feat(headless): add deterministic multi-session scheduler

src/AcDream.Headless/Hosting/HeadlessMonotonicTime.cs

@@ -0,0 +1,25 @@
+namespace AcDream.Headless.Hosting;
+
+internal static class HeadlessMonotonicTime
+{
+    internal static long Add(
+        TimeProvider timeProvider,
+        long timestamp,
+        TimeSpan duration)
+    {
+        ArgumentNullException.ThrowIfNull(timeProvider);
+        if (duration < TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(duration));
+
+        double ticks =
+            duration.TotalSeconds * timeProvider.TimestampFrequency;
+        if (!double.IsFinite(ticks) || ticks > long.MaxValue)
+            return long.MaxValue;
+        long delta = Math.Max(
+            1L,
+            checked((long)Math.Ceiling(ticks)));
+        return timestamp > long.MaxValue - delta
+            ? long.MaxValue
+            : timestamp + delta;
+    }
+}

src/AcDream.Headless/Hosting/HeadlessProcessHost.cs

@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using AcDream.Headless.Configuration;
 using AcDream.Headless.Credentials;
 using AcDream.Headless.Diagnostics;
@@ -10,12 +9,10 @@ namespace AcDream.Headless.Hosting;
 
 internal sealed class HeadlessProcessHost : IDisposable
 {
-    private static readonly TimeSpan K1TickInterval =
-        TimeSpan.FromMilliseconds(15);
-
-    private readonly HeadlessSessionHost _session;
+    private readonly HeadlessSessionHost[] _sessions;
+    private readonly HeadlessProcessScheduler _scheduler;
     private readonly HeadlessDiagnosticWriter _diagnostics;
-    private readonly string _sessionId;
+    private int _disposeIndex;
     private bool _disposed;
 
     internal HeadlessProcessHost(
@@ -30,28 +27,38 @@ internal sealed class HeadlessProcessHost : IDisposable
         ArgumentNullException.ThrowIfNull(paths);
         ArgumentNullException.ThrowIfNull(standardInput);
         ArgumentNullException.ThrowIfNull(diagnostics);
-        if (configuration.Sessions.Count != 1
-            || configuration.Sessions[0] is not { } descriptor)
+        if (configuration.Sessions.Count == 0)
         {
             throw new HeadlessConfigurationException(
-                "Slice K1 run mode requires exactly one session.");
+                "Headless run mode requires at least one session.");
         }
 
-        _sessionId = descriptor.Id;
         _diagnostics = new HeadlessDiagnosticWriter(diagnostics);
         var credentials = new HeadlessCredentialResolver(
             standardInput,
             paths.ConfigDirectory);
-        HeadlessCredentialSecret secret =
-            credentials.Resolve(descriptor.Id, descriptor.Credential);
+        var sessions = new List<HeadlessSessionHost>(
+            configuration.Sessions.Count);
         try
         {
-            _session = new HeadlessSessionHost(
+            foreach (HeadlessSessionDescriptor? candidate
+                in configuration.Sessions)
+            {
+                HeadlessSessionDescriptor descriptor = candidate
+                    ?? throw new HeadlessConfigurationException(
+                        "Headless run mode cannot contain a null session.");
+                HeadlessCredentialSecret secret =
+                    credentials.Resolve(
+                        descriptor.Id,
+                        descriptor.Credential);
+                try
+                {
+                    sessions.Add(new HeadlessSessionHost(
                         descriptor,
                         secret,
                         _diagnostics,
                         sessionOperations,
-                timeProvider);
+                        timeProvider));
                 }
                 catch
                 {
@@ -60,37 +67,57 @@ internal sealed class HeadlessProcessHost : IDisposable
                 }
             }
 
-    internal HeadlessSessionHost Session => _session;
+            _sessions = sessions.ToArray();
+            _scheduler = new HeadlessProcessScheduler(
+                _sessions,
+                timeProvider);
+            _disposeIndex = _sessions.Length - 1;
+        }
+        catch
+        {
+            for (int index = sessions.Count - 1; index >= 0; index--)
+                sessions[index].Dispose();
+            throw;
+        }
+    }
+
+    internal HeadlessSessionHost Session => _sessions.Length == 1
+        ? _sessions[0]
+        : throw new InvalidOperationException(
+            "The process owns more than one headless session.");
+    internal IReadOnlyList<HeadlessSessionHost> Sessions => _sessions;
+    internal HeadlessSchedulerSnapshot Scheduler =>
+        _scheduler.CaptureSnapshot();
 
     internal async Task<HeadlessExitCode> RunAsync(
         CancellationToken cancellationToken)
     {
         ObjectDisposedException.ThrowIf(_disposed, this);
-        RuntimeSessionStartResult started = _session.Start();
+        foreach (HeadlessSessionHost session in _sessions)
+        {
+            RuntimeSessionStartResult started = session.Start();
             if (started.Status != RuntimeSessionStartStatus.Connected)
             {
                 if (started.Error is { } error)
-                _diagnostics.Failure(_sessionId, "start", error);
+                {
+                    _diagnostics.Failure(
+                        session.SessionId,
+                        "start",
+                        error);
+                }
                 return HeadlessExitCode.ConnectionError;
             }
 
-        _diagnostics.Lifecycle(_sessionId, "running", _session.Runtime);
-        long previous = Stopwatch.GetTimestamp();
+            _diagnostics.Lifecycle(
+                session.SessionId,
+                "running",
+                session.Runtime);
+        }
+
         try
         {
-            while (!cancellationToken.IsCancellationRequested)
-            {
-                await Task.Delay(
-                    K1TickInterval,
-                    cancellationToken).ConfigureAwait(false);
-                long current = Stopwatch.GetTimestamp();
-                double deltaSeconds =
-                    (current - previous) / (double)Stopwatch.Frequency;
-                previous = current;
-                _session.Tick(deltaSeconds);
-                if (_session.IsPolicyComplete)
-                    break;
-            }
+            await _scheduler.RunAsync(cancellationToken)
+                .ConfigureAwait(false);
         }
         catch (OperationCanceledException)
             when (cancellationToken.IsCancellationRequested)
@@ -98,7 +125,7 @@ internal sealed class HeadlessProcessHost : IDisposable
         }
         catch (Exception error)
         {
-            _diagnostics.Failure(_sessionId, "tick", error);
+            _diagnostics.Failure("process", "tick", error);
             return HeadlessExitCode.RuntimeError;
         }
 
@@ -109,7 +136,11 @@ internal sealed class HeadlessProcessHost : IDisposable
     {
         if (_disposed)
             return;
-        _session.Dispose();
+        while (_disposeIndex >= 0)
+        {
+            _sessions[_disposeIndex].Dispose();
+            _disposeIndex--;
+        }
         _disposed = true;
     }
 }

src/AcDream.Headless/Hosting/HeadlessProcessScheduler.cs

@@ -0,0 +1,249 @@
+using AcDream.Runtime;
+
+namespace AcDream.Headless.Hosting;
+
+internal sealed class HeadlessProcessScheduler
+{
+    private sealed class Slot
+    {
+        internal Slot(
+            HeadlessSessionHost session,
+            long startTimestamp,
+            long periodTicks)
+        {
+            Session = session;
+            LastTurnTimestamp = startTimestamp;
+            NextDeadline = AddTicks(startTimestamp, periodTicks);
+        }
+
+        internal HeadlessSessionHost Session { get; }
+        internal long LastTurnTimestamp { get; set; }
+        internal long NextDeadline { get; set; }
+    }
+
+    private readonly TimeProvider _timeProvider;
+    private readonly long _periodTicks;
+    private readonly int _maximumCatchUpTurns;
+    private readonly Slot[] _slots;
+    private long _waitCount;
+    private long _turnCount;
+    private long _catchUpCollapseCount;
+
+    internal HeadlessProcessScheduler(
+        IReadOnlyList<HeadlessSessionHost> sessions,
+        TimeProvider? timeProvider = null,
+        TimeSpan? turnPeriod = null,
+        int maximumCatchUpTurns = 8)
+    {
+        ArgumentNullException.ThrowIfNull(sessions);
+        if (sessions.Count == 0)
+        {
+            throw new ArgumentException(
+                "A headless scheduler requires at least one session.",
+                nameof(sessions));
+        }
+        if (maximumCatchUpTurns <= 0)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(maximumCatchUpTurns));
+        }
+
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        TimeSpan configuredTurnPeriod =
+            turnPeriod ?? TimeSpan.FromMilliseconds(15);
+        if (configuredTurnPeriod <= TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(turnPeriod));
+        _periodTicks = DurationToTimestampTicks(
+            _timeProvider,
+            configuredTurnPeriod);
+        _maximumCatchUpTurns = maximumCatchUpTurns;
+
+        long start = _timeProvider.GetTimestamp();
+        _slots = new Slot[sessions.Count];
+        for (int index = 0; index < sessions.Count; index++)
+        {
+            _slots[index] = new Slot(
+                sessions[index]
+                    ?? throw new ArgumentException(
+                        "A headless scheduler session cannot be null.",
+                        nameof(sessions)),
+                start,
+                _periodTicks);
+        }
+    }
+
+    internal HeadlessSchedulerSnapshot CaptureSnapshot()
+    {
+        int activeSessionCount = 0;
+        for (int index = 0; index < _slots.Length; index++)
+        {
+            if (!_slots[index].Session.IsPolicyComplete)
+                activeSessionCount++;
+        }
+
+        return new HeadlessSchedulerSnapshot(
+            _slots.Length,
+            Interlocked.Read(ref _waitCount),
+            Interlocked.Read(ref _turnCount),
+            Interlocked.Read(ref _catchUpCollapseCount),
+            activeSessionCount,
+            NextDeadline());
+    }
+
+    internal async Task RunAsync(CancellationToken cancellationToken)
+    {
+        while (!cancellationToken.IsCancellationRequested
+            && HasActiveSession())
+        {
+            long now = _timeProvider.GetTimestamp();
+            if (DispatchDue(now))
+                continue;
+
+            long deadline = NextDeadline();
+            TimeSpan delay = deadline <= now
+                ? TimeSpan.Zero
+                : _timeProvider.GetElapsedTime(now, deadline);
+            Interlocked.Increment(ref _waitCount);
+            await Task.Delay(
+                    delay,
+                    _timeProvider,
+                    cancellationToken)
+                .ConfigureAwait(false);
+        }
+    }
+
+    internal bool DispatchDue(long nowTimestamp)
+    {
+        bool dispatched = false;
+        foreach (Slot slot in _slots)
+        {
+            HeadlessSessionHost session = slot.Session;
+            if (session.IsPolicyComplete)
+                continue;
+
+            if (session.IsReconnectPending)
+            {
+                if (nowTimestamp < session.ReconnectDeadline)
+                    continue;
+
+                RuntimeSessionStartResult result =
+                    session.CompletePendingReconnect(nowTimestamp);
+                if (result.Status
+                    is not RuntimeSessionStartStatus.Connected)
+                {
+                    throw result.Error
+                        ?? new InvalidOperationException(
+                            $"Headless session reconnect completed with {result.Status}.");
+                }
+                slot.LastTurnTimestamp = nowTimestamp;
+                slot.NextDeadline = AddTicks(
+                    nowTimestamp,
+                    _periodTicks);
+                dispatched = true;
+                continue;
+            }
+
+            if (nowTimestamp < slot.NextDeadline)
+                continue;
+
+            DispatchSessionDue(slot, nowTimestamp);
+            dispatched = true;
+        }
+        return dispatched;
+    }
+
+    private void DispatchSessionDue(Slot slot, long nowTimestamp)
+    {
+        long dueCount =
+            1L + ((nowTimestamp - slot.NextDeadline) / _periodTicks);
+        int turns = (int)Math.Min(
+            dueCount,
+            _maximumCatchUpTurns);
+        bool collapse = dueCount > _maximumCatchUpTurns;
+
+        for (int turn = 0; turn < turns; turn++)
+        {
+            if (slot.Session.IsPolicyComplete
+                || slot.Session.IsReconnectPending)
+            {
+                break;
+            }
+
+            bool finalCollapsedTurn =
+                collapse && turn == turns - 1;
+            long targetTimestamp = finalCollapsedTurn
+                ? nowTimestamp
+                : slot.NextDeadline;
+            double deltaSeconds = _timeProvider
+                .GetElapsedTime(
+                    slot.LastTurnTimestamp,
+                    targetTimestamp)
+                .TotalSeconds;
+            slot.Session.Tick(deltaSeconds);
+            Interlocked.Increment(ref _turnCount);
+            slot.LastTurnTimestamp = targetTimestamp;
+            slot.NextDeadline = AddTicks(
+                targetTimestamp,
+                _periodTicks);
+        }
+
+        if (collapse)
+            Interlocked.Increment(ref _catchUpCollapseCount);
+    }
+
+    private long NextDeadline()
+    {
+        long next = long.MaxValue;
+        foreach (Slot slot in _slots)
+        {
+            if (slot.Session.IsPolicyComplete)
+                continue;
+            long candidate = slot.Session.IsReconnectPending
+                ? slot.Session.ReconnectDeadline
+                : slot.NextDeadline;
+            if (candidate < next)
+                next = candidate;
+        }
+        return next;
+    }
+
+    private bool HasActiveSession()
+    {
+        for (int index = 0; index < _slots.Length; index++)
+        {
+            if (!_slots[index].Session.IsPolicyComplete)
+                return true;
+        }
+        return false;
+    }
+
+    private static long DurationToTimestampTicks(
+        TimeProvider timeProvider,
+        TimeSpan duration)
+    {
+        double ticks =
+            duration.TotalSeconds * timeProvider.TimestampFrequency;
+        if (!double.IsFinite(ticks)
+            || ticks <= 0d
+            || ticks > long.MaxValue)
+        {
+            throw new ArgumentOutOfRangeException(nameof(duration));
+        }
+        return Math.Max(
+            1L,
+            checked((long)Math.Ceiling(ticks)));
+    }
+
+    private static long AddTicks(long timestamp, long ticks) =>
+        timestamp > long.MaxValue - ticks
+            ? long.MaxValue
+            : timestamp + ticks;
+}
+
+internal readonly record struct HeadlessSchedulerSnapshot(
+    int SessionCount,
+    long WaitCount,
+    long TurnCount,
+    long CatchUpCollapseCount,
+    int ActiveSessionCount,
+    long NextDeadline);

src/AcDream.Headless/Hosting/HeadlessSessionHost.cs

@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using AcDream.Headless.Configuration;
 using AcDream.Headless.Credentials;
 using AcDream.Headless.Diagnostics;
@@ -47,12 +46,15 @@ internal sealed class HeadlessSessionHost : IDisposable
     private readonly HeadlessCredentialSecret _credential;
     private readonly HeadlessDiagnosticWriter _diagnostics;
     private readonly TimeSpan _reconnectQuiescence;
+    private readonly TimeProvider _timeProvider;
     private readonly HeadlessGenerationResetHost _resetHost = new();
     private readonly IDisposable _hostLease;
     private readonly IHeadlessBotPolicy _policy;
     private readonly IDisposable _policySubscription;
     private readonly LiveSessionHost _liveSession;
     private int _disposeStage;
+    private long _reconnectDeadline;
+    private bool _reconnectPending;
     private ulong _stoppedGeneration;
     private bool _disposed;
 
@@ -70,6 +72,7 @@ internal sealed class HeadlessSessionHost : IDisposable
             ?? throw new ArgumentNullException(nameof(credential));
         _diagnostics = diagnostics
             ?? throw new ArgumentNullException(nameof(diagnostics));
+        _timeProvider = timeProvider ?? TimeProvider.System;
         _reconnectQuiescence = reconnectQuiescence
             ?? (sessionOperations is null
                 ? TimeSpan.FromMilliseconds(2500)
@@ -92,7 +95,7 @@ internal sealed class HeadlessSessionHost : IDisposable
                 gameplay,
                 gameplay,
                 gameplay,
-                TimeProvider: timeProvider,
+                TimeProvider: _timeProvider,
                 Log: message => diagnostics.Message(
                     descriptor.Id,
                     message),
@@ -169,9 +172,15 @@ internal sealed class HeadlessSessionHost : IDisposable
 
     internal GameRuntime Runtime { get; }
     internal DirectGameRuntimeCommandAdapter Commands { get; }
+    internal string SessionId => _descriptor.Id;
     internal string ActiveCharacterName { get; private set; } =
         string.Empty;
     internal bool IsPolicyComplete => _policy.IsComplete;
+    internal bool IsReconnectPending => _reconnectPending;
+    internal long ReconnectDeadline => _reconnectPending
+        ? _reconnectDeadline
+        : throw new InvalidOperationException(
+            "The headless session has no pending reconnect.");
 
     internal RuntimeSessionStartResult Start() =>
         Commands.Session.Start(Runtime.Generation);
@@ -182,6 +191,8 @@ internal sealed class HeadlessSessionHost : IDisposable
     internal void Tick(double deltaSeconds)
     {
         ObjectDisposedException.ThrowIf(_disposed, this);
+        if (_reconnectPending)
+            return;
         _ = Runtime.Clock.Advance(deltaSeconds);
         Runtime.Session.Tick();
         Runtime.ActionOwner.CombatAttack.Tick();
@@ -191,6 +202,28 @@ internal sealed class HeadlessSessionHost : IDisposable
     internal RuntimeTeardownAcknowledgement Stop() =>
         Commands.Session.Stop(Runtime.Generation);
 
+    internal RuntimeSessionStartResult CompletePendingReconnect(
+        long nowTimestamp)
+    {
+        ObjectDisposedException.ThrowIf(_disposed, this);
+        if (!_reconnectPending)
+        {
+            return new RuntimeSessionStartResult(
+                RuntimeSessionStartStatus.Inactive,
+                Runtime.Generation);
+        }
+        if (nowTimestamp < _reconnectDeadline)
+        {
+            return new RuntimeSessionStartResult(
+                RuntimeSessionStartStatus.Deferred,
+                Runtime.Generation);
+        }
+
+        _reconnectPending = false;
+        _reconnectDeadline = 0L;
+        return StartLive(reconnect: true);
+    }
+
     public void Dispose()
     {
         if (_disposed)
@@ -202,6 +235,8 @@ internal sealed class HeadlessSessionHost : IDisposable
             {
                 case 0:
                 {
+                    _reconnectPending = false;
+                    _reconnectDeadline = 0L;
                     RuntimeTeardownAcknowledgement stopped = Stop();
                     if (!stopped.IsComplete)
                     {
@@ -266,6 +301,12 @@ internal sealed class HeadlessSessionHost : IDisposable
                 RuntimeSessionStartStatus.StaleGeneration,
                 Runtime.Generation);
         }
+        if (_reconnectPending)
+        {
+            return new RuntimeSessionStartResult(
+                RuntimeSessionStartStatus.Deferred,
+                Runtime.Generation);
+        }
 
         if (reconnect)
         {
@@ -281,19 +322,32 @@ internal sealed class HeadlessSessionHost : IDisposable
                             "The prior headless session did not quiesce before reconnect."));
             }
 
-            // ACE confirms CharacterLogOff before its account/session index
-            // releases the retiring socket. Starting a replacement in that
-            // short suffix makes ACE reject both sessions as Account In Use.
-            // This is a one-time connection-lifecycle wait, never a steady
-            // scheduler sleep; K2 moves it onto the absolute-deadline host.
             if (_reconnectQuiescence > TimeSpan.Zero)
             {
-                Task.Delay(_reconnectQuiescence)
-                    .GetAwaiter()
-                    .GetResult();
+                // ACE confirms CharacterLogOff before its account/session
+                // index releases the retiring socket. The process scheduler
+                // owns this monotonic deadline so other sessions keep moving
+                // and no command or worker thread blocks.
+                _reconnectDeadline = HeadlessMonotonicTime.Add(
+                    _timeProvider,
+                    _timeProvider.GetTimestamp(),
+                    _reconnectQuiescence);
+                _reconnectPending = true;
+                _diagnostics.Lifecycle(
+                    _descriptor.Id,
+                    "reconnect-deferred",
+                    Runtime);
+                return new RuntimeSessionStartResult(
+                    RuntimeSessionStartStatus.Deferred,
+                    Runtime.Generation);
             }
         }
 
+        return StartLive(reconnect);
+    }
+
+    private RuntimeSessionStartResult StartLive(bool reconnect)
+    {
         string password = _credential.Reveal();
         try
         {
@@ -366,8 +420,7 @@ internal sealed class HeadlessSessionHost : IDisposable
                 OnConfirmationRequest: null,
                 OnConfirmationDone: null,
                 ClientTime: () =>
-                    Stopwatch.GetTimestamp()
-                    / (double)Stopwatch.Frequency),
+                    Runtime.Clock.SimulationTimeSeconds),
             new LiveSocialSessionBindings(
                 Runtime.CommunicationOwner.Chat,
                 Runtime.CommunicationOwner.TurbineChat,

src/AcDream.Headless/Policies/HeadlessBotPolicy.cs

@@ -113,7 +113,9 @@ internal sealed class LifecycleSmokeHeadlessBotPolicy
                 RuntimeSessionStartResult reconnect =
                     commands.Session.Reconnect(view.Generation);
                 if (reconnect.Status
-                    != RuntimeSessionStartStatus.Connected)
+                    is not (
+                        RuntimeSessionStartStatus.Connected
+                        or RuntimeSessionStartStatus.Deferred))
                 {
                     throw new InvalidOperationException(
                         $"Lifecycle gate reconnect was rejected with {reconnect.Status}.");

tests/AcDream.Headless.Tests/HeadlessProcessSchedulerTests.cs

@@ -0,0 +1,280 @@
+using System.Net;
+using AcDream.Core.Net;
+using AcDream.Core.Net.Messages;
+using AcDream.Headless.Configuration;
+using AcDream.Headless.Credentials;
+using AcDream.Headless.Diagnostics;
+using AcDream.Headless.Hosting;
+using AcDream.Headless.Platform;
+using AcDream.Runtime;
+using AcDream.Runtime.Session;
+
+namespace AcDream.Headless.Tests;
+
+public sealed class HeadlessProcessSchedulerTests
+{
+    [Fact]
+    public void AbsoluteDeadlinesTickSessionsInStableElapsedTime()
+    {
+        var time = new ManualTimeProvider();
+        var operations = new FixtureSessionOperations();
+        using HeadlessSessionHost first =
+            CreateSession("first", time, operations);
+        using HeadlessSessionHost second =
+            CreateSession("second", time, operations);
+        Assert.Equal(
+            RuntimeSessionStartStatus.Connected,
+            first.Start().Status);
+        Assert.Equal(
+            RuntimeSessionStartStatus.Connected,
+            second.Start().Status);
+        var scheduler = new HeadlessProcessScheduler(
+            [first, second],
+            time);
+
+        Assert.False(scheduler.DispatchDue(time.GetTimestamp()));
+        time.Advance(TimeSpan.FromMilliseconds(15));
+
+        Assert.True(scheduler.DispatchDue(time.GetTimestamp()));
+        Assert.Equal(1UL, first.Runtime.Clock.FrameNumber);
+        Assert.Equal(1UL, second.Runtime.Clock.FrameNumber);
+        Assert.Equal(
+            0.015d,
+            first.Runtime.Clock.SimulationTimeSeconds,
+            precision: 9);
+        HeadlessSchedulerSnapshot snapshot =
+            scheduler.CaptureSnapshot();
+        Assert.Equal(2, snapshot.TurnCount);
+        Assert.Equal(0, snapshot.CatchUpCollapseCount);
+    }
+
+    [Fact]
+    public void LongPauseUsesBoundedCatchUpAndPreservesElapsedTime()
+    {
+        var time = new ManualTimeProvider();
+        var operations = new FixtureSessionOperations();
+        using HeadlessSessionHost session =
+            CreateSession("catch-up", time, operations);
+        Assert.Equal(
+            RuntimeSessionStartStatus.Connected,
+            session.Start().Status);
+        var scheduler = new HeadlessProcessScheduler(
+            [session],
+            time,
+            maximumCatchUpTurns: 4);
+
+        time.Advance(TimeSpan.FromSeconds(1));
+        Assert.True(scheduler.DispatchDue(time.GetTimestamp()));
+
+        Assert.Equal(4UL, session.Runtime.Clock.FrameNumber);
+        Assert.Equal(
+            1d,
+            session.Runtime.Clock.SimulationTimeSeconds,
+            precision: 9);
+        Assert.Equal(
+            1,
+            scheduler.CaptureSnapshot().CatchUpCollapseCount);
+    }
+
+    [Fact]
+    public void ReconnectQuiescenceIsAMonotonicDeadlineNotABlockingWait()
+    {
+        var time = new ManualTimeProvider();
+        var operations = new FixtureSessionOperations();
+        using HeadlessSessionHost session = CreateSession(
+            "reconnect",
+            time,
+            operations,
+            TimeSpan.FromMilliseconds(2500));
+        Assert.Equal(
+            RuntimeSessionStartStatus.Connected,
+            session.Start().Status);
+        var scheduler = new HeadlessProcessScheduler(
+            [session],
+            time);
+
+        RuntimeSessionStartResult reconnect = session.Reconnect();
+
+        Assert.Equal(
+            RuntimeSessionStartStatus.Deferred,
+            reconnect.Status);
+        Assert.True(session.IsReconnectPending);
+        Assert.Equal(1, operations.CreatedSessionCount);
+        Assert.Equal(1, operations.DisposedSessionCount);
+
+        time.Advance(TimeSpan.FromMilliseconds(2499));
+        Assert.False(scheduler.DispatchDue(time.GetTimestamp()));
+        Assert.Equal(1, operations.CreatedSessionCount);
+
+        time.Advance(TimeSpan.FromMilliseconds(1));
+        Assert.True(scheduler.DispatchDue(time.GetTimestamp()));
+        Assert.False(session.IsReconnectPending);
+        Assert.Equal(2, operations.CreatedSessionCount);
+        Assert.True(session.Runtime.Session.IsInWorld);
+    }
+
+    [Fact]
+    public async Task ProcessHostRunsMultipleSessionsOnOneScheduler()
+    {
+        var configuration = new HeadlessConfiguration
+        {
+            Version = 1,
+            Sessions =
+            [
+                Descriptor(
+                    "first",
+                    "first-stdin"),
+                Descriptor(
+                    "second",
+                    "second-stdin"),
+            ],
+        };
+        var operations = new FixtureSessionOperations();
+        using var diagnostics = new StringWriter();
+        using var host = new HeadlessProcessHost(
+            configuration,
+            HeadlessPathSet.Resolve(new HeadlessPathOverrides()),
+            new StringReader(
+                "first-password"
+                + Environment.NewLine
+                + "second-password"
+                + Environment.NewLine),
+            diagnostics,
+            operations);
+        using var cancellation = new CancellationTokenSource();
+        cancellation.Cancel();
+
+        HeadlessExitCode result =
+            await host.RunAsync(cancellation.Token);
+
+        Assert.Equal(HeadlessExitCode.Success, result);
+        Assert.Equal(2, host.Sessions.Count);
+        Assert.All(
+            host.Sessions,
+            session => Assert.True(session.Runtime.Session.IsInWorld));
+        Assert.Equal(2, operations.CreatedSessionCount);
+        Assert.DoesNotContain(
+            "password",
+            diagnostics.ToString(),
+            StringComparison.Ordinal);
+    }
+
+    private static HeadlessSessionHost CreateSession(
+        string id,
+        TimeProvider time,
+        ILiveSessionOperations operations,
+        TimeSpan? reconnectQuiescence = null)
+    {
+        var credential = new HeadlessCredentialSecret(
+            $"{id}-credential",
+            "password");
+        try
+        {
+            return new HeadlessSessionHost(
+                Descriptor(id, $"{id}-credential"),
+                credential,
+                new HeadlessDiagnosticWriter(TextWriter.Null),
+                operations,
+                time,
+                reconnectQuiescence);
+        }
+        catch
+        {
+            credential.Dispose();
+            throw;
+        }
+    }
+
+    private static HeadlessSessionDescriptor Descriptor(
+        string id,
+        string credentialReference) => new()
+    {
+        Id = id,
+        Endpoint = new HeadlessEndpointDescriptor
+        {
+            Host = "127.0.0.1",
+            Port = 9000,
+        },
+        Account = $"{id}-account",
+        Character = new HeadlessCharacterSelector
+        {
+            Index = 0,
+        },
+        Policy = new HeadlessBotPolicyDescriptor
+        {
+            Id = "idle",
+        },
+        Credential = new HeadlessCredentialReference
+        {
+            Provider = HeadlessCredentialProviderKind.StandardInput,
+            Reference = credentialReference,
+        },
+    };
+
+    private sealed class ManualTimeProvider : TimeProvider
+    {
+        private long _timestamp;
+
+        public override long TimestampFrequency =>
+            TimeSpan.TicksPerSecond;
+
+        public override long GetTimestamp() => _timestamp;
+
+        internal void Advance(TimeSpan duration) =>
+            _timestamp = checked(_timestamp + duration.Ticks);
+    }
+
+    private sealed class FixtureSessionOperations : ILiveSessionOperations
+    {
+        public int CreatedSessionCount { get; private set; }
+        public int DisposedSessionCount { get; private set; }
+
+        public IPEndPoint ResolveEndpoint(string host, int port) =>
+            new(IPAddress.Loopback, port);
+
+        public WorldSession CreateSession(IPEndPoint endpoint)
+        {
+            CreatedSessionCount++;
+            return new WorldSession(endpoint);
+        }
+
+        public void Connect(
+            WorldSession session,
+            string user,
+            string password)
+        {
+        }
+
+        public CharacterList.Parsed GetCharacters(
+            WorldSession session) =>
+            new(
+                0u,
+                [
+                    new CharacterList.Character(
+                        0x50000001u,
+                        "Headless",
+                        0u),
+                ],
+                [],
+                11,
+                "account",
+                true,
+                true);
+
+        public void EnterWorld(
+            WorldSession session,
+            int activeCharacterIndex)
+        {
+        }
+
+        public void Tick(WorldSession session)
+        {
+        }
+
+        public void DisposeSession(WorldSession session)
+        {
+            DisposedSessionCount++;
+            session.Dispose();
+        }
+    }
+}