Leases cache: manual refresh, staleness fallback, no duplicate reads

- ILeasesCache.Invalidate() so manual Refresh forces re-read; no watcher recreate
- GET api/leases?refresh=true; LeasesClient.GetLeasesAsync(forceRefresh); Dhcp Refresh button uses forceRefresh: true
- Staleness fallback: cache older than 2 min treated as dirty (fallback if FileSystemWatcher missed event)
- LeasesCache: _lastReadUtc, StaleCacheSeconds = 120

src/DnsmasqWebUI/Client/Http/Abstractions/ILeasesClient.cs

@@ -5,5 +5,6 @@ namespace DnsmasqWebUI.Client.Http.Abstractions;
 /// <summary>Typed client for GET api/leases.</summary>
 public interface ILeasesClient
 {
-    Task<LeasesResult> GetLeasesAsync(CancellationToken ct = default);
+    /// <param name="forceRefresh">When true, invalidates the server cache so the next read is from disk (e.g. after manual Refresh).</param>
+    Task<LeasesResult> GetLeasesAsync(bool forceRefresh = false, CancellationToken ct = default);
 }

src/DnsmasqWebUI/Client/Http/LeasesClient.cs

@@ -10,7 +10,10 @@ public sealed class LeasesClient : ILeasesClient
 
     public LeasesClient(HttpClient http) => _http = http;
 
-    public async Task<LeasesResult> GetLeasesAsync(CancellationToken ct = default) =>
-        await _http.GetFromJsonAsync<LeasesResult>("api/leases", ct)
+    public async Task<LeasesResult> GetLeasesAsync(bool forceRefresh = false, CancellationToken ct = default)
+    {
+        var url = forceRefresh ? "api/leases?refresh=true" : "api/leases";
+        return await _http.GetFromJsonAsync<LeasesResult>(url, ct)
             ?? throw new InvalidOperationException("Unexpected null from api/leases.");
+    }
 }

src/DnsmasqWebUI/Components/Pages/Dhcp.razor

@@ -252,7 +252,7 @@ else
         {
             <p class="text-muted">No leases yet. Leases appear here when DHCP clients obtain an address; use Refresh to update.</p>
             <div class="d-flex flex-wrap align-items-center gap-2 mt-2">
-                <button class="btn btn-primary" @onclick="() => LoadLeases(showLoading: true)" disabled="@_leasesLoading">Refresh</button>
+                <button class="btn btn-primary" @onclick="() => LoadLeases(showLoading: true, forceRefresh: true)" disabled="@_leasesLoading">Refresh</button>
             </div>
         }
         else
@@ -317,7 +317,7 @@ else
                 </div>
             </div>
             <div class="d-flex flex-wrap align-items-center gap-2 mt-2">
-                <button class="btn btn-primary" @onclick="() => LoadLeases(showLoading: true)" disabled="@_leasesLoading">Refresh</button>
+                <button class="btn btn-primary" @onclick="() => LoadLeases(showLoading: true, forceRefresh: true)" disabled="@_leasesLoading">Refresh</button>
             </div>
         }
         <SettingsModal IsVisible="_settingsModalOpen" Title="@_settingsModalTitle" SettingsContext="_settingsModalContext" OnClose="CloseSettingsModal" />
@@ -461,13 +461,13 @@ else
         await InvokeAsync(StateHasChanged);
     }
 
-    private async Task LoadLeases(bool showLoading = true)
+    private async Task LoadLeases(bool showLoading = true, bool forceRefresh = false)
     {
         if (_status == null || !_status.LeasesPathConfigured) return;
         if (showLoading) _leasesLoading = true;
         try
         {
-            var resp = await LeasesClient.GetLeasesAsync();
+            var resp = await LeasesClient.GetLeasesAsync(forceRefresh: forceRefresh);
             _leases = resp.Entries;
         }
         finally

src/DnsmasqWebUI/Controllers/LeasesController.cs

@@ -9,17 +9,21 @@ namespace DnsmasqWebUI.Controllers;
 public class LeasesController : ControllerBase
 {
     private readonly ILeasesFileService _leasesService;
+    private readonly ILeasesCache _cache;
 
-    public LeasesController(ILeasesFileService leasesService)
+    public LeasesController(ILeasesFileService leasesService, ILeasesCache cache)
     {
         _leasesService = leasesService;
+        _cache = cache;
     }
 
     [HttpGet]
-    public async Task<ActionResult<LeasesResult>> Get(CancellationToken ct)
+    public async Task<ActionResult<LeasesResult>> Get([FromQuery] bool refresh, CancellationToken ct)
     {
         try
         {
+            if (refresh)
+                _cache.Invalidate();
             var (available, entries) = await _leasesService.TryReadAsync(ct);
             if (!available)
                 return Ok(new LeasesResult(false, null, "Leases not configured."));

src/DnsmasqWebUI/Services/Abstractions/ILeasesCache.cs

@@ -7,5 +7,8 @@ namespace DnsmasqWebUI.Services.Abstractions;
 /// </summary>
 public interface ILeasesCache : IApplicationSingleton
 {
+    /// <summary>Forces the next <see cref="GetOrRefreshAsync"/> to re-read the file. Use for manual Refresh; no need to recreate the file watcher.</summary>
+    void Invalidate();
+
     Task<(bool Available, IReadOnlyList<LeaseEntry>? Entries)> GetOrRefreshAsync(CancellationToken ct = default);
 }

src/DnsmasqWebUI/Services/LeasesCache.cs

@@ -7,14 +7,19 @@ namespace DnsmasqWebUI.Services;
 
 /// <summary>
 /// Singleton cache for the leases file with a <see cref="FileSystemWatcher"/> so we re-read only when the file changes.
+/// Falls back to treating cache as dirty after <see cref="StaleCacheSeconds"/> so missed watcher events still get fresh data.
 /// </summary>
 public sealed class LeasesCache : ILeasesCache, IDisposable
 {
+    /// <summary>After this many seconds, cached data is treated as stale and re-read on next request (fallback if FileSystemWatcher missed an event).</summary>
+    private const int StaleCacheSeconds = 120;
+
     private readonly string? _path;
     private readonly ILogger<LeasesCache> _logger;
     private FileSystemWatcher? _watcher;
     private readonly object _lock = new();
     private (bool Available, IReadOnlyList<LeaseEntry>? Entries)? _cache;
+    private DateTime? _lastReadUtc;
     private bool _dirty = true;
 
     public LeasesCache(IDnsmasqConfigSetService configSetService, ILogger<LeasesCache> logger)
@@ -64,6 +69,14 @@ public sealed class LeasesCache : ILeasesCache, IDisposable
         }
     }
 
+    public void Invalidate()
+    {
+        lock (_lock)
+        {
+            _dirty = true;
+        }
+    }
+
     public (bool Available, IReadOnlyList<LeaseEntry>? Entries) GetOrRefresh(CancellationToken ct = default)
     {
         if (string.IsNullOrEmpty(_path))
@@ -71,13 +84,19 @@ public sealed class LeasesCache : ILeasesCache, IDisposable
 
         lock (_lock)
         {
-            if (!_dirty && _cache.HasValue)
-                return _cache.Value;
+            if (!_dirty && _cache.HasValue && _lastReadUtc.HasValue)
+            {
+                var ageSeconds = (DateTime.UtcNow - _lastReadUtc.Value).TotalSeconds;
+                if (ageSeconds < StaleCacheSeconds)
+                    return _cache.Value;
+                _dirty = true;
+            }
 
             if (!File.Exists(_path))
             {
                 _logger.LogDebug("Leases file not found: {Path}", _path);
                 _cache = (true, Array.Empty<LeaseEntry>());
+                _lastReadUtc = DateTime.UtcNow;
                 _dirty = false;
                 return _cache.Value;
             }
@@ -92,6 +111,7 @@ public sealed class LeasesCache : ILeasesCache, IDisposable
                         entries.Add(entry);
                 }
                 _cache = (true, entries);
+                _lastReadUtc = DateTime.UtcNow;
                 _dirty = false;
                 return _cache.Value;
             }
@@ -99,6 +119,7 @@ public sealed class LeasesCache : ILeasesCache, IDisposable
             {
                 _logger.LogWarning(ex, "Failed to read leases file: {Path}", _path);
                 _cache = (true, null);
+                _lastReadUtc = DateTime.UtcNow;
                 _dirty = false;
                 return _cache.Value;
             }