Handle MOVED error pointing to same endpoint.

src/StackExchange.Redis/ResultProcessor.cs

@@ -259,43 +259,49 @@ public virtual bool SetResult(PhysicalConnection connection, Message message, in
                     if (Format.TryParseInt32(parts[1], out int hashSlot)
                         && Format.TryParseEndPoint(parts[2], out var endpoint))
                     {
-                        // no point sending back to same server, and no point sending to a dead server
-                        if (!Equals(server?.EndPoint, endpoint))
+                        // Check if MOVED points to same endpoint
+                        bool isSameEndpoint = Equals(server?.EndPoint, endpoint);
+                        if (isSameEndpoint && isMoved)
                         {
-                            if (bridge is null)
-                            {
-                                // already toast
-                            }
-                            else if (bridge.Multiplexer.TryResend(hashSlot, message, endpoint, isMoved))
+                            // MOVED to same endpoint detected.
+                            // This occurs when Redis/Valkey servers are behind DNS records, load balancers, or proxies.
+                            // The MOVED error signals that the client should reconnect to allow the DNS/proxy/load balancer
+                            // to route the connection to a different underlying server host, then retry the command.
+                            bridge?.TryConnect(null)?.Dispose();
+                        }
+                        if (bridge is null)
+                        {
+                            // already toast
+                        }
+                        else if (bridge.Multiplexer.TryResend(hashSlot, message, endpoint, isMoved))
+                        {
+                            bridge.Multiplexer.Trace(message.Command + " re-issued to " + endpoint, isMoved ? "MOVED" : "ASK");
+                            return false;
+                        }
+                        else
+                        {
+                            if (isMoved && wasNoRedirect)
                             {
-                                bridge.Multiplexer.Trace(message.Command + " re-issued to " + endpoint, isMoved ? "MOVED" : "ASK");
-                                return false;
+                                if (bridge.Multiplexer.RawConfig.IncludeDetailInExceptions)
+                                {
+                                    err = $"Key has MOVED to Endpoint {endpoint} and hashslot {hashSlot} but CommandFlags.NoRedirect was specified - redirect not followed for {message.CommandAndKey}. ";
+                                }
+                                else
+                                {
+                                    err = "Key has MOVED but CommandFlags.NoRedirect was specified - redirect not followed. ";
+                                }
                             }
                             else
                             {
-                                if (isMoved && wasNoRedirect)
+                                unableToConnectError = true;
+                                if (bridge.Multiplexer.RawConfig.IncludeDetailInExceptions)
                                 {
-                                    if (bridge.Multiplexer.RawConfig.IncludeDetailInExceptions)
-                                    {
-                                        err = $"Key has MOVED to Endpoint {endpoint} and hashslot {hashSlot} but CommandFlags.NoRedirect was specified - redirect not followed for {message.CommandAndKey}. ";
-                                    }
-                                    else
-                                    {
-                                        err = "Key has MOVED but CommandFlags.NoRedirect was specified - redirect not followed. ";
-                                    }
+                                    err = $"Endpoint {endpoint} serving hashslot {hashSlot} is not reachable at this point of time. Please check connectTimeout value. If it is low, try increasing it to give the ConnectionMultiplexer a chance to recover from the network disconnect. "
+                                        + PerfCounterHelper.GetThreadPoolAndCPUSummary();
                                 }
                                 else
                                 {
-                                    unableToConnectError = true;
-                                    if (bridge.Multiplexer.RawConfig.IncludeDetailInExceptions)
-                                    {
-                                        err = $"Endpoint {endpoint} serving hashslot {hashSlot} is not reachable at this point of time. Please check connectTimeout value. If it is low, try increasing it to give the ConnectionMultiplexer a chance to recover from the network disconnect. "
-                                            + PerfCounterHelper.GetThreadPoolAndCPUSummary();
-                                    }
-                                    else
-                                    {
-                                        err = "Endpoint is not reachable at this point of time. Please check connectTimeout value. If it is low, try increasing it to give the ConnectionMultiplexer a chance to recover from the network disconnect. ";
-                                    }
+                                    err = "Endpoint is not reachable at this point of time. Please check connectTimeout value. If it is low, try increasing it to give the ConnectionMultiplexer a chance to recover from the network disconnect. ";
                                 }
                             }
                         }

tests/StackExchange.Redis.Tests/MovedTestServer.cs

@@ -0,0 +1,245 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Net;
+using System.Threading;
+using System.Threading.Tasks;
+using StackExchange.Redis.Server;
+
+namespace StackExchange.Redis.Tests;
+
+/// <summary>
+/// Test Redis/Valkey server that simulates MOVED errors pointing to the same endpoint.
+/// Used to verify client reconnection behavior when the server is behind DNS/load balancers/proxies.
+/// When a MOVED error points to the same endpoint, it signals the client to reconnect before retrying the command,
+/// allowing the DNS record/proxy/load balancer to route the connection to a different underlying server host.
+/// </summary>
+public class MovedTestServer : MemoryCacheRedisServer
+{
+    /// <summary>
+    /// Represents the simulated server host state behind a proxy/load balancer.
+    /// </summary>
+    private enum SimulatedHost
+    {
+        /// <summary>
+        /// Old server that returns MOVED errors for the trigger key (pre-migration state).
+        /// </summary>
+        OldServer,
+
+        /// <summary>
+        /// New server that handles requests normally (post-migration state).
+        /// </summary>
+        NewServer,
+    }
+
+    private int _setCmdCount = 0;
+    private int _movedResponseCount = 0;
+    private int _connectionCount = 0;
+    private SimulatedHost _currentServerHost = SimulatedHost.OldServer;
+    private readonly ConcurrentDictionary<RedisClient, SimulatedHost> _clientHostAssignments = new();
+    private readonly Func<string> _getEndpoint;
+    private readonly string _triggerKey;
+    private readonly int _hashSlot;
+    private EndPoint? _actualEndpoint;
+
+    public MovedTestServer(Func<string> getEndpoint, string triggerKey = "testkey", int hashSlot = 12345)
+    {
+        _getEndpoint = getEndpoint;
+        _triggerKey = triggerKey;
+        _hashSlot = hashSlot;
+    }
+
+    /// <summary>
+    /// Called when a new client connection is established.
+    /// Assigns the client to the current server host state (simulating proxy/load balancer routing).
+    /// </summary>
+    public override RedisClient CreateClient()
+    {
+        var client = base.CreateClient();
+        var assignedHost = _currentServerHost;
+        _clientHostAssignments[client] = assignedHost;
+        Interlocked.Increment(ref _connectionCount);
+        Log($"New client connection established (assigned to {assignedHost}, total connections: {_connectionCount}), endpoint: {_actualEndpoint}");
+        return client;
+    }
+
+    /// <summary>
+    /// Handles the INFO command, reporting cluster mode as enabled.
+    /// </summary>
+    protected override TypedRedisValue Info(RedisClient client, RedisRequest request)
+    {
+        // Override INFO to report cluster mode enabled
+        var section = request.Count >= 2 ? request.GetString(1) : null;
+
+        // Return cluster-enabled info
+        var infoResponse = section?.Equals("CLUSTER", StringComparison.OrdinalIgnoreCase) == true
+            ? "# Cluster\r\ncluster_enabled:1\r\n"
+            : "# Server\r\nredis_version:7.0.0\r\n# Cluster\r\ncluster_enabled:1\r\n";
+
+        Log($"Returning INFO response (cluster_enabled:1), endpoint: {_actualEndpoint}");
+
+        return TypedRedisValue.BulkString(infoResponse);
+    }
+
+    /// <summary>
+    /// Handles CLUSTER commands, supporting SLOTS and NODES subcommands for cluster mode simulation.
+    /// </summary>
+    protected override TypedRedisValue Cluster(RedisClient client, RedisRequest request)
+    {
+        if (request.Count < 2)
+        {
+            return TypedRedisValue.Error("ERR wrong number of arguments for 'cluster' command");
+        }
+
+        var subcommand = request.GetString(1);
+
+        // Handle CLUSTER SLOTS command to support cluster mode
+        if (subcommand.Equals("SLOTS", StringComparison.OrdinalIgnoreCase))
+        {
+            Log($"Returning CLUSTER SLOTS response, endpoint: {_actualEndpoint}");
+            return GetClusterSlotsResponse();
+        }
+
+        // Handle CLUSTER NODES command
+        if (subcommand.Equals("NODES", StringComparison.OrdinalIgnoreCase))
+        {
+            Log($"Returning CLUSTER NODES response, endpoint: {_actualEndpoint}");
+            return GetClusterNodesResponse();
+        }
+
+        return TypedRedisValue.Error($"ERR Unknown CLUSTER subcommand '{subcommand}'");
+    }
+
+    /// <summary>
+    /// Handles SET commands. Returns MOVED error for the trigger key when requested by clients
+    /// connected to the old server, simulating a server migration behind a proxy/load balancer.
+    /// </summary>
+    protected override TypedRedisValue Set(RedisClient client, RedisRequest request)
+    {
+        var key = request.GetKey(1);
+
+        // Increment SET command counter for every SET call
+        Interlocked.Increment(ref _setCmdCount);
+
+        // Get the client's assigned server host
+        if (!_clientHostAssignments.TryGetValue(client, out var clientHost))
+        {
+            throw new InvalidOperationException("Client host assignment not found - this indicates a test infrastructure error");
+        }
+
+        // Check if this is the trigger key from an old server client
+        if (key == _triggerKey && clientHost == SimulatedHost.OldServer)
+        {
+            // Transition server to new host (so future connections route to new server)
+            _currentServerHost = SimulatedHost.NewServer;
+
+            Interlocked.Increment(ref _movedResponseCount);
+            var endpoint = _getEndpoint();
+            Log($"Returning MOVED {_hashSlot} {endpoint} for key '{key}' from {clientHost} client, server transitioned to {SimulatedHost.NewServer}, actual endpoint: {_actualEndpoint}");
+
+            // Return MOVED error pointing to same endpoint
+            return TypedRedisValue.Error($"MOVED {_hashSlot} {endpoint}");
+        }
+
+        // Normal processing for new server clients or other keys
+        Log($"Processing SET normally for key '{key}' from {clientHost} client, endpoint: {_actualEndpoint}");
+        return base.Set(client, request);
+    }
+
+    /// <summary>
+    /// Returns a CLUSTER SLOTS response indicating this endpoint serves all slots (0-16383).
+    /// </summary>
+    private TypedRedisValue GetClusterSlotsResponse()
+    {
+        // Return a minimal CLUSTER SLOTS response indicating this endpoint serves all slots (0-16383)
+        // Format: Array of slot ranges, each containing:
+        // [start_slot, end_slot, [host, port, node_id]]
+        if (_actualEndpoint == null)
+        {
+            return TypedRedisValue.Error("ERR endpoint not set");
+        }
+
+        var endpoint = _getEndpoint();
+        var parts = endpoint.Split(':');
+        var host = parts.Length > 0 ? parts[0] : "127.0.0.1";
+        var port = parts.Length > 1 ? parts[1] : "6379";
+
+        // Build response: [[0, 16383, [host, port, node-id]]]
+        // Inner array: [host, port, node-id]
+        var hostPortArray = TypedRedisValue.MultiBulk((ICollection<TypedRedisValue>)new[]
+        {
+            TypedRedisValue.BulkString(host),
+            TypedRedisValue.Integer(int.Parse(port)),
+            TypedRedisValue.BulkString("test-node-id"),
+        });
+        // Slot range: [start_slot, end_slot, [host, port, node-id]]
+        var slotRange = TypedRedisValue.MultiBulk((ICollection<TypedRedisValue>)new[]
+        {
+            TypedRedisValue.Integer(0),      // start slot
+            TypedRedisValue.Integer(16383),  // end slot
+            hostPortArray,
+        });
+
+        // Outer array containing the single slot range
+        return TypedRedisValue.MultiBulk((ICollection<TypedRedisValue>)new[] { slotRange });
+    }
+
+    /// <summary>
+    /// Returns a CLUSTER NODES response.
+    /// </summary>
+    private TypedRedisValue GetClusterNodesResponse()
+    {
+        // Return CLUSTER NODES response
+        // Format: node-id host:port@cport flags master - ping-sent pong-recv config-epoch link-state slot-range
+        // Example: test-node-id 127.0.0.1:6379@16379 myself,master - 0 0 1 connected 0-16383
+        if (_actualEndpoint == null)
+        {
+            return TypedRedisValue.Error("ERR endpoint not set");
+        }
+
+        var endpoint = _getEndpoint();
+        var nodesInfo = $"test-node-id {endpoint}@1{endpoint.Split(':')[1]} myself,master - 0 0 1 connected 0-16383\r\n";
+
+        return TypedRedisValue.BulkString(nodesInfo);
+    }
+
+    /// <summary>
+    /// Gets the number of SET commands executed.
+    /// </summary>
+    public int SetCmdCount => _setCmdCount;
+
+    /// <summary>
+    /// Gets the number of times MOVED response was returned.
+    /// </summary>
+    public int MovedResponseCount => _movedResponseCount;
+
+    /// <summary>
+    /// Gets the number of client connections established.
+    /// </summary>
+    public int ConnectionCount => _connectionCount;
+
+    /// <summary>
+    /// Gets the actual endpoint the server is listening on.
+    /// </summary>
+    public EndPoint? ActualEndpoint => _actualEndpoint;
+
+    /// <summary>
+    /// Sets the actual endpoint the server is listening on.
+    /// This should be called externally after the server starts.
+    /// </summary>
+    public void SetActualEndpoint(EndPoint endPoint)
+    {
+        _actualEndpoint = endPoint;
+        Log($"MovedTestServer endpoint set to {endPoint}");
+    }
+
+    /// <summary>
+    /// Resets all counters for test reusability.
+    /// </summary>
+    public void ResetCounters()
+    {
+        Interlocked.Exchange(ref _setCmdCount, 0);
+        Interlocked.Exchange(ref _movedResponseCount, 0);
+        Interlocked.Exchange(ref _connectionCount, 0);
+    }
+}