feat: Purge slugs when they are expired

backend/src/Web/Features/Contents/Data/SlugPurger.cs

@@ -0,0 +1,41 @@
+namespace Hutopy.Web.Features.Contents.Data;
+
+public class SlugPurger(ContentDbContext context)
+{
+    private static readonly SemaphoreSlim _semaphore = new(1, 1);
+    private static DateTimeOffset _lastPurgeTime = DateTimeOffset.MinValue;
+    private static readonly TimeSpan _minTimeBetweenPurges = TimeSpan.FromSeconds(10);
+
+    public async Task PurgeExpiredSlugsAsync(CancellationToken ct)
+    {
+        // Try to acquire the semaphore
+        if (!await _semaphore.WaitAsync(0, ct))
+        {
+            // Another purge operation is in progress, skip this one
+            return;
+        }
+
+        try
+        {
+            var now = DateTimeOffset.UtcNow;
+            if (now - _lastPurgeTime < _minTimeBetweenPurges)
+            {
+                // Not enough time has passed since the last purge
+                return;
+            }
+
+            // Delete expired slugs that are not in use
+            await context
+                .Slugs
+                .Where(s => s.ReservedUntil < now && s.UsedBy == null)
+                .ExecuteDeleteAsync(ct);
+
+            // Update the last purge time regardless of whether we found expired slugs or not
+            _lastPurgeTime = now;
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+} 

backend/src/Web/Features/Contents/DependencyInjection.cs

@@ -12,6 +12,7 @@ public static class DependencyInjection
         builder.Services.AddDbContext<ContentDbContext>(configureAction);
         builder.Services.AddScoped<ContentDbContextInitializer>();
         builder.Services.Configure<ContentOptions>(builder.Configuration.GetSection(ContentOptions.ConfigurationSection));
+        builder.Services.AddScoped<SlugPurger>();
 
         return builder;
     }

backend/src/Web/Features/Contents/Handlers/ReserveSlug.cs

@@ -29,7 +29,8 @@ public sealed class ReserveSlugRequestValidator : Validator<ReserveSlugRequest>
 [PublicAPI]
 public sealed class ReserveSlug(
     ContentDbContext context,
-    IOptions<ContentOptions> opts)
+    IOptions<ContentOptions> opts,
+    SlugPurger slugPurger)
     : Endpoint<ReserveSlugRequest>
 {
     public override void Configure()
@@ -46,6 +47,9 @@ public sealed class ReserveSlug(
 
         try
         {
+            // First, purge any expired slugs
+            await slugPurger.PurgeExpiredSlugsAsync(ct);
+
             var reservation = await context.Slugs.FirstOrDefaultAsync(
                 s => s.Id == req.ReservationId && s.CreatedBy == User.GetUserId(), 
                 cancellationToken: ct);