feat(backend): global notices

Foxnouns.Backend/Services/Caching/NoticeCacheService.cs

@@ -0,0 +1,39 @@
+// Copyright (C) 2023-present sam/u1f320 (vulpine.solutions)
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published
+// by the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+using Foxnouns.Backend.Database;
+using Foxnouns.Backend.Database.Models;
+using Microsoft.EntityFrameworkCore;
+using NodaTime;
+
+namespace Foxnouns.Backend.Services.Caching;
+
+public class NoticeCacheService(IServiceProvider serviceProvider, IClock clock, ILogger logger)
+    : SingletonCacheService<Notice>(serviceProvider, clock, logger)
+{
+    public override Duration MaxAge { get; init; } = Duration.FromMinutes(5);
+
+    public override Func<
+        DatabaseContext,
+        CancellationToken,
+        Task<Notice?>
+    > FetchFunc { get; init; } =
+        async (db, ct) =>
+            await db
+                .Notices.Where(n =>
+                    n.StartTime < clock.GetCurrentInstant() && n.EndTime > clock.GetCurrentInstant()
+                )
+                .OrderByDescending(n => n.Id)
+                .FirstOrDefaultAsync(ct);
+}

Foxnouns.Backend/Services/Caching/SingletonCacheService.cs

@@ -0,0 +1,63 @@
+// Copyright (C) 2023-present sam/u1f320 (vulpine.solutions)
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published
+// by the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+using Foxnouns.Backend.Database;
+using NodaTime;
+
+namespace Foxnouns.Backend.Services.Caching;
+
+public abstract class SingletonCacheService<T>(
+    IServiceProvider serviceProvider,
+    IClock clock,
+    ILogger logger
+)
+    where T : class
+{
+    private T? _item;
+    private Instant _lastUpdated = Instant.MinValue;
+    private readonly SemaphoreSlim _semaphore = new(1, 1);
+    private readonly ILogger _logger = logger.ForContext<SingletonCacheService<T>>();
+
+    public virtual Duration MaxAge { get; init; } = Duration.FromMinutes(5);
+
+    public virtual Func<DatabaseContext, CancellationToken, Task<T?>> FetchFunc { get; init; } =
+        (_, __) => Task.FromResult<T?>(null);
+
+    public async Task<T?> GetAsync(CancellationToken ct = default)
+    {
+        await _semaphore.WaitAsync(ct);
+        try
+        {
+            if (_lastUpdated > clock.GetCurrentInstant() - MaxAge)
+            {
+                return _item;
+            }
+
+            _logger.Debug("Cached item of type {Type} is expired, fetching it.", typeof(T));
+
+            await using AsyncServiceScope scope = serviceProvider.CreateAsyncScope();
+            await using DatabaseContext db =
+                scope.ServiceProvider.GetRequiredService<DatabaseContext>();
+
+            T? item = await FetchFunc(db, ct);
+            _item = item;
+            _lastUpdated = clock.GetCurrentInstant();
+            return item;
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+}