject, "after-restart", cts2.Token);
    }

    // ---------------------------------------------------------------------------
    // Helpers
    // ---------------------------------------------------------------------------

    /// <summary>
    /// Publishes <paramref name="payload"/> from <paramref name="publisher"/> in a retry loop
    /// until <paramref name="sub"/> receives it, confirming gateway interest has propagated.
    /// PingAsync flushes server dispatch so TryRead can check the channel without blocking,
    /// eliminating the need for a try/catch around a bounded ReadAsync.
    /// </summary>
    private static async Task WaitForPropagationAsync(
        NatsConnection publisher,
        INatsSub<string> sub,
        string subject,
        string payload,
        CancellationToken ct)
    {
        await publisher.ConnectAsync();

        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(200));
        while (await timer.WaitForNextTickAsync(ct).ConfigureAwait(false))
        {
            await publisher.PublishAsync(subject, payload, cancellationToken: ct);

            // PingAsync is a round-trip to the server: when it returns, any message the
            // server dispatched before the ping is already buffered in sub.Msgs.
            await publisher.PingAsync(ct);

            if (sub.Msgs.TryRead(out var msg))
            {
                msg.Data.ShouldBe(payload);
                return;
            }
        }
    }
}