feat: add GatewayPairFixture for failover tests

tests/NATS.E2E.Cluster.Tests/Infrastructure/GatewayPairFixture.cs

@@ -0,0 +1,167 @@
+using System.Net.Http.Json;
+using System.Text.Json.Serialization;
+using NATS.Client.Core;
+
+namespace NATS.E2E.Cluster.Tests.Infrastructure;
+
+public sealed class GatewayPairFixture : IAsyncLifetime
+{
+    private readonly NatsServerProcess?[] _servers = new NatsServerProcess?[2];
+    private readonly int[] _clientPorts = new int[2];
+    private readonly int[] _gatewayPorts = new int[2];
+    private readonly int[] _monitorPorts = new int[2];
+    private readonly string[] _configs = new string[2];
+
+    public async Task InitializeAsync()
+    {
+        // Pre-allocate all ports upfront so they can be reused across kill/restart
+        for (var i = 0; i < 2; i++)
+        {
+            _clientPorts[i] = NatsServerProcess.AllocateFreePort();
+            _gatewayPorts[i] = NatsServerProcess.AllocateFreePort();
+            _monitorPorts[i] = NatsServerProcess.AllocateFreePort();
+        }
+
+        _configs[0] = $$"""
+            server_name: gw-a
+            gateway {
+                name: cluster-a
+                listen: 127.0.0.1:{{_gatewayPorts[0]}}
+                gateways: [
+                    { name: cluster-b, url: nats://127.0.0.1:{{_gatewayPorts[1]}} }
+                ]
+            }
+            """;
+
+        _configs[1] = $$"""
+            server_name: gw-b
+            gateway {
+                name: cluster-b
+                listen: 127.0.0.1:{{_gatewayPorts[1]}}
+                gateways: [
+                    { name: cluster-a, url: nats://127.0.0.1:{{_gatewayPorts[0]}} }
+                ]
+            }
+            """;
+
+        for (var i = 0; i < 2; i++)
+        {
+            _servers[i] = new NatsServerProcess(
+                _clientPorts[i],
+                configContent: _configs[i],
+                enableMonitoring: true,
+                monitorPort: _monitorPorts[i]);
+        }
+
+        await Task.WhenAll(_servers.Select(s => s!.StartAsync()));
+
+        await WaitForGatewayConnectionAsync();
+    }
+
+    /// <summary>
+    /// Kills the server at the given index (0=A, 1=B) by disposing it.
+    /// </summary>
+    public async Task KillNode(int index)
+    {
+        if (_servers[index] is { } server)
+        {
+            await server.DisposeAsync();
+            _servers[index] = null;
+        }
+    }
+
+    /// <summary>
+    /// Restarts the server at the given index on its originally allocated ports.
+    /// </summary>
+    public async Task RestartNode(int index)
+    {
+        // Dispose any existing process first (idempotent)
+        if (_servers[index] is not null)
+            await KillNode(index);
+
+        _servers[index] = new NatsServerProcess(
+            _clientPorts[index],
+            configContent: _configs[index],
+            enableMonitoring: true,
+            monitorPort: _monitorPorts[index]);
+
+        await _servers[index]!.StartAsync();
+    }
+
+    /// <summary>
+    /// Polls /gatewayz on alive nodes until NumGateways >= 1 for each.
+    /// Times out after 30 seconds.
+    /// </summary>
+    public async Task WaitForGatewayConnectionAsync()
+    {
+        using var http = new HttpClient();
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(200));
+
+        while (await timer.WaitForNextTickAsync(timeout.Token).ConfigureAwait(false))
+        {
+            var allConnected = true;
+
+            for (var i = 0; i < 2; i++)
+            {
+                // Skip killed nodes
+                if (_servers[i] is null)
+                    continue;
+
+                try
+                {
+                    var gatewayz = await http.GetFromJsonAsync<Gatewayz>(
+                        $"http://127.0.0.1:{_monitorPorts[i]}/gatewayz",
+                        timeout.Token);
+
+                    if (gatewayz?.NumGateways < 1)
+                    {
+                        allConnected = false;
+                        break;
+                    }
+                }
+                catch (HttpRequestException)
+                {
+                    // Monitor not yet ready — retry on next tick
+                    allConnected = false;
+                    break;
+                }
+            }
+
+            if (allConnected)
+                return;
+        }
+
+        throw new TimeoutException("Gateways did not connect to each other within 30s.");
+    }
+
+    /// <summary>
+    /// Creates a NatsConnection pointed at server A (index 0).
+    /// </summary>
+    public NatsConnection CreateClientA()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{_clientPorts[0]}" });
+
+    /// <summary>
+    /// Creates a NatsConnection pointed at server B (index 1).
+    /// </summary>
+    public NatsConnection CreateClientB()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{_clientPorts[1]}" });
+
+    public async Task DisposeAsync()
+    {
+        var disposeTasks = _servers
+            .Where(s => s is not null)
+            .Select(s => s!.DisposeAsync().AsTask());
+
+        await Task.WhenAll(disposeTasks);
+    }
+
+    private sealed class Gatewayz
+    {
+        [JsonPropertyName("num_gateways")]
+        public int NumGateways { get; init; }
+    }
+}
+
+[CollectionDefinition("E2E-GatewayPair")]
+public class GatewayPairCollection : ICollectionFixture<GatewayPairFixture>;