make quick connect authorize atomic and stop the fallback shadowing redis

Move the pending requests and authorized secrets out of process so the
initiate, authorize and exchange legs can land on different replicas.

.woodpecker/ci.yaml

@@ -47,6 +47,7 @@ steps:
   # Its data directory lives on the step's ephemeral storage, not on the workspace volume.
   # Both projects attach to it through JELLYFIN_TEST_POSTGRES and give every test a database of
   # its own, so nothing here depends on a docker daemon.
+  # Valkey runs in the step for the same reason, attached through JELLYFIN_TEST_REDIS.
   - name: postgres-migration-chain
     image: mcr.microsoft.com/dotnet/sdk:10.0
     depends_on:
@@ -55,13 +56,16 @@ steps:
       DOTNET_CLI_TELEMETRY_OPTOUT: "1"
       DOTNET_NOLOGO: "1"
       JELLYFIN_TEST_POSTGRES: "Host=127.0.0.1;Port=5432;Database=postgres;Username=postgres"
+      JELLYFIN_TEST_REDIS: "127.0.0.1:6379"
     commands:
       - apt-get -o Acquire::Retries=3 update || apt-get -o Acquire::Retries=3 update
-      - DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends postgresql
+      - DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends postgresql valkey-server
       - install -d -o postgres -g postgres /tmp/pgdata /tmp/pgrun
       - PGBIN=$(ls -d /usr/lib/postgresql/*/bin | tail -1)
       - su postgres -c "$PGBIN/initdb -D /tmp/pgdata -A trust -U postgres"
       - su postgres -c "$PGBIN/pg_ctl -D /tmp/pgdata -o \"-c listen_addresses=127.0.0.1 -k /tmp/pgrun\" -l /tmp/pg.log -w start"
+      - valkey-server --daemonize yes --bind 127.0.0.1 --port 6379 --save ''
+      - valkey-cli ping
       - dotnet build tests/Jellyfin.Server.Tests/Jellyfin.Server.Tests.csproj -c Release
       - dotnet build tests/Jellyfin.Database.Tests.PostgreSQL/Jellyfin.Database.Tests.PostgreSQL.csproj -c Release
       - dotnet test tests/Jellyfin.Server.Tests/Jellyfin.Server.Tests.csproj -c Release --no-build --verbosity minimal --filter "Category=RequiresDocker"

Emby.Server.Implementations/QuickConnect/QuickConnectManager.cs

@@ -1,7 +1,5 @@
 using System;
-using System.Collections.Concurrent;
 using System.Globalization;
-using System.Linq;
 using System.Security.Cryptography;
 using System.Threading.Tasks;
 using MediaBrowser.Common.Extensions;
@@ -30,12 +28,10 @@ namespace Emby.Server.Implementations.QuickConnect
         /// </summary>
         private const int Timeout = 10;
 
-        private readonly ConcurrentDictionary<string, QuickConnectResult> _currentRequests = new();
-        private readonly ConcurrentDictionary<string, (DateTime Timestamp, AuthenticationResult AuthenticationResult)> _authorizedSecrets = new();
-
         private readonly IServerConfigurationManager _config;
         private readonly ILogger<QuickConnectManager> _logger;
         private readonly ISessionManager _sessionManager;
+        private readonly IQuickConnectStore _store;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="QuickConnectManager"/> class.
@@ -44,14 +40,17 @@ namespace Emby.Server.Implementations.QuickConnect
         /// <param name="config">Configuration.</param>
         /// <param name="logger">Logger.</param>
         /// <param name="sessionManager">Session Manager.</param>
+        /// <param name="store">Quick connect store.</param>
         public QuickConnectManager(
             IServerConfigurationManager config,
             ILogger<QuickConnectManager> logger,
-            ISessionManager sessionManager)
+            ISessionManager sessionManager,
+            IQuickConnectStore store)
         {
             _config = config;
             _logger = logger;
             _sessionManager = sessionManager;
+            _store = store;
         }
 
         /// <inheritdoc />
@@ -69,7 +68,7 @@ namespace Emby.Server.Implementations.QuickConnect
         }
 
         /// <inheritdoc/>
-        public QuickConnectResult TryConnect(AuthorizationInfo authorizationInfo)
+        public async Task<QuickConnectResult> TryConnect(AuthorizationInfo authorizationInfo)
         {
             ArgumentException.ThrowIfNullOrEmpty(authorizationInfo.DeviceId);
             ArgumentException.ThrowIfNullOrEmpty(authorizationInfo.Device);
@@ -77,7 +76,6 @@ namespace Emby.Server.Implementations.QuickConnect
             ArgumentException.ThrowIfNullOrEmpty(authorizationInfo.Version);
 
             AssertActive();
-            ExpireRequests();
 
             var secret = GenerateSecureRandom();
             var code = GenerateCode();
@@ -90,19 +88,17 @@ namespace Emby.Server.Implementations.QuickConnect
                 authorizationInfo.Client,
                 authorizationInfo.Version);
 
-            _currentRequests[code] = result;
+            await _store.SetRequestAsync(result, ExpiryOf(result)).ConfigureAwait(false);
             return result;
         }
 
         /// <inheritdoc/>
-        public QuickConnectResult CheckRequestStatus(string secret)
+        public async Task<QuickConnectResult> CheckRequestStatus(string secret)
         {
             AssertActive();
-            ExpireRequests();
 
-            string code = _currentRequests.Where(x => x.Value.Secret == secret).Select(x => x.Value.Code).DefaultIfEmpty(string.Empty).First();
-
-            if (!_currentRequests.TryGetValue(code, out QuickConnectResult? result))
+            var result = await _store.GetRequestBySecretAsync(secret).ConfigureAwait(false);
+            if (result is null)
             {
                 throw new ResourceNotFoundException("Unable to find request with provided secret");
             }
@@ -136,9 +132,9 @@ namespace Emby.Server.Implementations.QuickConnect
         public async Task<bool> AuthorizeRequest(Guid userId, string code)
         {
             AssertActive();
-            ExpireRequests();
 
-            if (!_currentRequests.TryGetValue(code, out QuickConnectResult? result))
+            var result = await _store.GetRequestByCodeAsync(code).ConfigureAwait(false);
+            if (result is null)
             {
                 throw new ResourceNotFoundException("Unable to find request");
             }
@@ -151,6 +147,12 @@ namespace Emby.Server.Implementations.QuickConnect
             // Change the time on the request so it expires one minute into the future. It can't expire immediately as otherwise some clients wouldn't ever see that they have been authenticated.
             result.DateAdded = DateTime.UtcNow.Add(TimeSpan.FromMinutes(1));
 
+            // The guard above is a read on shared state, so it cannot settle a race between instances; the claim can.
+            if (!await _store.TryClaimAuthorizationAsync(result.Secret, ExpiryOf(result)).ConfigureAwait(false))
+            {
+                throw new InvalidOperationException("Request is already authorized");
+            }
+
             var authenticationResult = await _sessionManager.AuthenticateDirect(new AuthenticationRequest
             {
                 UserId = userId,
@@ -160,9 +162,10 @@ namespace Emby.Server.Implementations.QuickConnect
                 AppVersion = result.AppVersion
             }).ConfigureAwait(false);
 
-            _authorizedSecrets[result.Secret] = (DateTime.UtcNow, authenticationResult);
             result.Authenticated = true;
-            _currentRequests[code] = result;
+
+            await _store.SetAuthorizationAsync(result.Secret, authenticationResult, DateTime.UtcNow.AddMinutes(Timeout)).ConfigureAwait(false);
+            await _store.SetRequestAsync(result, ExpiryOf(result)).ConfigureAwait(false);
 
             _logger.LogDebug("Authorizing device with code {Code} to login as user {UserId}", code, userId);
 
@@ -170,19 +173,21 @@ namespace Emby.Server.Implementations.QuickConnect
         }
 
         /// <inheritdoc/>
-        public AuthenticationResult GetAuthorizedRequest(string secret)
+        public async Task<AuthenticationResult> GetAuthorizedRequest(string secret)
         {
             AssertActive();
-            ExpireRequests();
 
-            if (!_authorizedSecrets.TryGetValue(secret, out var result))
+            var result = await _store.TryConsumeAuthorizationAsync(secret).ConfigureAwait(false);
+            if (result is null)
             {
                 throw new ResourceNotFoundException("Unable to find request");
             }
 
-            return result.AuthenticationResult;
+            return result;
         }
 
+        private static DateTime ExpiryOf(QuickConnectResult request) => request.DateAdded.AddMinutes(Timeout);
+
         private string GenerateSecureRandom(int length = 32)
         {
             Span<byte> bytes = stackalloc byte[length];
@@ -190,42 +195,5 @@ namespace Emby.Server.Implementations.QuickConnect
 
             return Convert.ToHexString(bytes);
         }
-
-        /// <summary>
-        /// Expire quick connect requests that are over the time limit. If <paramref name="expireAll"/> is true, all requests are unconditionally expired.
-        /// </summary>
-        /// <param name="expireAll">If true, all requests will be expired.</param>
-        private void ExpireRequests(bool expireAll = false)
-        {
-            // All requests before this timestamp have expired
-            var minTime = DateTime.UtcNow.AddMinutes(-Timeout);
-
-            // Expire stale connection requests
-            foreach (var (_, currentRequest) in _currentRequests)
-            {
-                if (expireAll || currentRequest.DateAdded < minTime)
-                {
-                    var code = currentRequest.Code;
-                    _logger.LogDebug("Removing expired request {Code}", code);
-
-                    if (!_currentRequests.TryRemove(code, out _))
-                    {
-                        _logger.LogWarning("Request {Code} already expired", code);
-                    }
-                }
-            }
-
-            foreach (var (secret, (timestamp, _)) in _authorizedSecrets)
-            {
-                if (expireAll || timestamp < minTime)
-                {
-                    _logger.LogDebug("Removing expired secret {Secret}", secret);
-                    if (!_authorizedSecrets.TryRemove(secret, out _))
-                    {
-                        _logger.LogWarning("Secret {Secret} already expired", secret);
-                    }
-                }
-            }
-        }
     }
 }

Emby.Server.Implementations/QuickConnect/RedisQuickConnectStore.cs

@@ -0,0 +1,175 @@
+using System;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using Jellyfin.Extensions.Json;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Controller.QuickConnect;
+using MediaBrowser.Model.QuickConnect;
+using Microsoft.Extensions.Logging;
+using StackExchange.Redis;
+
+namespace Emby.Server.Implementations.QuickConnect;
+
+/// <summary>
+/// A Redis-backed <see cref="IQuickConnectStore"/> that lets the initiate, authorize and exchange legs
+/// of a quick connect flow land on different instances. Expiry is the key TTL, an authorization is
+/// claimed with a Lua check-and-set and consumed with <c>GETDEL</c>, so only one instance can ever mint
+/// a given secret's access token and only one can ever hand it out.
+/// </summary>
+/// <remarks>
+/// A pending request survives an unreachable Redis through a process-local fallback, because a second
+/// copy of it is harmless. An authorization has none: a second copy of it is a second access token, and
+/// a write whose response timed out may well have been applied, so a transport failure on that path is
+/// surfaced rather than degraded.
+/// </remarks>
+public sealed class RedisQuickConnectStore : IQuickConnectStore
+{
+    private const string KeyPrefix = "jellyfin:quickconnect:";
+
+    /// <summary>
+    /// Lua script for the atomic claim of the sole right to authorize a request: the request has to
+    /// exist and not already be authorized, and the claim marker is taken with <c>SET NX</c>, so of two
+    /// instances racing on one code exactly one goes on to mint an access token.
+    /// </summary>
+    private const string ClaimAuthorizationScript = @"
+local raw = redis.call('GET', KEYS[1])
+if not raw then return 0 end
+if cjson.decode(raw)['Authenticated'] then return 0 end
+if redis.call('SET', KEYS[2], '1', 'NX', 'PX', ARGV[1]) then return 1 end
+return 0";
+
+    private readonly IDatabase _db;
+    private readonly InMemoryQuickConnectStore _fallback;
+    private readonly ILogger<RedisQuickConnectStore> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="RedisQuickConnectStore"/> class.
+    /// </summary>
+    /// <param name="redis">The Redis connection multiplexer.</param>
+    /// <param name="logger">The logger.</param>
+    public RedisQuickConnectStore(IConnectionMultiplexer redis, ILogger<RedisQuickConnectStore> logger)
+    {
+        ArgumentNullException.ThrowIfNull(redis);
+
+        _db = redis.GetDatabase();
+        _fallback = new InMemoryQuickConnectStore();
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task<QuickConnectResult?> GetRequestBySecretAsync(string secret, CancellationToken cancellationToken = default)
+    {
+        RedisValue raw;
+        try
+        {
+            raw = await _db.StringGetAsync(RequestKey(secret)).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (IsTransportFailure(ex))
+        {
+            LogDegraded(ex);
+            return await _fallback.GetRequestBySecretAsync(secret, cancellationToken).ConfigureAwait(false);
+        }
+
+        // A miss is an answer rather than a transport failure, so the fallback is not consulted for it.
+        return raw.HasValue ? JsonSerializer.Deserialize<QuickConnectResult>(raw.ToString(), JsonDefaults.Options) : null;
+    }
+
+    /// <inheritdoc />
+    public async Task<QuickConnectResult?> GetRequestByCodeAsync(string code, CancellationToken cancellationToken = default)
+    {
+        RedisValue secret;
+        try
+        {
+            secret = await _db.StringGetAsync(CodeKey(code)).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (IsTransportFailure(ex))
+        {
+            LogDegraded(ex);
+            return await _fallback.GetRequestByCodeAsync(code, cancellationToken).ConfigureAwait(false);
+        }
+
+        return secret.HasValue
+            ? await GetRequestBySecretAsync(secret.ToString(), cancellationToken).ConfigureAwait(false)
+            : null;
+    }
+
+    /// <inheritdoc />
+    public async Task SetRequestAsync(QuickConnectResult request, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        var ttl = expiresUtc - DateTime.UtcNow;
+        if (ttl <= TimeSpan.Zero)
+        {
+            return;
+        }
+
+        try
+        {
+            var json = JsonSerializer.Serialize(request, JsonDefaults.Options);
+            await _db.StringSetAsync(RequestKey(request.Secret), json, ttl).ConfigureAwait(false);
+            await _db.StringSetAsync(CodeKey(request.Code), request.Secret, ttl).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (IsTransportFailure(ex))
+        {
+            LogDegraded(ex);
+            await _fallback.SetRequestAsync(request, expiresUtc, cancellationToken).ConfigureAwait(false);
+        }
+    }
+
+    /// <inheritdoc />
+    public async Task<bool> TryClaimAuthorizationAsync(string secret, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        var ttl = expiresUtc - DateTime.UtcNow;
+        if (ttl <= TimeSpan.Zero)
+        {
+            return false;
+        }
+
+        var claimed = (long?)await _db.ScriptEvaluateAsync(
+            ClaimAuthorizationScript,
+            keys: new RedisKey[] { RequestKey(secret), ClaimKey(secret) },
+            values: new RedisValue[] { (long)ttl.TotalMilliseconds }).ConfigureAwait(false);
+
+        return claimed == 1;
+    }
+
+    /// <inheritdoc />
+    public async Task SetAuthorizationAsync(string secret, AuthenticationResult authenticationResult, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        var ttl = expiresUtc - DateTime.UtcNow;
+        if (ttl <= TimeSpan.Zero)
+        {
+            return;
+        }
+
+        var json = JsonSerializer.Serialize(authenticationResult, JsonDefaults.Options);
+        await _db.StringSetAsync(AuthorizationKey(secret), json, ttl).ConfigureAwait(false);
+    }
+
+    /// <inheritdoc />
+    public async Task<AuthenticationResult?> TryConsumeAuthorizationAsync(string secret, CancellationToken cancellationToken = default)
+    {
+        var raw = await _db.StringGetDeleteAsync(AuthorizationKey(secret)).ConfigureAwait(false);
+
+        return raw.HasValue
+            ? JsonSerializer.Deserialize<AuthenticationResult>(raw.ToString(), JsonDefaults.Options)
+            : null;
+    }
+
+    // Deliberately excludes a malformed stored value, which is a fault of its own rather than a reason
+    // to answer from this instance.
+    private static bool IsTransportFailure(Exception exception) => exception is RedisException or TimeoutException;
+
+    private static string RequestKey(string secret) => KeyPrefix + "request:" + secret;
+
+    private static string CodeKey(string code) => KeyPrefix + "code:" + code;
+
+    private static string ClaimKey(string secret) => KeyPrefix + "claim:" + secret;
+
+    private static string AuthorizationKey(string secret) => KeyPrefix + "auth:" + secret;
+
+    private void LogDegraded(Exception exception)
+        => _logger.LogWarning(exception, "Quick connect request state could not be shared through Redis; falling back to this instance only.");
+}

Jellyfin.Api/Controllers/QuickConnectController.cs

@@ -59,7 +59,7 @@ public class QuickConnectController : BaseJellyfinApiController
         try
         {
             var auth = await _authContext.GetAuthorizationInfo(Request).ConfigureAwait(false);
-            return _quickConnect.TryConnect(auth);
+            return await _quickConnect.TryConnect(auth).ConfigureAwait(false);
         }
         catch (AuthenticationException)
         {
@@ -77,11 +77,11 @@ public class QuickConnectController : BaseJellyfinApiController
     [HttpGet("Connect")]
     [ProducesResponseType(StatusCodes.Status200OK)]
     [ProducesResponseType(StatusCodes.Status404NotFound)]
-    public ActionResult<QuickConnectResult> GetQuickConnectState([FromQuery, Required] string secret)
+    public async Task<ActionResult<QuickConnectResult>> GetQuickConnectState([FromQuery, Required] string secret)
     {
         try
         {
-            return _quickConnect.CheckRequestStatus(secret);
+            return await _quickConnect.CheckRequestStatus(secret).ConfigureAwait(false);
         }
         catch (ResourceNotFoundException)
         {

Jellyfin.Api/Controllers/UserController.cs

@@ -245,11 +245,11 @@ public class UserController : BaseJellyfinApiController
     [HttpPost("AuthenticateWithQuickConnect")]
     [ProducesResponseType(StatusCodes.Status200OK)]
     [Tags("Authentication")]
-    public ActionResult<AuthenticationResult> AuthenticateWithQuickConnect([FromBody, Required] QuickConnectDto request)
+    public async Task<ActionResult<AuthenticationResult>> AuthenticateWithQuickConnect([FromBody, Required] QuickConnectDto request)
     {
         try
         {
-            return _quickConnectManager.GetAuthorizedRequest(request.Secret);
+            return await _quickConnectManager.GetAuthorizedRequest(request.Secret).ConfigureAwait(false);
         }
         catch (SecurityException e)
         {

Jellyfin.Server/CoreAppHost.cs

@@ -116,6 +116,10 @@ namespace Jellyfin.Server
             // to the other instances. Redis-backed when configured, no-op otherwise.
             serviceCollection.AddConfigurationInvalidationBus(_startupConfig, Logger);
 
+            // Quick connect store: shares in-flight quick connect requests so the initiate, authorize and
+            // exchange legs can land on different instances. Redis-backed when configured, local otherwise.
+            serviceCollection.AddQuickConnectStore(_startupConfig, Logger);
+
             foreach (var type in GetExportTypes<ILyricProvider>())
             {
                 serviceCollection.AddSingleton(typeof(ILyricProvider), type);

Jellyfin.Server/Extensions/QuickConnectStoreServiceCollectionExtensions.cs

@@ -0,0 +1,72 @@
+using System;
+using Emby.Server.Implementations.QuickConnect;
+using MediaBrowser.Controller.MediaEncoding;
+using MediaBrowser.Controller.QuickConnect;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using StackExchange.Redis;
+
+namespace Jellyfin.Server.Extensions;
+
+/// <summary>
+/// Extensions for registering the quick connect store.
+/// </summary>
+public static class QuickConnectStoreServiceCollectionExtensions
+{
+    /// <summary>
+    /// Registers the quick connect store, Redis-backed when a connection string is configured and
+    /// process-local otherwise, and reports the selected store at <see cref="LogLevel.Information"/>.
+    /// </summary>
+    /// <remarks>
+    /// The connection string is only set for a multi-instance deployment, which is the only shape where
+    /// the initiate, authorize and exchange legs of one flow can land on different instances.
+    /// </remarks>
+    /// <param name="serviceCollection">The service collection.</param>
+    /// <param name="configuration">The configuration to read the Redis connection string from.</param>
+    /// <param name="logger">The logger to report the selected store on.</param>
+    /// <returns>The updated service collection.</returns>
+    public static IServiceCollection AddQuickConnectStore(
+        this IServiceCollection serviceCollection,
+        IConfiguration configuration,
+        ILogger logger)
+    {
+        ArgumentNullException.ThrowIfNull(configuration);
+        ArgumentNullException.ThrowIfNull(logger);
+
+        if (string.IsNullOrEmpty(configuration[TranscodeStoreOptions.RedisConnectionStringKey]))
+        {
+            logger.LogInformation(
+                "Quick connect store: {Store}. A quick connect flow has to complete against one instance; set {Key} to share it.",
+                nameof(InMemoryQuickConnectStore),
+                TranscodeStoreOptions.RedisConnectionStringKey);
+
+            return serviceCollection.AddSingleton<IQuickConnectStore, InMemoryQuickConnectStore>();
+        }
+
+        logger.LogInformation(
+            "Quick connect store: {Store}. Quick connect flows complete across any instance.",
+            nameof(RedisQuickConnectStore));
+
+        return serviceCollection.AddSingleton<IQuickConnectStore>(sp =>
+        {
+            try
+            {
+                return new RedisQuickConnectStore(
+                    sp.GetRequiredService<IConnectionMultiplexer>(),
+                    sp.GetRequiredService<ILogger<RedisQuickConnectStore>>());
+            }
+            catch (Exception ex)
+            {
+                // Fail open: an unreachable Redis degrades to the single-instance behaviour of a flow
+                // having to complete against one instance, rather than taking quick connect down.
+                sp.GetRequiredService<ILogger<CoreAppHost>>().LogError(
+                    ex,
+                    "Redis is configured but unavailable, so quick connect flows will not complete across instances. Check {Key}.",
+                    TranscodeStoreOptions.RedisConnectionStringKey);
+
+                return new InMemoryQuickConnectStore();
+            }
+        });
+    }
+}

MediaBrowser.Controller/QuickConnect/IQuickConnect.cs

@@ -21,14 +21,14 @@ namespace MediaBrowser.Controller.QuickConnect
         /// </summary>
         /// <param name="authorizationInfo">The initiator authorization info.</param>
         /// <returns>A quick connect result with tokens to proceed or throws an exception if not active.</returns>
-        QuickConnectResult TryConnect(AuthorizationInfo authorizationInfo);
+        Task<QuickConnectResult> TryConnect(AuthorizationInfo authorizationInfo);
 
         /// <summary>
         /// Checks the status of an individual request.
         /// </summary>
         /// <param name="secret">Unique secret identifier of the request.</param>
         /// <returns>Quick connect result.</returns>
-        QuickConnectResult CheckRequestStatus(string secret);
+        Task<QuickConnectResult> CheckRequestStatus(string secret);
 
         /// <summary>
         /// Authorizes a quick connect request to connect as the calling user.
@@ -43,6 +43,6 @@ namespace MediaBrowser.Controller.QuickConnect
         /// </summary>
         /// <param name="secret">The secret.</param>
         /// <returns>The authentication result.</returns>
-        AuthenticationResult GetAuthorizedRequest(string secret);
+        Task<AuthenticationResult> GetAuthorizedRequest(string secret);
     }
 }

MediaBrowser.Controller/QuickConnect/IQuickConnectStore.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Model.QuickConnect;
+
+namespace MediaBrowser.Controller.QuickConnect;
+
+/// <summary>
+/// Holds the state of in-flight quick connect requests. The three legs of a quick connect flow -
+/// initiate, authorize and exchange - can each land on a different instance, so the state has to be
+/// reachable from all of them.
+/// </summary>
+public interface IQuickConnectStore
+{
+    /// <summary>
+    /// Looks up a pending request by the secret handed to the initiating client.
+    /// </summary>
+    /// <param name="secret">The request secret.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>The request, or <c>null</c> when it is unknown or has expired.</returns>
+    Task<QuickConnectResult?> GetRequestBySecretAsync(string secret, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Looks up a pending request by the code shown to the user.
+    /// </summary>
+    /// <param name="code">The user facing code.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>The request, or <c>null</c> when it is unknown or has expired.</returns>
+    Task<QuickConnectResult?> GetRequestByCodeAsync(string code, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Stores a new or updated request until <paramref name="expiresUtc"/>.
+    /// </summary>
+    /// <param name="request">The request to store.</param>
+    /// <param name="expiresUtc">The instant the request stops being resolvable.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    Task SetRequestAsync(QuickConnectResult request, DateTime expiresUtc, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Atomically claims the sole right to authorize the request behind <paramref name="secret"/>, so
+    /// that two instances racing on one code cannot both mint an access token. The claim is never
+    /// released: a mint that failed after writing its token would otherwise be retried into a second one.
+    /// </summary>
+    /// <param name="secret">The request secret.</param>
+    /// <param name="expiresUtc">The instant the claim lapses, after which the request can be authorized again.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns><c>true</c> when this caller may go on to authorize the request; <c>false</c> when it is unknown, expired, already authorized or being authorized elsewhere.</returns>
+    Task<bool> TryClaimAuthorizationAsync(string secret, DateTime expiresUtc, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Stores the authentication minted for an authorized request until <paramref name="expiresUtc"/>.
+    /// </summary>
+    /// <param name="secret">The request secret the client exchanges.</param>
+    /// <param name="authenticationResult">The authentication to hand out.</param>
+    /// <param name="expiresUtc">The instant the authentication stops being exchangeable.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    Task SetAuthorizationAsync(string secret, AuthenticationResult authenticationResult, DateTime expiresUtc, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Atomically takes the authentication for <paramref name="secret"/> and removes it, so that two
+    /// instances racing on the same secret cannot both hand out an access token.
+    /// </summary>
+    /// <param name="secret">The request secret.</param>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>The authentication, or <c>null</c> when the secret is unknown, expired or already exchanged.</returns>
+    Task<AuthenticationResult?> TryConsumeAuthorizationAsync(string secret, CancellationToken cancellationToken = default);
+}

MediaBrowser.Controller/QuickConnect/InMemoryQuickConnectStore.cs

@@ -0,0 +1,110 @@
+using System;
+using System.Collections.Concurrent;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Model.QuickConnect;
+
+namespace MediaBrowser.Controller.QuickConnect;
+
+/// <summary>
+/// A process-local <see cref="IQuickConnectStore"/>. It is the single-instance default, and the
+/// fallback a shared store degrades to while its backend is unreachable, so quick connect keeps
+/// working for clients whose three legs happen to land on one instance.
+/// </summary>
+public sealed class InMemoryQuickConnectStore : IQuickConnectStore
+{
+    private readonly ConcurrentDictionary<string, Entry<QuickConnectResult>> _requests = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<string, Entry<AuthenticationResult>> _authorizations = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<string, DateTime> _authorizationClaims = new(StringComparer.Ordinal);
+
+    /// <inheritdoc />
+    public Task<QuickConnectResult?> GetRequestBySecretAsync(string secret, CancellationToken cancellationToken = default)
+    {
+        Expire();
+        return Task.FromResult(_requests.TryGetValue(secret, out var entry) ? entry.Value : null);
+    }
+
+    /// <inheritdoc />
+    public Task<QuickConnectResult?> GetRequestByCodeAsync(string code, CancellationToken cancellationToken = default)
+    {
+        Expire();
+        return Task.FromResult(_requests.Values
+            .Select(entry => entry.Value)
+            .FirstOrDefault(request => string.Equals(request.Code, code, StringComparison.Ordinal)));
+    }
+
+    /// <inheritdoc />
+    public Task SetRequestAsync(QuickConnectResult request, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        Expire();
+        _requests[request.Secret] = new Entry<QuickConnectResult>(expiresUtc, request);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task<bool> TryClaimAuthorizationAsync(string secret, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        Expire();
+        if (!_requests.TryGetValue(secret, out var entry) || entry.Value.Authenticated)
+        {
+            return Task.FromResult(false);
+        }
+
+        return Task.FromResult(_authorizationClaims.TryAdd(secret, expiresUtc));
+    }
+
+    /// <inheritdoc />
+    public Task SetAuthorizationAsync(string secret, AuthenticationResult authenticationResult, DateTime expiresUtc, CancellationToken cancellationToken = default)
+    {
+        Expire();
+        _authorizations[secret] = new Entry<AuthenticationResult>(expiresUtc, authenticationResult);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task<AuthenticationResult?> TryConsumeAuthorizationAsync(string secret, CancellationToken cancellationToken = default)
+    {
+        Expire();
+        if (!_authorizations.TryRemove(secret, out var entry) || entry.ExpiresUtc <= DateTime.UtcNow)
+        {
+            return Task.FromResult<AuthenticationResult?>(null);
+        }
+
+        return Task.FromResult<AuthenticationResult?>(entry.Value);
+    }
+
+    private void Expire()
+    {
+        var now = DateTime.UtcNow;
+
+        foreach (var (secret, entry) in _requests)
+        {
+            if (entry.ExpiresUtc <= now)
+            {
+                _requests.TryRemove(secret, out _);
+            }
+        }
+
+        foreach (var (secret, entry) in _authorizations)
+        {
+            if (entry.ExpiresUtc <= now)
+            {
+                _authorizations.TryRemove(secret, out _);
+            }
+        }
+
+        foreach (var (secret, expiresUtc) in _authorizationClaims)
+        {
+            if (expiresUtc <= now)
+            {
+                _authorizationClaims.TryRemove(secret, out _);
+            }
+        }
+    }
+
+    private sealed record Entry<T>(DateTime ExpiresUtc, T Value);
+}

tests/Jellyfin.Server.Implementations.Tests/QuickConnect/QuickConnectManagerTests.cs

@@ -8,6 +8,7 @@ using MediaBrowser.Common.Extensions;
 using MediaBrowser.Controller.Authentication;
 using MediaBrowser.Controller.Configuration;
 using MediaBrowser.Controller.Net;
+using MediaBrowser.Controller.QuickConnect;
 using MediaBrowser.Model.Configuration;
 using Moq;
 using Xunit;
@@ -40,6 +41,8 @@ namespace Jellyfin.Server.Implementations.Tests.QuickConnect
                 ConfigureMembers = true
             }).Inject(configManager.Object);
 
+            _fixture.Inject<IQuickConnectStore>(new InMemoryQuickConnectStore());
+
             // User object contains circular references.
             _fixture.Behaviors.OfType<ThrowingRecursionBehavior>().ToList()
                 .ForEach(b => _fixture.Behaviors.Remove(b));
@@ -60,8 +63,8 @@ namespace Jellyfin.Server.Implementations.Tests.QuickConnect
         [InlineData("Device", "", "Client", "1.0.0")]
         [InlineData("Device", "DeviceId", "", "1.0.0")]
         [InlineData("Device", "DeviceId", "Client", "")]
-        public void TryConnect_InvalidAuthorizationInfo_ThrowsArgumentException(string device, string deviceId, string client, string version)
-            => Assert.Throws<ArgumentException>(() => _quickConnectManager.TryConnect(
+        public async Task TryConnect_InvalidAuthorizationInfo_ThrowsArgumentException(string device, string deviceId, string client, string version)
+            => await Assert.ThrowsAsync<ArgumentException>(() => _quickConnectManager.TryConnect(
                 new AuthorizationInfo
                 {
                     Device = device,
@@ -71,17 +74,17 @@ namespace Jellyfin.Server.Implementations.Tests.QuickConnect
                 }));
 
         [Fact]
-        public void TryConnect_QuickConnectUnavailable_ThrowsAuthenticationException()
+        public async Task TryConnect_QuickConnectUnavailable_ThrowsAuthenticationException()
         {
             _config.QuickConnectAvailable = false;
-            Assert.Throws<AuthenticationException>(() => _quickConnectManager.TryConnect(_quickConnectAuthInfo));
+            await Assert.ThrowsAsync<AuthenticationException>(() => _quickConnectManager.TryConnect(_quickConnectAuthInfo));
         }
 
         [Fact]
-        public void CheckRequestStatus_QuickConnectUnavailable_ThrowsAuthenticationException()
+        public async Task CheckRequestStatus_QuickConnectUnavailable_ThrowsAuthenticationException()
         {
             _config.QuickConnectAvailable = false;
-            Assert.Throws<AuthenticationException>(() => _quickConnectManager.CheckRequestStatus(string.Empty));
+            await Assert.ThrowsAsync<AuthenticationException>(() => _quickConnectManager.CheckRequestStatus(string.Empty));
         }
 
         [Fact]
@@ -92,10 +95,10 @@ namespace Jellyfin.Server.Implementations.Tests.QuickConnect
         }
 
         [Fact]
-        public void GetAuthorizedRequest_QuickConnectUnavailable_ThrowsAuthenticationException()
+        public async Task GetAuthorizedRequest_QuickConnectUnavailable_ThrowsAuthenticationException()
         {
             _config.QuickConnectAvailable = false;
-            Assert.Throws<AuthenticationException>(() => _quickConnectManager.GetAuthorizedRequest(string.Empty));
+            await Assert.ThrowsAsync<AuthenticationException>(() => _quickConnectManager.GetAuthorizedRequest(string.Empty));
         }
 
         [Fact]
@@ -106,34 +109,71 @@ namespace Jellyfin.Server.Implementations.Tests.QuickConnect
         }
 
         [Fact]
-        public void CheckRequestStatus_QuickConnectAvailable_Success()
+        public async Task CheckRequestStatus_QuickConnectAvailable_Success()
         {
             _config.QuickConnectAvailable = true;
-            var res1 = _quickConnectManager.TryConnect(_quickConnectAuthInfo);
-            var res2 = _quickConnectManager.CheckRequestStatus(res1.Secret);
-            Assert.Equal(res1, res2);
+            var res1 = await _quickConnectManager.TryConnect(_quickConnectAuthInfo);
+            var res2 = await _quickConnectManager.CheckRequestStatus(res1.Secret);
+            Assert.Equal(res1.Secret, res2.Secret);
+            Assert.Equal(res1.Code, res2.Code);
         }
 
         [Fact]
-        public void CheckRequestStatus_UnknownSecret_ThrowsResourceNotFoundException()
+        public async Task CheckRequestStatus_UnknownSecret_ThrowsResourceNotFoundException()
         {
             _config.QuickConnectAvailable = true;
-            Assert.Throws<ResourceNotFoundException>(() => _quickConnectManager.CheckRequestStatus("Unknown secret"));
+            await Assert.ThrowsAsync<ResourceNotFoundException>(() => _quickConnectManager.CheckRequestStatus("Unknown secret"));
         }
 
         [Fact]
-        public void GetAuthorizedRequest_UnknownSecret_ThrowsResourceNotFoundException()
+        public async Task GetAuthorizedRequest_UnknownSecret_ThrowsResourceNotFoundException()
         {
             _config.QuickConnectAvailable = true;
-            Assert.Throws<ResourceNotFoundException>(() => _quickConnectManager.GetAuthorizedRequest("Unknown secret"));
+            await Assert.ThrowsAsync<ResourceNotFoundException>(() => _quickConnectManager.GetAuthorizedRequest("Unknown secret"));
         }
 
         [Fact]
         public async Task AuthorizeRequest_QuickConnectAvailable_Success()
         {
             _config.QuickConnectAvailable = true;
-            var res = _quickConnectManager.TryConnect(_quickConnectAuthInfo);
+            var res = await _quickConnectManager.TryConnect(_quickConnectAuthInfo);
             Assert.True(await _quickConnectManager.AuthorizeRequest(Guid.Empty, res.Code));
         }
+
+        [Fact]
+        public async Task AuthorizeRequest_RacedOnOneCode_SucceedsOnce()
+        {
+            _config.QuickConnectAvailable = true;
+            var res = await _quickConnectManager.TryConnect(_quickConnectAuthInfo);
+
+            var outcomes = await Task.WhenAll(
+                Task.Run(() => AuthorizeAsync(res.Code)),
+                Task.Run(() => AuthorizeAsync(res.Code)));
+
+            Assert.Single(outcomes, authorized => authorized);
+        }
+
+        [Fact]
+        public async Task GetAuthorizedRequest_SecondExchange_ThrowsResourceNotFoundException()
+        {
+            _config.QuickConnectAvailable = true;
+            var res = await _quickConnectManager.TryConnect(_quickConnectAuthInfo);
+            await _quickConnectManager.AuthorizeRequest(Guid.Empty, res.Code);
+
+            Assert.NotNull(await _quickConnectManager.GetAuthorizedRequest(res.Secret));
+            await Assert.ThrowsAsync<ResourceNotFoundException>(() => _quickConnectManager.GetAuthorizedRequest(res.Secret));
+        }
+
+        private async Task<bool> AuthorizeAsync(string code)
+        {
+            try
+            {
+                return await _quickConnectManager.AuthorizeRequest(Guid.Empty, code).ConfigureAwait(false);
+            }
+            catch (InvalidOperationException)
+            {
+                return false;
+            }
+        }
     }
 }

tests/Jellyfin.Server.Tests/HighAvailability/JellyfinSectionConfigurationTests.cs

@@ -13,6 +13,7 @@ namespace Jellyfin.Server.Tests.HighAvailability;
 /// startup configuration the host reads them from must therefore accept that form; when it does not,
 /// a correctly set variable is dropped and the feature it configures stays off without any error.
 /// </summary>
+[Collection("JellyfinSectionConfiguration")]
 public sealed class JellyfinSectionConfigurationTests : IDisposable
 {
     private const string RedisKey = "Jellyfin:TranscodeStore:RedisConnectionString";

tests/Jellyfin.Server.Tests/HighAvailability/RedisFaultProxy.cs

@@ -0,0 +1,170 @@
+using System;
+using System.Collections.Concurrent;
+using System.Globalization;
+using System.IO;
+using System.Net;
+using System.Net.Sockets;
+using System.Threading;
+using System.Threading.Tasks;
+using StackExchange.Redis;
+
+namespace Jellyfin.Server.Tests.HighAvailability;
+
+/// <summary>
+/// A loopback TCP proxy in front of a Redis server. Cutting it drops every connection through it and
+/// refuses new ones, so a test can take Redis away from one instance mid-flow - and give it back - the
+/// way a restarted valkey does, and watch what a real StackExchange.Redis client makes of it.
+/// </summary>
+public sealed class RedisFaultProxy : IAsyncDisposable
+{
+    private readonly ConcurrentDictionary<TcpClient, byte> _live = new();
+    private readonly CancellationTokenSource _cts = new();
+    private readonly TcpListener _listener;
+    private readonly string _targetHost;
+    private readonly int _targetPort;
+    private readonly int _port;
+
+    private volatile bool _cut;
+
+    private RedisFaultProxy(TcpListener listener, int port, string targetHost, int targetPort)
+    {
+        _listener = listener;
+        _port = port;
+        _targetHost = targetHost;
+        _targetPort = targetPort;
+    }
+
+    /// <summary>
+    /// Gets a connection string pointing at the proxy. The timeouts are short so a cut surfaces as a
+    /// failure in seconds rather than in the library's minute-scale defaults.
+    /// </summary>
+    public string ConnectionString => string.Create(
+        CultureInfo.InvariantCulture,
+        $"127.0.0.1:{_port},abortConnect=false,connectTimeout=500,syncTimeout=2000,connectRetry=1");
+
+    /// <summary>
+    /// Starts a proxy in front of the server named by <paramref name="target"/>.
+    /// </summary>
+    /// <param name="target">The connection string of the server to forward to.</param>
+    /// <returns>The running proxy.</returns>
+    public static RedisFaultProxy Start(string target)
+    {
+        var endpoint = ConfigurationOptions.Parse(target).EndPoints[0];
+        var (host, port) = endpoint switch
+        {
+            DnsEndPoint dns => (dns.Host, dns.Port),
+            IPEndPoint ip => (ip.Address.ToString(), ip.Port),
+            _ => throw new NotSupportedException("Unsupported endpoint " + endpoint)
+        };
+
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+
+        var proxy = new RedisFaultProxy(listener, ((IPEndPoint)listener.LocalEndpoint).Port, host, port);
+        _ = Task.Run(proxy.AcceptAsync);
+        return proxy;
+    }
+
+    /// <summary>
+    /// Takes Redis away from everything connected through the proxy.
+    /// </summary>
+    public void Cut()
+    {
+        _cut = true;
+        DropLiveConnections();
+    }
+
+    /// <summary>
+    /// Lets connections through again. Clients reconnect on their own schedule, so callers have to wait
+    /// for the connection to come back rather than assume it already has.
+    /// </summary>
+    public void Restore() => _cut = false;
+
+    /// <inheritdoc/>
+    public async ValueTask DisposeAsync()
+    {
+        _cut = true;
+        await _cts.CancelAsync().ConfigureAwait(false);
+        _listener.Stop();
+        DropLiveConnections();
+        _cts.Dispose();
+    }
+
+    private void DropLiveConnections()
+    {
+        foreach (var client in _live.Keys)
+        {
+            if (_live.TryRemove(client, out _))
+            {
+                client.Dispose();
+            }
+        }
+    }
+
+    private async Task AcceptAsync()
+    {
+        while (!_cts.IsCancellationRequested)
+        {
+            TcpClient client;
+            try
+            {
+                client = await _listener.AcceptTcpClientAsync(_cts.Token).ConfigureAwait(false);
+            }
+            catch (Exception exception) when (exception is OperationCanceledException or SocketException or ObjectDisposedException)
+            {
+                return;
+            }
+
+            if (_cut)
+            {
+                client.Dispose();
+                continue;
+            }
+
+            _ = Task.Run(() => ForwardAsync(client));
+        }
+    }
+
+    private async Task ForwardAsync(TcpClient client)
+    {
+        TcpClient? upstream = null;
+        try
+        {
+            upstream = new TcpClient();
+            await upstream.ConnectAsync(_targetHost, _targetPort, _cts.Token).ConfigureAwait(false);
+
+            _live[client] = 0;
+            _live[upstream] = 0;
+
+            var clientStream = client.GetStream();
+            var upstreamStream = upstream.GetStream();
+            await Task.WhenAny(
+                CopyAsync(clientStream, upstreamStream),
+                CopyAsync(upstreamStream, clientStream)).ConfigureAwait(false);
+        }
+        catch (Exception exception) when (exception is IOException or SocketException or OperationCanceledException or ObjectDisposedException)
+        {
+        }
+        finally
+        {
+            _live.TryRemove(client, out _);
+            client.Dispose();
+            if (upstream is not null)
+            {
+                _live.TryRemove(upstream, out _);
+                upstream.Dispose();
+            }
+        }
+    }
+
+    private async Task CopyAsync(NetworkStream from, NetworkStream to)
+    {
+        try
+        {
+            await from.CopyToAsync(to, _cts.Token).ConfigureAwait(false);
+        }
+        catch (Exception exception) when (exception is IOException or SocketException or OperationCanceledException or ObjectDisposedException)
+        {
+        }
+    }
+}

tests/Jellyfin.Server.Tests/HighAvailability/RedisTestServer.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Threading.Tasks;
+using StackExchange.Redis;
+using Testcontainers.Redis;
+
+namespace Jellyfin.Server.Tests.HighAvailability;
+
+/// <summary>
+/// Hands out a Redis server for the tests that need one. A server named by <c>JELLYFIN_TEST_REDIS</c> is
+/// used as is, so CI can run one beside the step instead of a docker daemon of its own; without it a
+/// container is started through testcontainers.
+/// </summary>
+public sealed class RedisTestServer : IAsyncDisposable
+{
+    /// <summary>
+    /// The connection string of an already running server.
+    /// </summary>
+    public const string ConnectionStringVariable = "JELLYFIN_TEST_REDIS";
+
+    private readonly RedisContainer? _container;
+
+    private RedisTestServer(RedisContainer? container, string connectionString)
+    {
+        _container = container;
+        ConnectionString = connectionString;
+    }
+
+    /// <summary>
+    /// Gets the connection string of the running server.
+    /// </summary>
+    public string ConnectionString { get; }
+
+    /// <summary>
+    /// Starts or attaches to a Redis server and waits until it accepts connections.
+    /// </summary>
+    /// <returns>The running server.</returns>
+    public static async Task<RedisTestServer> StartAsync()
+    {
+        var provided = Environment.GetEnvironmentVariable(ConnectionStringVariable);
+        if (!string.IsNullOrWhiteSpace(provided))
+        {
+            var attached = new RedisTestServer(null, provided);
+            await attached.WaitUntilReadyAsync().ConfigureAwait(false);
+            return attached;
+        }
+
+        var container = new RedisBuilder("redis:7-alpine").Build();
+        await container.StartAsync().ConfigureAwait(false);
+
+        var started = new RedisTestServer(container, container.GetConnectionString());
+        await started.WaitUntilReadyAsync().ConfigureAwait(false);
+        return started;
+    }
+
+    /// <summary>
+    /// Opens a connection of its own, so each in-process stand-in for a replica talks to the server the
+    /// way a separate pod would.
+    /// </summary>
+    /// <returns>A new multiplexer.</returns>
+    public async Task<IConnectionMultiplexer> ConnectAsync()
+        => await ConnectionMultiplexer.ConnectAsync(ConnectionString).ConfigureAwait(false);
+
+    /// <inheritdoc/>
+    public async ValueTask DisposeAsync()
+    {
+        if (_container is not null)
+        {
+            await _container.DisposeAsync().ConfigureAwait(false);
+        }
+    }
+
+    private async Task WaitUntilReadyAsync()
+    {
+        for (var attempt = 1; ; attempt++)
+        {
+            try
+            {
+                var connection = await ConnectionMultiplexer.ConnectAsync(ConnectionString).ConfigureAwait(false);
+                await using (connection.ConfigureAwait(false))
+                {
+                    await connection.GetDatabase().PingAsync().ConfigureAwait(false);
+                    return;
+                }
+            }
+            catch (RedisException) when (attempt < 60)
+            {
+                await Task.Delay(TimeSpan.FromSeconds(1)).ConfigureAwait(false);
+            }
+        }
+    }
+}

tests/Jellyfin.Server.Tests/Jellyfin.Server.Tests.csproj

@@ -12,6 +12,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="Npgsql" />
     <PackageReference Include="Testcontainers.PostgreSql" />
+    <PackageReference Include="Testcontainers.Redis" />
     <PackageReference Include="xunit.v3" />
     <PackageReference Include="xunit.runner.visualstudio">
       <PrivateAssets>all</PrivateAssets>

tests/Jellyfin.Server.Tests/QuickConnect/QuickConnectReplicaTests.cs

@@ -0,0 +1,369 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Threading;
+using System.Threading.Tasks;
+using Emby.Server.Implementations.QuickConnect;
+using Jellyfin.Data.Queries;
+using Jellyfin.Database.Implementations;
+using Jellyfin.Database.Implementations.DbConfiguration;
+using Jellyfin.Database.Implementations.Entities;
+using Jellyfin.Database.Implementations.Entities.Security;
+using Jellyfin.Database.Implementations.Locking;
+using Jellyfin.Database.Providers.PostgreSQL;
+using Jellyfin.Server.Implementations.Devices;
+using Jellyfin.Server.Tests.HighAvailability;
+using Jellyfin.Server.Tests.Migrations;
+using MediaBrowser.Common.Extensions;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Controller.Configuration;
+using MediaBrowser.Controller.Devices;
+using MediaBrowser.Controller.Library;
+using MediaBrowser.Controller.Net;
+using MediaBrowser.Controller.QuickConnect;
+using MediaBrowser.Controller.Session;
+using MediaBrowser.Model.Configuration;
+using MediaBrowser.Model.Dto;
+using MediaBrowser.Model.QuickConnect;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using Npgsql;
+using StackExchange.Redis;
+using Xunit;
+
+namespace Jellyfin.Server.Tests.QuickConnect;
+
+/// <summary>
+/// Three independently constructed <see cref="QuickConnectManager"/> instances over one PostgreSQL
+/// database and one Redis are the in-process stand-in for three replicas without sticky sessions: the
+/// initiate, authorize and exchange legs of one flow each land on a different one.
+/// </summary>
+[Trait("Category", "RequiresDocker")]
+public sealed class QuickConnectReplicaTests : IAsyncLifetime
+{
+    private static readonly AuthorizationInfo _authorizationInfo = new AuthorizationInfo
+    {
+        Device = "Living Room TV",
+        DeviceId = "device-1",
+        Client = "Jellyfin Web",
+        Version = "1.0.0"
+    };
+
+    private readonly List<IConnectionMultiplexer> _connections = new();
+
+    private PostgreSqlTestServer _postgres = null!;
+    private RedisTestServer _redis = null!;
+
+    /// <inheritdoc/>
+    public async ValueTask InitializeAsync()
+    {
+        _postgres = await PostgreSqlTestServer.StartAsync().ConfigureAwait(false);
+        _redis = await RedisTestServer.StartAsync().ConfigureAwait(false);
+    }
+
+    /// <inheritdoc/>
+    public async ValueTask DisposeAsync()
+    {
+        foreach (var connection in _connections)
+        {
+            await connection.DisposeAsync().ConfigureAwait(false);
+        }
+
+        await _redis.DisposeAsync().ConfigureAwait(false);
+        await _postgres.DisposeAsync().ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// The three legs of a quick connect flow land on three different replicas, and the token the third
+    /// one hands out is the one the second one minted into the shared database.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task InitiateAuthorizeExchange_AcrossThreeReplicas_Succeeds()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var connectionString = await _postgres.CreateDatabaseAsync("quickconnect_replica_flow", cancellationToken);
+
+        await using var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        var user = await CreateSchemaWithUserAsync(dataSource, cancellationToken);
+
+        var replicaA = await CreateReplicaAsync(dataSource, user);
+        var replicaB = await CreateReplicaAsync(dataSource, user);
+        var replicaC = await CreateReplicaAsync(dataSource, user);
+
+        var initiated = await replicaA.Manager.TryConnect(_authorizationInfo);
+
+        // The code is shown to the user on whichever replica serves the dashboard.
+        Assert.True(await replicaB.Manager.AuthorizeRequest(user.Id, initiated.Code));
+
+        var polled = await replicaC.Manager.CheckRequestStatus(initiated.Secret);
+        Assert.True(polled.Authenticated);
+        Assert.Equal(initiated.Code, polled.Code);
+        Assert.Equal(_authorizationInfo.DeviceId, polled.DeviceId);
+
+        var exchanged = await replicaC.Manager.GetAuthorizedRequest(initiated.Secret);
+
+        Assert.False(string.IsNullOrEmpty(exchanged.AccessToken));
+        Assert.Equal(user.Id, exchanged.User.Id);
+
+        var devices = await replicaA.Devices.GetDevices(new DeviceQuery { AccessToken = exchanged.AccessToken });
+        Assert.Equal(user.Id, Assert.Single(devices.Items).UserId);
+    }
+
+    /// <summary>
+    /// A secret is single use across the whole deployment: two replicas racing to exchange it must not
+    /// both hand out an access token. One scheduling of one race settles nothing either way, so the race
+    /// is run repeatedly.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Exchange_RacedOnTwoReplicas_SucceedsOnce()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var connectionString = await _postgres.CreateDatabaseAsync("quickconnect_replica_race", cancellationToken);
+
+        await using var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        var user = await CreateSchemaWithUserAsync(dataSource, cancellationToken);
+
+        var replicaA = await CreateReplicaAsync(dataSource, user);
+        var replicaB = await CreateReplicaAsync(dataSource, user);
+        var replicaC = await CreateReplicaAsync(dataSource, user);
+
+        for (var attempt = 0; attempt < 25; attempt++)
+        {
+            var initiated = await replicaA.Manager.TryConnect(AuthorizationInfoFor(attempt));
+            await replicaB.Manager.AuthorizeRequest(user.Id, initiated.Code);
+
+            var outcomes = await Task.WhenAll(
+                Task.Run(() => ExchangeAsync(replicaA.Manager, initiated.Secret), cancellationToken),
+                Task.Run(() => ExchangeAsync(replicaC.Manager, initiated.Secret), cancellationToken));
+
+            Assert.Single(outcomes, outcome => outcome is not null);
+
+            // And it stays consumed for every later attempt, on any replica.
+            await Assert.ThrowsAsync<ResourceNotFoundException>(() => replicaB.Manager.GetAuthorizedRequest(initiated.Secret));
+        }
+    }
+
+    /// <summary>
+    /// Two replicas authorizing one code at the same time mint one access token between them. A second
+    /// one would be live, attached to the same device and reachable by nobody.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Authorize_RacedOnTwoReplicas_MintsOneAccessToken()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var connectionString = await _postgres.CreateDatabaseAsync("quickconnect_replica_authorize_race", cancellationToken);
+
+        await using var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        var user = await CreateSchemaWithUserAsync(dataSource, cancellationToken);
+
+        var replicaA = await CreateReplicaAsync(dataSource, user);
+        var replicaB = await CreateReplicaAsync(dataSource, user);
+        var replicaC = await CreateReplicaAsync(dataSource, user);
+
+        for (var attempt = 0; attempt < 20; attempt++)
+        {
+            var authorizationInfo = AuthorizationInfoFor(attempt);
+            var initiated = await replicaA.Manager.TryConnect(authorizationInfo);
+
+            var outcomes = await Task.WhenAll(
+                Task.Run(() => AuthorizeAsync(replicaB.Manager, user.Id, initiated.Code), cancellationToken),
+                Task.Run(() => AuthorizeAsync(replicaC.Manager, user.Id, initiated.Code), cancellationToken));
+
+            Assert.Single(outcomes, authorized => authorized);
+
+            var devices = await replicaA.Devices.GetDevices(new DeviceQuery { DeviceId = authorizationInfo.DeviceId });
+            var device = Assert.Single(devices.Items);
+
+            var exchanged = await replicaA.Manager.GetAuthorizedRequest(initiated.Secret);
+            Assert.Equal(device.AccessToken, exchanged.AccessToken);
+        }
+    }
+
+    /// <summary>
+    /// An expired request is rejected on a replica that never saw it created, rather than resolving to a
+    /// stale authorization.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task ExpiredRequest_IsRejectedOnEveryReplica()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var connectionString = await _postgres.CreateDatabaseAsync("quickconnect_replica_expiry", cancellationToken);
+
+        await using var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        var user = await CreateSchemaWithUserAsync(dataSource, cancellationToken);
+
+        var replicaA = await CreateReplicaAsync(dataSource, user);
+        var replicaB = await CreateReplicaAsync(dataSource, user);
+
+        var initiated = await replicaA.Manager.TryConnect(_authorizationInfo);
+        Assert.NotNull(await replicaB.Manager.CheckRequestStatus(initiated.Secret));
+
+        // Shorten the stored expiry instead of waiting out the ten minute timeout.
+        await replicaA.Store.SetRequestAsync(initiated, DateTime.UtcNow.AddSeconds(1), cancellationToken);
+        await Task.Delay(TimeSpan.FromSeconds(2), cancellationToken);
+
+        await Assert.ThrowsAsync<ResourceNotFoundException>(() => replicaB.Manager.CheckRequestStatus(initiated.Secret));
+        await Assert.ThrowsAsync<ResourceNotFoundException>(() => replicaB.Manager.AuthorizeRequest(user.Id, initiated.Code));
+    }
+
+    /// <summary>
+    /// An authorization that was never exchanged expires too, so a code authorized and then abandoned
+    /// cannot be redeemed later from another replica.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task ExpiredAuthorization_IsRejectedOnEveryReplica()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var connectionString = await _postgres.CreateDatabaseAsync("quickconnect_replica_auth_expiry", cancellationToken);
+
+        await using var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        var user = await CreateSchemaWithUserAsync(dataSource, cancellationToken);
+
+        var replicaA = await CreateReplicaAsync(dataSource, user);
+        var replicaB = await CreateReplicaAsync(dataSource, user);
+
+        var initiated = await replicaA.Manager.TryConnect(_authorizationInfo);
+        await replicaA.Manager.AuthorizeRequest(user.Id, initiated.Code);
+
+        var stored = await replicaA.Store.GetRequestBySecretAsync(initiated.Secret, cancellationToken);
+        Assert.True(stored?.Authenticated);
+
+        await replicaA.Store.SetAuthorizationAsync(
+            initiated.Secret,
+            new AuthenticationResult { AccessToken = "stale" },
+            DateTime.UtcNow.AddSeconds(1),
+            cancellationToken);
+        await Task.Delay(TimeSpan.FromSeconds(2), cancellationToken);
+
+        await Assert.ThrowsAsync<ResourceNotFoundException>(() => replicaB.Manager.GetAuthorizedRequest(initiated.Secret));
+    }
+
+    private static AuthorizationInfo AuthorizationInfoFor(int attempt) => new AuthorizationInfo
+    {
+        Device = _authorizationInfo.Device,
+        DeviceId = string.Create(CultureInfo.InvariantCulture, $"device-{attempt}"),
+        Client = _authorizationInfo.Client,
+        Version = _authorizationInfo.Version
+    };
+
+    private static async Task<bool> AuthorizeAsync(IQuickConnect manager, Guid userId, string code)
+    {
+        try
+        {
+            return await manager.AuthorizeRequest(userId, code).ConfigureAwait(false);
+        }
+        catch (InvalidOperationException)
+        {
+            return false;
+        }
+    }
+
+    private static async Task<AuthenticationResult?> ExchangeAsync(IQuickConnect manager, string secret)
+    {
+        try
+        {
+            return await manager.GetAuthorizedRequest(secret).ConfigureAwait(false);
+        }
+        catch (ResourceNotFoundException)
+        {
+            return null;
+        }
+    }
+
+    private static async Task<User> CreateSchemaWithUserAsync(NpgsqlDataSource dataSource, CancellationToken cancellationToken)
+    {
+        var context = CreateContext(dataSource);
+        await using (context.ConfigureAwait(false))
+        {
+            await context.Database.EnsureCreatedAsync(cancellationToken).ConfigureAwait(false);
+
+            var user = new User("quickconnect-user", "provider", "provider");
+            context.Users.Add(user);
+            await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+
+            return user;
+        }
+    }
+
+    private static JellyfinDbContext CreateContext(NpgsqlDataSource dataSource)
+    {
+        var optionsBuilder = new DbContextOptionsBuilder<JellyfinDbContext>();
+        var provider = new PostgreSqlDatabaseProvider(dataSource);
+        provider.Initialise(optionsBuilder, new DatabaseConfigurationOptions { DatabaseType = "PostgreSQL" });
+        return new JellyfinDbContext(
+            optionsBuilder.Options,
+            NullLogger<JellyfinDbContext>.Instance,
+            provider,
+            new NoLockBehavior(NullLogger<NoLockBehavior>.Instance));
+    }
+
+    private async Task<Replica> CreateReplicaAsync(NpgsqlDataSource dataSource, User user)
+    {
+        var connection = await _redis.ConnectAsync().ConfigureAwait(false);
+        _connections.Add(connection);
+
+        var userManager = new Mock<IUserManager>();
+        userManager.Setup(manager => manager.GetUserById(user.Id)).Returns(user);
+        var deviceManager = new DeviceManager(new DataSourceContextFactory(dataSource), userManager.Object);
+
+        var configManager = new Mock<IServerConfigurationManager>();
+        configManager.Setup(manager => manager.Configuration).Returns(new ServerConfiguration { QuickConnectAvailable = true });
+
+        // Stands in for SessionManager.AuthenticateDirect: the token has to be minted into the shared
+        // database, because the replica that exchanges the secret is not the one that authorized it.
+        var sessionManager = new Mock<ISessionManager>();
+        sessionManager
+            .Setup(manager => manager.AuthenticateDirect(It.IsAny<AuthenticationRequest>()))
+            .Returns<AuthenticationRequest>(async request =>
+            {
+                var device = await deviceManager.CreateDevice(
+                    new Device(request.UserId, request.App, request.AppVersion, request.DeviceName, request.DeviceId)).ConfigureAwait(false);
+
+                return new AuthenticationResult
+                {
+                    AccessToken = device.AccessToken,
+                    ServerId = "server-1",
+                    User = new UserDto { Id = user.Id, Name = user.Username, ServerId = "server-1" },
+                    SessionInfo = new SessionInfoDto
+                    {
+                        Id = device.Id.ToString(CultureInfo.InvariantCulture),
+                        UserId = user.Id,
+                        UserName = user.Username,
+                        Client = request.App,
+                        DeviceId = request.DeviceId,
+                        DeviceName = request.DeviceName,
+                        ApplicationVersion = request.AppVersion
+                    }
+                };
+            });
+
+        var store = new RedisQuickConnectStore(connection, NullLogger<RedisQuickConnectStore>.Instance);
+        var manager = new QuickConnectManager(
+            configManager.Object,
+            NullLogger<QuickConnectManager>.Instance,
+            sessionManager.Object,
+            store);
+
+        return new Replica(manager, store, deviceManager);
+    }
+
+    private sealed record Replica(IQuickConnect Manager, IQuickConnectStore Store, IDeviceManager Devices);
+
+    private sealed class DataSourceContextFactory : IDbContextFactory<JellyfinDbContext>
+    {
+        private readonly NpgsqlDataSource _dataSource;
+
+        public DataSourceContextFactory(NpgsqlDataSource dataSource)
+        {
+            _dataSource = dataSource;
+        }
+
+        public JellyfinDbContext CreateDbContext() => CreateContext(_dataSource);
+    }
+}

tests/Jellyfin.Server.Tests/QuickConnect/QuickConnectStoreWiringTests.cs

@@ -0,0 +1,142 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using Emby.Server.Implementations.QuickConnect;
+using Jellyfin.Server.Extensions;
+using Jellyfin.Server.Tests.HighAvailability;
+using MediaBrowser.Common.Configuration;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Controller.QuickConnect;
+using MediaBrowser.Model.QuickConnect;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using StackExchange.Redis;
+using Xunit;
+
+namespace Jellyfin.Server.Tests.QuickConnect;
+
+/// <summary>
+/// Drives the whole configuration path a deployment uses: a bare
+/// <c>Jellyfin__TranscodeStore__RedisConnectionString</c> environment variable, the server's own
+/// configuration builder, the store registration, and a quick connect flow against a real valkey.
+/// </summary>
+[Trait("Category", "RequiresDocker")]
+[Collection("JellyfinSectionConfiguration")]
+public sealed class QuickConnectStoreWiringTests : IAsyncLifetime
+{
+    private const string RedisConnectionStringVariable = "Jellyfin__TranscodeStore__RedisConnectionString";
+
+    private RedisTestServer _redis = null!;
+    private string _configDirectory = string.Empty;
+
+    /// <inheritdoc/>
+    public async ValueTask InitializeAsync()
+    {
+        _redis = await RedisTestServer.StartAsync().ConfigureAwait(false);
+        _configDirectory = Directory.CreateTempSubdirectory("jellyfin-quickconnect-wiring").FullName;
+        await File.WriteAllTextAsync(Path.Combine(_configDirectory, "logging.default.json"), "{}").ConfigureAwait(false);
+    }
+
+    /// <inheritdoc/>
+    public async ValueTask DisposeAsync()
+    {
+        Environment.SetEnvironmentVariable(RedisConnectionStringVariable, null);
+
+        if (_configDirectory.Length > 0)
+        {
+            Directory.Delete(_configDirectory, true);
+        }
+
+        await _redis.DisposeAsync().ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// The variable form deployments set selects the shared store, and that store really talks to valkey.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task ManifestStyleEnvironmentVariable_SelectsTheSharedStore()
+    {
+        Environment.SetEnvironmentVariable(RedisConnectionStringVariable, _redis.ConnectionString + ",abortConnect=false");
+
+        await using var provider = BuildProvider();
+
+        var store = provider.GetRequiredService<IQuickConnectStore>();
+        Assert.IsType<RedisQuickConnectStore>(store);
+
+        var request = NewRequest();
+        await store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), TestContext.Current.CancellationToken);
+
+        Assert.True(await store.TryClaimAuthorizationAsync(request.Secret, DateTime.UtcNow.AddMinutes(10), TestContext.Current.CancellationToken));
+
+        var redis = provider.GetRequiredService<IConnectionMultiplexer>();
+        Assert.True(await redis.GetDatabase().KeyExistsAsync("jellyfin:quickconnect:request:" + request.Secret));
+    }
+
+    /// <summary>
+    /// Without the variable the deployment is single-instance and gets the process-local store.
+    /// </summary>
+    [Fact]
+    public void NoEnvironmentVariable_SelectsTheProcessLocalStore()
+    {
+        Environment.SetEnvironmentVariable(RedisConnectionStringVariable, null);
+
+        using var provider = BuildProvider();
+
+        Assert.IsType<InMemoryQuickConnectStore>(provider.GetRequiredService<IQuickConnectStore>());
+    }
+
+    /// <summary>
+    /// A configured but unreachable Redis degrades to the single-instance behaviour of a flow having to
+    /// complete against one instance, rather than taking quick connect down at startup.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task UnreachableRedisAtStartup_DegradesToTheProcessLocalStore()
+    {
+        Environment.SetEnvironmentVariable(RedisConnectionStringVariable, "127.0.0.1:1,connectTimeout=250,connectRetry=0");
+
+        await using var provider = BuildProvider();
+
+        var store = provider.GetRequiredService<IQuickConnectStore>();
+        Assert.IsType<InMemoryQuickConnectStore>(store);
+
+        // Quick connect still works, it just cannot span instances.
+        var request = NewRequest();
+        await store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), TestContext.Current.CancellationToken);
+        Assert.True(await store.TryClaimAuthorizationAsync(request.Secret, DateTime.UtcNow.AddMinutes(10), TestContext.Current.CancellationToken));
+        await store.SetAuthorizationAsync(
+            request.Secret,
+            new AuthenticationResult { AccessToken = "token-1" },
+            DateTime.UtcNow.AddMinutes(10),
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal("token-1", (await store.TryConsumeAuthorizationAsync(request.Secret, TestContext.Current.CancellationToken))?.AccessToken);
+        Assert.Null(await store.TryConsumeAuthorizationAsync(request.Secret, TestContext.Current.CancellationToken));
+    }
+
+    private static QuickConnectResult NewRequest() => new QuickConnectResult(
+        Guid.NewGuid().ToString("N"),
+        Guid.NewGuid().ToString("N").Substring(0, 6),
+        DateTime.UtcNow,
+        "device-1",
+        "Living Room TV",
+        "Jellyfin Web",
+        "1.0.0");
+
+    private ServiceProvider BuildProvider()
+    {
+        var appPaths = new Mock<IApplicationPaths>();
+        appPaths.Setup(paths => paths.ConfigurationDirectoryPath).Returns(_configDirectory);
+        IConfiguration configuration = Jellyfin.Server.Program.CreateAppConfiguration(new StartupOptions(), appPaths.Object);
+
+        var services = new ServiceCollection();
+        services.AddLogging();
+        services.AddTranscodeSessionStore(configuration, NullLogger.Instance);
+        services.AddQuickConnectStore(configuration, NullLogger.Instance);
+
+        return services.BuildServiceProvider();
+    }
+}

tests/Jellyfin.Server.Tests/QuickConnect/RedisQuickConnectStoreDegradedTests.cs

@@ -0,0 +1,308 @@
+using System;
+using System.Collections.Generic;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using Emby.Server.Implementations.QuickConnect;
+using Jellyfin.Server.Tests.HighAvailability;
+using MediaBrowser.Controller.Authentication;
+using MediaBrowser.Model.QuickConnect;
+using Microsoft.Extensions.Logging.Abstractions;
+using StackExchange.Redis;
+using Xunit;
+
+namespace Jellyfin.Server.Tests.QuickConnect;
+
+/// <summary>
+/// What a <see cref="RedisQuickConnectStore"/> does while its Redis is unreachable. Each instance talks
+/// to the one real server through a proxy of its own, so an outage can be given to one instance and not
+/// the others, and then taken back.
+/// </summary>
+[Trait("Category", "RequiresDocker")]
+public sealed class RedisQuickConnectStoreDegradedTests : IAsyncLifetime
+{
+    private readonly List<RedisFaultProxy> _proxies = new();
+    private readonly List<IConnectionMultiplexer> _connections = new();
+
+    private RedisTestServer _redis = null!;
+
+    private static CancellationToken CancellationToken => TestContext.Current.CancellationToken;
+
+    /// <inheritdoc/>
+    public async ValueTask InitializeAsync()
+    {
+        _redis = await RedisTestServer.StartAsync().ConfigureAwait(false);
+    }
+
+    /// <inheritdoc/>
+    public async ValueTask DisposeAsync()
+    {
+        foreach (var connection in _connections)
+        {
+            await connection.DisposeAsync().ConfigureAwait(false);
+        }
+
+        foreach (var proxy in _proxies)
+        {
+            await proxy.DisposeAsync().ConfigureAwait(false);
+        }
+
+        await _redis.DisposeAsync().ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// A request stored while Redis is unreachable is still resolvable on the instance that stored it,
+    /// so a flow whose three legs happen to land on one instance keeps working through the outage.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task PendingRequest_SurvivesAnOutage_OnTheInstanceThatStoredIt()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+
+        instance.Proxy.Cut();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+
+        Assert.Equal(request.Secret, (await instance.Store.GetRequestBySecretAsync(request.Secret, CancellationToken))?.Secret);
+        Assert.Equal(request.Secret, (await instance.Store.GetRequestByCodeAsync(request.Code, CancellationToken))?.Secret);
+    }
+
+    /// <summary>
+    /// Once Redis answers again it is the only authority: a miss is a miss, not a reason to serve the
+    /// copy this instance kept while it was unreachable.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task PendingRequest_StoredDuringAnOutage_IsNotServedOnceRedisAnswersAgain()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+
+        instance.Proxy.Cut();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+        Assert.NotNull(await instance.Store.GetRequestBySecretAsync(request.Secret, CancellationToken));
+
+        await RestoreAsync(instance);
+
+        Assert.Null(await instance.Store.GetRequestBySecretAsync(request.Secret, CancellationToken));
+        Assert.Null(await instance.Store.GetRequestByCodeAsync(request.Code, CancellationToken));
+    }
+
+    /// <summary>
+    /// A malformed stored value is a fault of its own, not a transport failure, so it is surfaced rather
+    /// than answered from the copy this instance happens to hold.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task PendingRequest_ThatIsMalformedInRedis_SurfacesInsteadOfDegrading()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+
+        instance.Proxy.Cut();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+        await RestoreAsync(instance);
+
+        await instance.Connection.GetDatabase().StringSetAsync(
+            "jellyfin:quickconnect:request:" + request.Secret,
+            "{ not json",
+            TimeSpan.FromMinutes(10));
+
+        await Assert.ThrowsAsync<JsonException>(() => instance.Store.GetRequestBySecretAsync(request.Secret, CancellationToken));
+    }
+
+    /// <summary>
+    /// An authorization write that failed leaves nothing behind on the instance, because the response
+    /// that never arrived may still have been applied and a second copy of an authorization is a second
+    /// access token.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Authorization_ThatFailedToStore_LeavesNothingOnTheInstance()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+
+        instance.Proxy.Cut();
+        await AssertTransportFailureAsync(() => instance.Store.SetAuthorizationAsync(
+            request.Secret,
+            new AuthenticationResult { AccessToken = "token-1" },
+            DateTime.UtcNow.AddMinutes(10),
+            CancellationToken));
+
+        await RestoreAsync(instance);
+
+        Assert.Null(await instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken));
+    }
+
+    /// <summary>
+    /// The instance whose authorization write failed while the write landed anyway still hands the token
+    /// out exactly once, rather than once from Redis and again from a copy of its own.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Authorization_IsHandedOutOnce_EvenAfterAFailedWriteOnTheSameInstance()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+        var authentication = new AuthenticationResult { AccessToken = "token-1" };
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+
+        instance.Proxy.Cut();
+        await AssertTransportFailureAsync(() => instance.Store.SetAuthorizationAsync(
+            request.Secret,
+            authentication,
+            DateTime.UtcNow.AddMinutes(10),
+            CancellationToken));
+
+        await RestoreAsync(instance);
+
+        // Stands in for that write having been applied before the response was lost.
+        await instance.Store.SetAuthorizationAsync(request.Secret, authentication, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+
+        Assert.Equal("token-1", (await instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken))?.AccessToken);
+        Assert.Null(await instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken));
+    }
+
+    /// <summary>
+    /// Exchanging during an outage fails loudly and spends nothing, so the token is still there to be
+    /// handed out once when Redis comes back.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Exchange_DuringAnOutage_SurfacesTheFailureAndLeavesTheTokenUnspent()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+        await instance.Store.SetAuthorizationAsync(
+            request.Secret,
+            new AuthenticationResult { AccessToken = "token-1" },
+            DateTime.UtcNow.AddMinutes(10),
+            CancellationToken);
+
+        instance.Proxy.Cut();
+        await AssertTransportFailureAsync(() => instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken));
+
+        await RestoreAsync(instance);
+
+        Assert.Equal("token-1", (await instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken))?.AccessToken);
+        Assert.Null(await instance.Store.TryConsumeAuthorizationAsync(request.Secret, CancellationToken));
+    }
+
+    /// <summary>
+    /// Authorizing during an outage fails loudly rather than claiming locally, because a claim only this
+    /// instance knows about does not stop another one minting a second access token.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Claim_DuringAnOutage_SurfacesTheFailure()
+    {
+        var instance = await CreateInstanceAsync();
+        var request = NewRequest();
+        await instance.Store.SetRequestAsync(request, DateTime.UtcNow.AddMinutes(10), CancellationToken);
+
+        instance.Proxy.Cut();
+        await AssertTransportFailureAsync(() => instance.Store.TryClaimAuthorizationAsync(
+            request.Secret,
+            DateTime.UtcNow.AddMinutes(10),
+            CancellationToken));
+    }
+
+    /// <summary>
+    /// Two instances racing to authorize one request: exactly one of them may go on to mint a token.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Claim_RacedOnTwoInstances_SucceedsOnce()
+    {
+        var first = await CreateInstanceAsync();
+        var second = await CreateInstanceAsync();
+
+        for (var attempt = 0; attempt < 25; attempt++)
+        {
+            var request = NewRequest();
+            var expiresUtc = DateTime.UtcNow.AddMinutes(10);
+            await first.Store.SetRequestAsync(request, expiresUtc, CancellationToken);
+
+            var claims = await Task.WhenAll(
+                Task.Run(() => first.Store.TryClaimAuthorizationAsync(request.Secret, expiresUtc, CancellationToken), CancellationToken),
+                Task.Run(() => second.Store.TryClaimAuthorizationAsync(request.Secret, expiresUtc, CancellationToken), CancellationToken));
+
+            Assert.Single(claims, claimed => claimed);
+        }
+    }
+
+    /// <summary>
+    /// A request that is unknown, already claimed or already authorized cannot be claimed.
+    /// </summary>
+    /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+    [Fact]
+    public async Task Claim_IsRefused_ForUnknownClaimedAndAuthorizedRequests()
+    {
+        var instance = await CreateInstanceAsync();
+        var expiresUtc = DateTime.UtcNow.AddMinutes(10);
+
+        Assert.False(await instance.Store.TryClaimAuthorizationAsync("unknown-secret", expiresUtc, CancellationToken));
+
+        var request = NewRequest();
+        await instance.Store.SetRequestAsync(request, expiresUtc, CancellationToken);
+        Assert.True(await instance.Store.TryClaimAuthorizationAsync(request.Secret, expiresUtc, CancellationToken));
+        Assert.False(await instance.Store.TryClaimAuthorizationAsync(request.Secret, expiresUtc, CancellationToken));
+
+        var authorized = NewRequest();
+        authorized.Authenticated = true;
+        await instance.Store.SetRequestAsync(authorized, expiresUtc, CancellationToken);
+        Assert.False(await instance.Store.TryClaimAuthorizationAsync(authorized.Secret, expiresUtc, CancellationToken));
+    }
+
+    private static QuickConnectResult NewRequest() => new QuickConnectResult(
+        Guid.NewGuid().ToString("N"),
+        Guid.NewGuid().ToString("N").Substring(0, 6),
+        DateTime.UtcNow,
+        "device-1",
+        "Living Room TV",
+        "Jellyfin Web",
+        "1.0.0");
+
+    private static async Task AssertTransportFailureAsync(Func<Task> operation)
+    {
+        var exception = await Record.ExceptionAsync(operation);
+
+        Assert.NotNull(exception);
+        Assert.True(exception is RedisException or TimeoutException, exception.ToString());
+    }
+
+    private static async Task RestoreAsync(Instance instance)
+    {
+        instance.Proxy.Restore();
+
+        for (var attempt = 1; ; attempt++)
+        {
+            try
+            {
+                await instance.Connection.GetDatabase().PingAsync();
+                return;
+            }
+            catch (Exception exception) when (exception is RedisException or TimeoutException && attempt < 60)
+            {
+                await Task.Delay(TimeSpan.FromMilliseconds(500), CancellationToken);
+            }
+        }
+    }
+
+    private async Task<Instance> CreateInstanceAsync()
+    {
+        var proxy = RedisFaultProxy.Start(_redis.ConnectionString);
+        _proxies.Add(proxy);
+
+        var connection = await ConnectionMultiplexer.ConnectAsync(proxy.ConnectionString).ConfigureAwait(false);
+        _connections.Add(connection);
+
+        return new Instance(proxy, connection, new RedisQuickConnectStore(connection, NullLogger<RedisQuickConnectStore>.Instance));
+    }
+
+    private sealed record Instance(RedisFaultProxy Proxy, IConnectionMultiplexer Connection, RedisQuickConnectStore Store);
+}